Arbuscular mycorrhizal symbiosis ameliorates the optimum quantum yield of photosystem II and reduces non-photochemical quenching in rice plants subjected to salt stress

Mycorrhizas Affect Physiological Performance, Antioxidant System, Photosynthesis, Endogenous Hormones, and Water Content in Cotton under Salt Stress

Saline-alkali stress seriously endangers the normal growth of cotton (Gossypium hirsutum). Arbuscular mycorrhizal fungi (AMF) could enhance salt tolerance by establishing symbiotic relationships with plants. Based on it, a pot experiment was conducted to simulate a salt environment in which cotton was inoculated with Paraglomus occultum to explore its effects on the saline-alkali tolerance of cotton. Our results showed that salt stress noticeably decreased cotton seedling growth parameters (such as plant height, number of leaves, dry weight, root system architecture, etc.), while AMF exhibited a remarkable effect on promoting growth. It was noteworthy that AMF significantly mitigated the inhibitory effect of salt on cotton seedlings. However, AMF colonization in root and soil hyphal length were collectively descended via salt stress. With regard to osmotic regulating substances, Pro and MDA values in roots were significantly increased when seedlings were exposed to salt stress, while AMF only partially mitigated these reactions. Salt stress increased ROS levels in the roots of cotton seedlings and enhanced antioxidant enzyme activity (SOD, POD, and CAT), while AMF mitigated the increases in ROS levels but further strengthened antioxidant enzyme activity. AMF inoculation increased the photosynthesis parameters of cotton seedling leaves to varying degrees, while salt stress decreased them dramatically. When inoculated with AMF under a salt stress environment, only partial mitigation of these photosynthesis values was observed. Under saline-alkali stress, AMF improved the leaf fluorescence parameters (φPSII, Fv'/Fm', and qP) of cotton seedlings, leaf chlorophyll levels, and root endogenous hormones (IAA and BR); promoted the absorption of water; and maintained nitrogen balance, thus alleviating the damage from salt stress on the growth of cotton plants to some extent. In summary, mycorrhizal cotton seedlings may exhibit mechanisms involving root system architecture, the antioxidant system, photosynthesis, leaf fluorescence, endogenous hormones, water content, and nitrogen balance that increase their resistance to saline-alkali environments. This study provide a theoretical basis for further exploring the application of AMF to enhance the salt tolerance of cotton.

The role of arbuscular mycorrhizal symbiosis in plant abiotic stress

Arbuscular mycorrhizal fungi (AMF) can penetrate plant root cortical cells, establish a symbiosis with most land plant species, and form branched structures (known as arbuscules) for nutrient exchange. Plants have evolved a complete plant-AMF symbiosis system to sustain their growth and development under various types of abiotic stress. Here, we highlight recent studies of AM symbiosis and the regulation of symbiosis process. The roles of mycorrhizal symbiosis and host plant interactions in enhancing drought resistance, increasing mineral nutrient uptake, regulating hormone synthesis, improving salt resistance, and alleviating heavy metal stress were also discussed. Overall, studies of AM symbiosis and a variety of abiotic stresses will aid applications of AMF in sustainable agriculture and can improve plant production and environmental safety.

Effects of polystyrene nanoplastics on growth and hemolysin production of microalgae Karlodinium veneficum

There are few studies on the effects of nanoplastics on growth and hemolysin production of harmful algal bloom species at present. In this study, Karlodinium veneficum was exposed to different concentrations (0, 5, 25, 50, 75 mg/L) of polystyrene nanoplastics (PS-NPs, 100 nm) for 96 h. The effects of PS-NPs on growth of K. veneficum were investigated by measuring algal cell abundance, growth inhibition rate (IR), total protein (TP), malondialdehyde (MDA), glutathione reductase (GSH), superoxide dismutase (SOD), ATPase activity (Na+/K+ ATPase and Ca2+/Mg2+ ATPase). Scanning electron microscope and transmission electron microscope (SEM and TEM) images of microalgae with or without nanoplastics were also observed. The effects of PS-NPs on hemolysin production of K. veneficum were studied by measuring the changes of hemolytic toxin production of K. veneficum exposed to PS-NPs on 1, 3, 5 and 7 days. High concentrations (50 and 75 mg/L) of PS-NPs seriously affected the growth of K. veneficum and different degrees of damage to cell morphology and ultrastructure were found. Excessive free radicals and other oxidants were produced in the cells, which disrupted the intracellular redox balance state and caused oxidative damage to the cells, and the basic activities such as photosynthesis and energy metabolism were weakened. The athletic ability of K. veneficum was decreased, but the ability to produce hemolysin was enhanced. It was suggested that the presence of nanoplastics in seawater may strengthen the threat of harmful algal bloom species to aquatic ecosystems and human health.

GmPLP1 negatively regulates soybean resistance to high light stress by modulating photosynthetic capacity and reactive oxygen species accumulation in a blue light-dependent manner

High light stress is an important factor limiting crop yield. Light receptors play an important role in the response to high light stress, but their mechanisms are still poorly understood. Here, we found that the abundance of GmPLP1, a positive blue light receptor protein, was significantly inhibited by high light stress and mainly responded to high blue light. GmPLP1 RNA-interference soybean lines exhibited higher light energy utilization ability and less light damage and reactive oxygen species (ROS) accumulation in leaves under high light stress, while the phenotype of GmPLP1:GmPLP1-Flag overexpression soybean showed the opposite characteristics. Then, we identified a protein-protein interaction between GmPLP1 and GmVTC2, and the intensity of this interaction was primarily affected by sensing the intensity of blue light. More importantly, overexpression of GmVTC2b improved soybean tolerance to high light stress by enhancing the ROS scavenging capability through increasing the biosynthesis of ascorbic acid. This regulation was significantly enhanced after interfering with a GmPLP1-interference fragment in GmVTC2b-ox soybean leaves, but was weakened when GmPLP1 was transiently overexpressed. These findings demonstrate that GmPLP1 regulates the photosynthetic capacity and ROS accumulation of soybean to adapt to changes in light intensity by sensing blue light. In summary, this study discovered a new mechanism through which GmPLP1 participates in high light stress in soybean, which has great significance for improving soybean yield and the adaptability of soybean to high light.

Functions of arbuscular mycorrhizal fungi in regulating endangered species Heptacodium miconioides growth and drought stress tolerance

KEY MESSAGE

The important values of AMF in regulating endangered species Heptacodium miconioides growth and drought stress tolerance. The wild endangered tree Heptacodium miconioides is distributed sporadically in mountainous areas and often subjected to various abiotic stresses, such as drought. The mutualistic association between plants and arbuscular mycorrhizal fungi (AMF) is known to have a significant impact on plant growth and their ability to withstand drought conditions. However, the role of AMF in H. miconioides seedlings in regulating drought tolerance remains unknown. This study investigated the ability of AMF symbionts to mitigate drought and their underlying mechanism on H. miconioides leaves. The results showed that drought stress dramatically decreased the leaf biomass and damaged the chloroplast structure in seedlings. Conversely, inoculation with AMF noticeably alleviated the deleterious effects of drought stress by restoring leaf morphology and improving the photosynthetic capacity. Moreover, plants inoculated with AMF enhanced the proportion of palisade tissue to spongy tissue in the leaves and the size of starch grains and number of plastoglobules in the chloroplast ultrastructure. A transcriptomic analysis showed that 2157 genes (691 upregulated and 1466 downregulated) were differentially expressed between drought stress with AMF inoculation and drought treatment. Further examination demonstrated that the genes exhibiting differential expression were predominantly associated with the advancement of photosynthesis, sucrose and starch metabolism, nitrogen metabolism, chloroplast development, and phenylpropanoid biosynthetic pathways, and the key potential genes were screened. These findings conclusively provided the physiological and molecular mechanisms that underlie improved drought resistance in H. miconioides in the presence of AMF, which could contribute to improving the survival and species conservation of H. miconioides.

Arbuscular mycorrhizal fungi enhance active ingredients of medicinal plants: a quantitative analysis

Medicinal plants are invaluable resources for mankind and play a crucial role in combating diseases. Arbuscular mycorrhizal fungi (AMF) are widely recognized for enhancing the production of medicinal active ingredients in medicinal plants. However, there is still a lack of comprehensive understanding regarding the quantitative effects of AMF on the accumulation of medicinal active ingredients. Here we conducted a comprehensive global analysis using 233 paired observations to investigate the impact of AMF inoculation on the accumulation of medicinal active ingredients. This study revealed that AMF inoculation significantly increased the contents of medicinal active ingredients by 27%, with a particularly notable enhancement observed in flavonoids (68%) and terpenoids (53%). Furthermore, the response of medicinal active ingredients in belowground organs (32%) to AMF was more pronounced than that in aboveground organs (18%). Notably, the AMF genus Rhizophagus exhibited the strongest effect in improving the contents of medicinal active ingredients, resulting in an increase of over 50% in both aboveground and belowground organs. Additionally, the promotion of medicinal active ingredients by AMF was attributed to improvements in physiological factors, such as chlorophyll, stomatal conductance and net photosynthetic rate. Collectively, this research substantially advanced our comprehension of the pivotal role of AMF in improving the medicinal active ingredients of plants and provided valuable insights into the potential mechanisms driving these enhancements.

Seed priming with NaCl helps to improve tissue tolerance, potassium retention ability of plants, and protects the photosynthetic ability in two different legumes, chickpea and lentil, under salt stress

MAIN CONCLUSION

Seed priming with NaCl mimicked the conditions of natural priming to improve the tissue tolerance nature of sensitive legumes, which helps to maintain survivability and yield in mildly saline areas. Seed priming with NaCl is a seed invigoration technique that helps to improve plant growth by altering Na⁺ and K⁺ content under salt stress. Legumes are overall sensitive to salt and salinity hampers their growth and yield. Therefore, a priming (50 mM NaCl) experiment was performed with two different legume members [Cicer arietinum cv. Anuradha and Lens culinaris cv. Ranjan] and different morpho-physiological, biochemical responses at 50 mM, 100 mM, and 150 mM NaCl and molecular responses at 150 mM NaCl were studied in hydroponically grown nonprimed and primed members. Similarly, a pot experiment was performed at 80 mM Na⁺, to check the yield. Tissue Na⁺ and K⁺ content suggested NaCl-priming did not significantly alter the accumulation of Na⁺ among nonprimed and primed members but retained more K⁺ in cells, thus maintaining a lower cellular Na⁺/K⁺ ratio. Low osmolyte content (e.g., proline) in primed members suggested priming could minimize their overall osmolytic requirement. Altogether, these implied tissue tolerance (TT) nature might have improved in case of NaCl-priming as was also reflected by a better TT score (LC₅₀ value). An improved TT nature enabled the primed plants to maintain a significantly higher photosynthetic rate through better stomatal conductance. Along with this, a higher level of chlorophyll content and competent functioning of the photosynthetic subunits improved photosynthetic performance that ensured yield under stress. Overall, this study explores the potential of NaCl-priming and creates possibilities for considerably sensitive members; those in their nonprimed forms have no prospect in mildly saline agriculture.

Arbuscular mycorrhizal fungi alleviates salt stress in Xanthoceras sorbifolium through improved osmotic tolerance, antioxidant activity, and photosynthesis

Mycorrhizal inoculation was widely reported to alleviate the damage resulting from NaCl by various physiological ways. However, the symbiotic benefit under distant NaCl concentrations and the relationship among different responsive physiological processes were elusive. In this study, saline resistant plant Xanthoceras sorbifolium was selected as the experimental material and five concentrations of NaCl in the presence or absence of Arbuscular Mycorrhiza Fungi (AMF) were conducted, in order to understand the differences and similarities on the photosynthesis, antioxidant activity, and osmotic adjustment between arbuscular mycorrhizal (AM) plants and non-arbuscular mycorrhizal (NM) plants. Under low salt stress, X. sorbifolium can adapt to salinity by accumulating osmotic adjustment substances, such as soluble protein and proline, increasing superoxide dismutase (SOD), catalase (CAT) activity, and glutathione (GSH). However, under high concentrations of NaCl [240 and 320 mM (mmol·L−1)], the resistant ability of the plants significantly decreased, as evidenced by the significant downregulation of photosynthetic capacity and biomass compared with the control plants in both AM and NM groups. This demonstrates that the regulatory capacity of X. sorbifolium was limiting, and it played a crucial role mainly under the conditions of 0–160 mM NaCl. After inoculation of AMF, the concentration of Na+ in roots was apparently lower than that of NM plants, while Gs (Stomatal conductance) and Ci (Intercellular CO2 concentration) increased, leading to increases in Pn (Net photosynthetic rate) as well. Moreover, under high salt stress, proline, soluble protein, GSH, and reduced ascorbic acid (ASA) in AM plants are higher in comparison with NM plants, revealing that mycorrhizal symbiotic benefits are more crucial against severe salinity toxicity. Meanwhile, X. sorbifolium itself has relatively high tolerance to salinity, and AMF inoculation can significantly increase the resistant ability against NaCl, whose function was more important under high concentrations.

Dual Inoculation with Rhizophagus irregularis and Bacillus megaterium Improves Maize Tolerance to Combined Drought and High Temperature Stress by Enhancing Root Hydraulics, Photosynthesis and Hormonal Responses

Climate change is leading to combined drought and high temperature stress in many areas, drastically reducing crop production, especially for high-water-consuming crops such as maize. This study aimed to determine how the co-inoculation of an arbuscular mycorrhizal (AM) fungus (Rhizophagus irregularis) and the PGPR Bacillus megaterium (Bm) alters the radial water movement and physiology in maize plants in order to cope with combined drought and high temperature stress. Thus, maize plants were kept uninoculated or inoculated with R. irregularis (AM), with B. megaterium (Bm) or with both microorganisms (AM + Bm) and subjected or not to combined drought and high temperature stress (D + T). We measured plant physiological responses, root hydraulic parameters, aquaporin gene expression and protein abundances and sap hormonal content. The results showed that dual AM + Bm inoculation was more effective against combined D + T stress than single inoculation. This was related to a synergistic enhancement of efficiency of the phytosystem II, stomatal conductance and photosynthetic activity. Moreover, dually inoculated plants maintained higher root hydraulic conductivity, which was related to regulation of the aquaporins ZmPIP1;3, ZmTIP1.1, ZmPIP2;2 and GintAQPF1 and levels of plant sap hormones. This study demonstrates the usefulness of combining beneficial soil microorganisms to improve crop productivity under the current climate-change scenario.

Transcriptome changes induced by Arbuscular mycorrhizal symbiosis in leaves of durum wheat (Triticum durum Desf.) promote higher salt tolerance

The salinity of soil is a relevant environmental problem around the world, with climate change raising its relevance, particularly in arid and semiarid areas. Arbuscular Mycorrhizal Fungi (AMF) positively affect plant growth and health by mitigating biotic and abiotic stresses, including salt stress. The mechanisms through which these benefits manifest are, however, still unclear. This work aimed to identify key genes involved in the response to salt stress induced by AMF using RNA-Seq analysis on durum wheat (Triticum turgidum L. subsp. durum Desf. Husn.). Five hundred sixty-three differentially expressed genes (DEGs), many of which involved in pathways related to plant stress responses, were identified. The expression of genes involved in trehalose metabolism, RNA processing, vesicle trafficking, cell wall organization, and signal transduction was significantly enhanced by the AMF symbiosis. A downregulation of genes involved in both enzymatic and non-enzymatic oxidative stress responses as well as amino acids, lipids, and carbohydrates metabolisms was also detected, suggesting a lower oxidative stress condition in the AMF inoculated plants. Interestingly, many transcription factor families, including WRKY, NAC, and MYB, already known for their key role in plant abiotic stress response, were found differentially expressed between treatments. This study provides valuable insights on AMF-induced gene expression modulation and the beneficial effects of plant-AMF interaction in durum wheat under salt stress.

Cold acclimation alleviates cold stress-induced PSII inhibition and oxidative damage in tobacco leaves

This study aimed to explore how cold acclimation (CA) modulates cold stress in tobacco leaves and reveal the relationship between CA and cold stress resistance, and the mechanism of CA-induced plant resistance to cold stress. This study examined the effects of CA treatment (at 8-10℃ for 2 d) on the cold tolerance of tobacco leaves under 4°C cold stress treatment using seedlings without CA treatment as the control (NA). In both CA and NA leaves, cold stress treatment resulted in a decrease in maximum photochemical efficiency of PSII (Fv/Fm), increase in relative variable fluorescence (VJ) at 2 ms on the standardized OJIP curve, inhibition of PSII activity, and impairment of electron transfer on the acceptor side. Besides increasing the malondialdehyde (MDA) content and electrolyte leakage rate, the cold stress exacerbated the degree of membrane peroxidation. The CA treatment also induced the accumulation of reactive oxygen species (ROS), including superoxide anion (O2·-) and H2O2, and increased the activities of antioxidant enzymes, such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbic acid peroxidase (APX). The CA treatment also enhanced the accumulation of soluble sugar (SS) and soluble protein (SP), cyclic electron flow (CEF), and the proportion of regulatory energy dissipation Y(NPQ). Moreover, CA+ cold stress treatment significantly reduced CEF and Y(NPQ) in tobacco leaves than under NA+ cold stress treatment, thus significantly alleviating the degree of PSII photoinhibition. In conclusion, CA treatment significantly alleviated PSII photoinhibition and oxidative damage in tobacco leaves under cold stress treatment. Improvement in cold resistance of tobacco leaves is associated with the induction of antioxidant enzyme activity, accumulation of osmoregulation substances, and initiation of photoprotective mechanisms.

Transcriptome analysis reveals the mechanisms for mycorrhiza-enhanced salt tolerance in rice

More than half of the global population relies on rice as a staple food, but salinization of soil presents a great threat to rice cultivation. Although previous studies have addressed the possible benefits of arbuscular mycorrhizal (AM) symbiosis for rice under salinity stress, the underlying molecular mechanisms are still unclear. In this study, we found that mycorrhizal rice had better shoot and reproductive growth and a significantly higher K⁺/Na⁺ ratio in the shoot. The reactive oxygen species (ROS) scavenging capacity in rice shoots was also improved by AM symbiosis. To elucidate the molecular mechanisms required for AM-improved salt tolerance, transcriptome analysis revealing the differentially expressed genes (DEGs) based on the response to AM symbiosis, salinity or specific tissue was performed. Thirteen percent of DEGs showed tissue-preferred responses to both AM symbiosis and salt stress and might be the key genes contributing to AM-enhanced salt tolerance. Gene Ontology (GO) enrichment analysis identified GO terms specifically appearing in this category, including cell wall, oxidoreductase activity, reproduction and ester-related terms. Interestingly, GO terms related to phosphate (Pi) homeostasis were also found, suggesting the possible role of the Pi-related signaling pathway involved in AM-enhanced salt tolerance. Intriguingly, under nonsaline conditions, AM symbiosis influenced the expression of these genes in a similar way as salinity, especially in the shoots. Overall, our results indicate that AM symbiosis may possibly use a multipronged approach to influence gene expression in a way similar to salinity, and this modification could help plants be prepared for salt stress.

Microbial Diversity and Adaptation under Salt-Affected Soils: A Review

The salinization of soil is responsible for the reduction in the growth and development of plants. As the global population increases day by day, there is a decrease in the cultivation of farmland due to the salinization of soil, which threatens food security. Salt-affected soils occur all over the world, especially in arid and semi-arid regions. The total area of global salt-affected soil is 1 billion ha, and in India, an area of nearly 6.74 million ha−1 is salt-stressed, out of which 2.95 million ha−1 are saline soil (including coastal) and 3.78 million ha−1 are alkali soil. The rectification and management of salt-stressed soils require specific approaches for sustainable crop production. Remediating salt-affected soil by chemical, physical and biological methods with available resources is recommended for agricultural purposes. Bioremediation is an eco-friendly approach compared to chemical and physical methods. The role of microorganisms has been documented by many workers for the bioremediation of such problematic soils. Halophilic Bacteria, Arbuscular mycorrhizal fungi, Cyanobacteria, plant growth-promoting rhizobacteria and microbial inoculation have been found to be effective for plant growth promotion under salt-stress conditions. The microbial mediated approaches can be adopted for the mitigation of salt-affected soil and help increase crop productivity. A microbial product consisting of beneficial halophiles maintains and enhances the soil health and the yield of the crop in salt-affected soil. This review will focus on the remediation of salt-affected soil by using microorganisms and their mechanisms in the soil and interaction with the plants.

Multi-Omics and Integrative Approach towards Understanding Salinity Tolerance in Rice: A Review

Rice (Oryza sativa L.) plants are simultaneously encountered by environmental stressors, most importantly salinity stress. Salinity is the major hurdle that can negatively impact growth and crop yield. Understanding the salt stress and its associated complex trait mechanisms for enhancing salt tolerance in rice plants would ensure future food security. The main aim of this review is to provide insights and impacts of molecular-physiological responses, biochemical alterations, and plant hormonal signal transduction pathways in rice under saline stress. Furthermore, the review highlights the emerging breakthrough in multi-omics and computational biology in identifying the saline stress-responsive candidate genes and transcription factors (TFs). In addition, the review also summarizes the biotechnological tools, genetic engineering, breeding, and agricultural practicing factors that can be implemented to realize the bottlenecks and opportunities to enhance salt tolerance and develop salinity tolerant rice varieties. Future studies pinpointed the augmentation of powerful tools to dissect the salinity stress-related novel players, reveal in-depth mechanisms and ways to incorporate the available literature, and recent advancements to throw more light on salinity responsive transduction pathways in plants. Particularly, this review unravels the whole picture of salinity stress tolerance in rice by expanding knowledge that focuses on molecular aspects.

Arbuscular Mycorrhizal Fungi Symbiosis to Enhance Plant–Soil Interaction

Arbuscular mycorrhizal fungi (AMF) form a symbiotic relationship with plants; a symbiotic relationship is one in which both partners benefit from each other. Fungi benefit plants by improving uptake of water and nutrients, especially phosphorous, while plants provide 10–20% of their photosynthates to fungus. AMF tend to make associations with 85% of plant families and play a significant role in the sustainability of an ecosystem. Plants’ growth and productivity are negatively affected by various biotic and abiotic stresses. AMF proved to enhance plants’ tolerance against various stresses, such as drought, salinity, high temperature, and heavy metals. There are some obstacles impeding the beneficial formation of AMF communities, such as heavy tillage practices, high fertilizer rates, unchecked pesticide application, and monocultures. Keeping in view the stress-extenuation potential of AMF, the present review sheds light on their role in reducing erosion, nutrient leaching, and tolerance to abiotic stresses. In addition, recent advances in commercial production of AMF are discussed.

The arbuscular mycorrhizal fungus Rhizophagus clarus improves physiological tolerance to drought stress in soybean plants

Soybean (Glycine max L.) is an economically important crop, and is cultivated worldwide, although increasingly long periods of drought have reduced the productivity of this plant. Research has shown that inoculation with arbuscular mycorrhizal fungi (AMF) provides a potential alternative strategy for the mitigation of drought stress. In the present study, we measured the physiological and morphological performance of two soybean cultivars in symbiosis with Rhizophagus clarus that were subjected to drought stress (DS). The soybean cultivars Anta82 and Desafio were grown in pots inoculated with R. clarus. Drought stress was imposed at the V3 development stage and maintained for 7 days. A control group, with well-irrigated plants and no AMF, was established simultaneously in the greenhouse. The mycorrhizal colonization rate, and the physiological, morphological, and nutritional traits of the plants were recorded at days 3 and 7 after drought stress conditions were implemented. The Anta82 cultivar presented the highest percentage of AMF colonization, and N and K in the leaves, whereas the DS group of the Desafio cultivar had the highest water potential and water use efficiency, and the DS + AMF group had thermal dissipation that permitted higher values of Fv/Fm, A, and plant height. The results of the principal components analysis demonstrated that both cultivars inoculated with AMF performed similarly under DS to the well-watered plants. These findings indicate that AMF permitted the plant to reduce the impairment of growth and physiological traits caused by drought conditions.

A, A. WA, Bhubalan K, S. SNM, C. I. MS, Ng LC. Setting a Plausible Route for Saline Soil-Based Crop Cultivations by Application of Beneficial Halophyte-Associated Bacteria: A Review

The global scale of land salinization has always been a considerable concern for human livelihoods, mainly regarding the food-producing agricultural industries. The latest update suggested that the perpetual salinity problem claimed up to 900 million hectares of agricultural land worldwide, inducing salinity stress among salt-sensitive crops and ultimately reducing productivity and yield. Moreover, with the constant growth of the human population, sustainable solutions are vital to ensure food security and social welfare. Despite that, the current method of crop augmentations via selective breeding and genetic engineering only resulted in mild success. Therefore, using the biological approach of halotolerant plant growth-promoting bacteria (HT-PGPB) as bio-inoculants provides a promising crop enhancement strategy. HT-PGPB has been proven capable of forming a symbiotic relationship with the host plant by instilling induced salinity tolerance (IST) and multiple plant growth-promoting traits (PGP). Nevertheless, the mechanisms and prospects of HT-PGPB application of glycophytic rice crops remains incomprehensively reported. Thus, this review describes a plausible strategy of halophyte-associated HT-PGPB as the future catalyst for rice crop production in salt-dominated land and aims to meet the global Sustainable Development Goals (SDGs) of zero hunger.

Tandem application of endophytic fungus Serendipita indica and phosphorus synergistically recuperate arsenic induced stress in rice

In the context of eco-sustainable acquisition of food security, arsenic (As) acts as a deterring factor, which easily infiltrates our food chain via plant uptake. Therefore, devising climate-smart strategies becomes exigent for minimizing the imposed risks. Pertinently, Serendipita indica (S. indica) is well reputed for its post-symbiotic stress alleviatory and phyto-promotive potential. Management of phosphorus (P) is acclaimed for mitigating arsenic toxicity in plants by inhibiting the uptake of As molecules due to the competitive cationic exchange in the rhizosphere. The current study was designed to investigate the tandem effects of S. indica and P in combating As toxicity employing two rice genotypes, i.e., Guodao-6 (GD-6; As-sensitive genotype) and Zhongzhe You-1 (ZZY-1; As-tolerant genotype). After successful fungal colonization, alone and combined arsenic (10  μ M L^-1) and phosphorus (50  μ M L^-1) treatments were applied. Results displayed that the recuperating effects of combined S. indica and P treatment were indeed much profound than their alone treatments; however, most of the beneficial influences were harnessed by ZZY-1 in comparison with GD-6. Distinct genotypic differences were observed for antioxidant enzyme activities, which were induced slightly higher in S. indica-colonized ZZY-1 plants, with or without additional P, as compared to GD-6. Ultrastructure images of root and shoot exhibited ravages of As in the form of chloroplasts-, nuclei-and cell wall-damage with enlarged vacuole area, mellowed mostly by the combined treatment of S. indica and P in both genotypes. Gene expression of PHTs family transporters was regulated at different levels in almost all treatments across genotypes. Conclusively, the results of this study validated the promising role of S. indica and additional P in mitigating As stress, albeit corroborated that the extent of relevant benefit exploitation is highly genotype-dependent. Verily, unlocking the potential of nature-friendly solutions will mend the anthropogenic damage already been done to our environment.

Sex-specific photosynthetic capacity and Na+ homeostasis in Populus euphratica exposed to NaCl stress and AMF inoculation

Soil salinity and associated land degradation are major ecological problems. Excess Na⁺ ions in soil impede the plant photosynthetic process and Na⁺ homeostasis status. Arbuscular mycorrhizal fungi (AMF) can alleviate salt stress in host plants. Although a number of studies have demonstrated that Na⁺ accumulation is decreased by mycorrhizae, the molecular mechanisms involved have received little attention from researchers. Populus euphratica is a typical natural woody tree with excellent salt tolerance. Due to its symbiosis forming capability with AMF, we explored the influence of Funneliformis mosseae on the growth, photosynthesis, and expression of three genes involved in Na⁺ homeostasis within dioecious P. euphratica under salt stress. The results indicated that salt stress significantly increases Na⁺ contents and inhibits growth status and photosynthetic capacity, especially in females. However, AMF had positive effects on the growth status, photosynthetic capacity and Na⁺ homeostasis, especially in males. The expression levels of NHX1 in shoots and HKT1 and SOS1 in roots, all of which are involved in Na⁺ homeostasis, were upregulated by F. mosseae under salt stress. For males, the beneficial effect of AMF centered on extruding, sequestering and long-distance transporting of Na⁺ ions . For females, the beneficial effect of AMF centered on extruding excessive Na⁺.

Plant Growth Promoting Rhizobacteria, Arbuscular Mycorrhizal Fungi and Their Synergistic Interactions to Counteract the Negative Effects of Saline Soil on Agriculture: Key Macromolecules and Mechanisms

Soil saltiness is a noteworthy issue as it results in loss of profitability and development of agrarian harvests and decline in soil health. Microorganisms associated with plants contribute to their growth promotion and salinity tolerance by employing a multitude of macromolecules and pathways. Plant growth promoting rhizobacteria (PGPR) have an immediate impact on improving profitability based on higher crop yield. Some PGPR produce 1-aminocyclopropane-1-carboxylic (ACC) deaminase (EC 4.1.99.4), which controls ethylene production by diverting ACC into α-ketobutyrate and ammonia. ACC deaminase enhances germination rate and growth parameters of root and shoot in different harvests with and without salt stress. Arbuscular mycorrhizal fungi (AMF) show a symbiotic relationship with plants, which helps in efficient uptake of mineral nutrients and water by the plants and also provide protection to the plants against pathogens and various abiotic stresses. The dual inoculation of PGPR and AMF enhances nutrient uptake and productivity of several crops compared to a single inoculation in both normal and stressed environments. Positively interacting PGPR + AMF combination is an efficient and cost-effective recipe for improving plant tolerance against salinity stress, which can be an extremely useful approach for sustainable agriculture.

Mitigation of saline conditions in watermelon with mycorrhiza and silicon application

•

Salt stress effects agronomic traits and uptake of minerals.

•

Salt stress also enhanced the oxidative stress biomarkers like hydrogen peroxide (H₂O₂).

•

Supplementation of Mycorrhiza enhances the agronomical traits and alleviates slat stress.

•

Silicon application also mitigates the salt stress through modulating antioxidant enzymes.

•

The combination of Mycorrhiza and Silicon were more effective than their individual effect.

Citrullus lanatus L. is critical vegetable for salinity stress. Arbuscular mycorrhizal fungi (AMF) and silicon treatments are known to help as bio-ameliorator of saline soils that can improve salinity tolerance in plants. But their combined effect has never been examined on watermelon therefore, present study investigated the effect of inoculation with the Arbuscular mycorrhizal fungi (AMF) along with silicon on the growth and yield parameters, antioxidant enzyme activities, pigment and mineral content of Citrullus lanatus L. plants grown during salt stress conditions. Outcomes from the study point out that salt stressed watermelon plants showed the best morphological and biochemical values when inoculated with Silicon (4 mM) + Glomus mosseae + Gigaspora gigantean. In addition, the plants inoculated by similar treatment demonstrated less osmotic activity, electrolyte leakage, as well as peroxide content. Treatments comprising Silicon (4 mM) with either Glomus mosseae and Gigaspora gigantean also performed significantly similar for most of the traits studied in the present investigation and better than the treatment only with either one of Glomus mosseae and Gigaspora gigantean. Antioxidant efficiency of melon was certainly appreciably enhanced after incubation with AMF and Si combination in salinity stress. Overall, the application of mycorrhiza and silicon can be considered to overcome the salinity stress in watermelon.

Physiological Alteration in Sunflower Plants (Helianthus annuus L.) Exposed to High CO2 and Arbuscular Mycorrhizal Fungi

Sunflower plants (Helianthus annuus L.) in a CO2-enriched atmosphere (eCO2) were used herein to examine the developmental and physiological effects of biofertilization with mycorrhizae (Rhizophagus irregularis). The eCO2 environment stimulated colonization using R. irregularis mycorrhizal fungi, as compared to plants grown under ambient CO2 conditions (aCO2). This colonization promotes plant growth due to an increased nutrient content (P, K, Mg, and B), which favors a greater synthesis of photosynthetic pigments. Biofertilized plants (M) under eCO2 conditions have a higher concentration of carbon compounds in their leaves, as compared to non-biofertilized eCO2 plants (NM). The biofertilization (M) of sunflowers with R. irregularis decreased the C/N ratio, as compared to the NM plants, decreasing the hydrogen peroxide content and increasing the antioxidant enzyme activity (catalase and APX). These results suggest that sunflower symbiosis with R. irregularis improves the absorption of N, while also decreasing the plant’s oxidative stress. It may be concluded that biofertilization with mycorrhizae (R. irregularis) may potentially replace the chemical fertilization of sunflower plants (H. annuus L.), resulting in more environmentally friendly agricultural practices. This information is essential to our understanding of the mechanisms influencing the C and N dynamic in future climate change scenarios, in which high CO2 levels are expected.

Arbuscular mycorrhizal fungi can ameliorate salt stress in Elaeagnus angustifolia by improving leaf photosynthetic function and ultrastructure

Arbuscular mycorrhizal fungi (AMF) can form symbiosis with Elaeagnus angustifolia, allowing this species to tolerate salt stress. However, the physiological mechanism through which AMF improve E. angustifolia tolerance is still unclear. In this study, we examined E. angustifolia inoculated with AMF Rhizophagus irregularis (M) or inactivated inoculum (NM) under 0 and 300 mM NaCl stress for the determination of photosynthetic gas exchange, pigment content, chlorophyll fluorescence, antioxidant capacity and chloroplast ultrastructural in leaves. Photosynthetic gas exchange parameters in the leaves of M and NM decreased significantly under salt stress, while the M treatment significantly reduced the effect of salt stress compared with NM. Various chlorophyll components in the M treatment were two- to three-fold higher than in NM, together with a much more complex chloroplast structure and higher number of plastoglobules. The total flavonoid and proline content in leaves of M increased significantly, while the concentration of malondialdehyde (MDA) decreased significantly under salt stress. Chlorophyll fluorescence data also showed good PSII function in the M treatment, together with salt stress reduction of photochemical reactions and sharp enhancements in non-photosynthetic quenching (NPQ). AMF inoculation ameliorated the inhibition on the actual PSII efficiency (ФPSII) and the photochemical quenching coefficient (q_P ) by 10-15%. Our results clearly demonstrate that R. irregularis can improve the salt tolerance of plants by improving leaf photosynthetic performance, PSII function, antioxidant capacity and leaf chloroplast ultrastructure, and that E. angustifolia inoculated with AMF could enhance saline soil rehabilitation.

Contribution of Rhizobium–Legume Symbiosis in Salt Stress Tolerance in Medicago truncatula Evaluated through Photosynthesis, Antioxidant Enzymes, and Compatible Solutes Accumulation

The effects of salt stress on the growth, nodulation, and nitrogen (N) fixation of legumes are well known, but the relationship between symbiotic nitrogen fixation (SNF) driven by rhizobium–legume symbiosis and salt tolerance in Medicago truncatula is not well studied. The effects of the active nodulation process on salt stress tolerance of Medicago truncatula were evaluated by quantifying the compatible solutes, soluble sugars, and antioxidants enzymes, as well as growth and survival rate of plants. Eight weeks old plants, divided in three groups: (i) no nodules (NN), (ii) inactive nodules (IN), and (iii) active nodules (AN), were exposed to 150 mM of NaCl salt stress for 0, 8, 16, 24, 32, 40, and 48 h in hydroponic system. AN plants showed a higher survival rate (30.83% and 38.35%), chlorophyll contents (37.18% and 44.51%), and photosynthesis compared to IN and NN plants, respectively. Improved salt tolerance in AN plants was linked with higher activities of enzymatic and nonenzymatic antioxidants and higher K+ (20.45% and 39.21%) and lower Na+ accumulations (17.54% and 24.51%) when compared with IN and NN plants, respectively. Additionally, higher generation of reactive oxygen species (ROS) was indicative of salt stress, causing membrane damage as revealed by higher electrolyte leakage and lipid peroxidation. All such effects were significantly ameliorated in AN plants, showing higher compatible solutes (proline, free amino acids, glycine betaine, soluble sugars, and proteins) and maintaining higher relative water contents (61.34%). This study advocates positive role of Rhizobium meliloti inoculation against salt stress through upregulation of antioxidant system and a higher concentration of compatible solutes.

Effect of co-inoculation with arbuscular mycorrhizal fungi and phosphate solubilizing fungi on nutrient uptake and photosynthesis of beach palm under salt stress environment

Beach plum (Prunus maritima) is an ornamental plant, famous for its strong salt and drought stress tolerance. However, the poor growth rate of transplanted seedlings has seriously restricted its application in salinized soil. This study investigated the effects of inoculation with arbuscular mycorrhizal fungus (AMF), Funneliformis mosseae, and phosphate-solubilizing fungus (PSF), Apophysomyces spartima, on the growth, nutrient (N, P, and K) uptake, and photosynthesis of beach plum under saline (170 mM NaCl) and non-saline (0 mM NaCl) conditions. We aimed to find measures to increase the growth rate of beach plum in saline-alkali land and to understand the reasons for this increase. The results showed that salinization adversely affected colonization by AMF but positively increased PSF populations (increased by 33.9-93.3% over non-NaCl treatment). The dual application of AMF and PSF mitigated the effects of salt stress on all growth parameters and nutrient uptake, significantly for roots (dry weight and P and N contents increased by 91.0%, 68.9%, and 40%, respectively, over non-NaCl treatment). Salinization caused significant reductions in net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (E), and intercellular CO2 concentration (Ci) value, while inoculation with AMF and PSF inoculations significantly abated such reductions. The maximum efficiency of photosystem II (PSII) (Fv/Fm), the photochemical quenching coefficient (qP), and the nonphotochemical quenching (NPQ) values were affected little by inoculation with AMF, PSF, or both under non-NaCl treatments. However, plants inoculated with AMF and/or PSF had higher Fv/Fm, qP, and ФPSII values (increased by 72.5-188.1%) than the control under NaCl treatment, but not a higher NPQ value. We concluded that inoculation with AMF or PSF increased nutrient uptake and improved the gas-exchange and Chl fluorescence parameters of beach plum under salt stress environment. These effects could be strengthened by the combination of AMF and PSF, especially for nutrient uptake, root growth, and Pn, thereby alleviating the deleterious effects of NaCl stress on beach plum growth.

Aquaporins and cation transporters are differentially regulated by two arbuscular mycorrhizal fungi strains in lettuce cultivars growing under salinity conditions

The aim was to identify the effects of AM symbiosis on the expression patterns of genes associated with K+ and Na+ compartmentalization and translocation and on K+/Na+ homeostasis in some lettuce (Lactuca sativa) cultivars as well as the effects of the relative abundance of plant AQPs on plant water status. Two AM fungi species (Funneliformis mosseae and Claroideoglomus lamellosum) isolated from the hyper-arid Atacama Desert (northern Chile) were inoculated to two lettuce cultivars (Grand Rapids and Lollo Bionda), and watered with 0 and 60 mM NaCl. At 60 days of plant growth, the AM symbiotic development, biomass production, nutrient content (Pi, Na+, K+), physiological parameters, gene expressions of ion channels and transporters (NHX and HKT1), and aquaporins proteins abundance (phosphorylated and non-phosphorylated) were evaluated. Salinity increased the AM root colonization by both inocula. AM lettuce plants showed an improved growth, increased relative water content and improved of K/Na ratio in root. In Grand Rapids cultivar, the high efficiency of photosystem II was higher than Lollo Bionda cultivar; on the contrary, stomatal conductance was higher in Lollo Bionda. Nevertheless, both parameters were increased by AM colonization. In the same way, LsaHKT1;1, LsaHKT1;6, LsaNHX2, LsaNHX4, LsaNHX6 and LsaNHX8 genes and aquaporins PIP2 were up-regulated differentially by both AM fungi. The improved plant growth was closely related to a higher water status due to increased PIP2 abundance, as well as to the upregulation of LsaNHX gene expression, which concomitantly improved plant nutrition and K+/Na+ homeostasis maintenance.

Arbuscular mycorrhizal fungus-mediated amelioration of NO2-induced phytotoxicity in tomato

Atmospheric nitrogen dioxide (NO₂) negatively affects plant (crop) growth and development, as well the yield and quality in some regions or environments. Arbuscular mycorrhizal fungus (AMF)-mediated amelioration of NO₂-induced plant damage has been reported, but the underlying mechanisms remained unclear. This study explored the beneficial effect of AMF symbiosis on tomato plant responses to NO₂ at physiology, biochemistry, and gene expression, with an emphasis on nitrate metabolism, antioxidative defense, and photosynthetic performance. Pot-grown plants were used in the experiments, which were performed in laboratory from February to November 2019. NO₂ fumigation with a dose of 10 ± 1 ppm was carried out after 50 d of plant growth, and data were collected following 8 h of fumigation. NO₂ fumigation (+NO₂) and AMF inoculation (+AMF), alone and especially in combination (NO₂ + AMF), increased the gene expression of nitrate- and nitrite reductase, and their enzymatic activity in leaves, such as by 61%, 27%, and 126% for the activity of nitrate reductase, and by 95%, 37%, and 188% for nitrite reductase, respectively, in +NO₂, +AMF, and AMF + NO₂ plants relative the control (-NO₂, -AMF) levels. Following NO₂ exposure, +AMF leaves displayed stronger activities of superoxide dismutase, peroxidase and catalase, and higher content of glutathione and ratio of its reduced form to oxidized form, as compared with -AMF ones. Correspondingly, lesser oxidative damage was detected in +AMF than in -AMF plants, as indicated by the contents of H₂O₂ and malondialdehyde, electrolyte leakage, also by in situ visualization for the formation of H₂O₂, superoxide anion, and dead cells. The increased antioxidative capacity in +AMF plants was correlated with enhanced expression of antioxidation-related genes. Exposure to NO₂ substantially impaired photosynthetic processes in both + AMF and -AMF plants, but an obvious mitigation was observed in the former than in the latter. For example, the total chlorophyll, net photosynthetic rate, stomatal conductance, and ribulose-1,5-bisphosphate carboxylase activity were 18%, 27%, 26%, and 40% higher, respectively, in +AMF than in -AMF plants under NO₂ stress. The differential photosynthetic performance was also revealed by chlorophyll fluorescence imaging. We analyzed the expression patterns of some genes related to photosynthesis and carbon metabolisms, and found that all of them exclusively presented a higher expression level in +AMF plants relative to -AMF ones under NO₂ stress. Taken together, this study provided evidence that AMF symbiosis played a positively regulatory role in host plant responses to NO₂, probably by increasing leaf nitrate metabolism and antioxidative defense, and maintaining the photosynthetic efficiency to some extent, wherein the transcription regulation might be a main target.

Arbuscular Mycorrhizas Regulate Photosynthetic Capacity and Antioxidant Defense Systems to Mediate Salt Tolerance in Maize

Salt stress inhibits photosynthetic process and triggers excessive formation of reactive oxygen species (ROS). This study examined the role of arbuscular mycorrhizal (AM) association in regulating photosynthetic capacity and antioxidant activity in leaves of two maize genotypes (salt-tolerant JD52 and salt-sensitive FSY1) exposed to salt stress (100 mM NaCl) in soils for 21 days. The leaf water content, chlorophyll content, and photosynthetic capacity in non-mycorrhizal (NM) plants were decreased by salt stress, especially in FSY1, with less reduction in AM plants than NM plants. Salinity increased the activities of antioxidant enzymes (superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and glutathione reductase (GR)) in both genotypes regardless of AM inoculation, but decreased the contents of non-enzymatic antioxidants (reduced glutathione (GSH) and ascorbate (AsA)), especially in FSY1, with less decrease in AM plants than NM plants. The AM plants, especially JD52, maintained higher photosynthetic capacity, CO2 fixation efficiency, and ability to preserve membrane integrity than NM plants under salt stress, as also indicated by the higher antioxidant contents and lower malondialdehyde (MDA)/electrolyte leakage in leaves. To conclude, the higher salt tolerance in AM plants correlates with the alleviation of salinity-induced oxidative stress and membrane damage, and the better performance of photosynthesis could have also contributed to this effect through reduced ROS formation. The greater improvements in photosynthetic processes and antioxidant defense systems by AM fungi in FSY1 than JD52 under salinity demonstrate genotypic variation in antioxidant defenses for mycorrhizal amelioration of salt stress.

Insights Into Microbially Induced Salt Tolerance and Endurance Mechanisms (STEM) in Plants

Salt stress threatens the achievement of sustainable global food security goals by inducing secondary stresses, such as osmotic, ionic, and oxidative stress, that are detrimental to plant growth and productivity. Various studies have reported the beneficial roles of microbes in ameliorating salt stress in plants. This review emphasizes salt tolerance and endurance mechanisms (STEM) in microbially inoculated (MI) plants that ensure plant growth and survival. Well-established STEM have been documented in MI plants and include conglomeration of osmolytes, antioxidant barricading, recuperating nutritional status, and ionic homeostasis. This is achieved via involvement of P solubilization, siderophore production, nitrogen fixation, selective ion absorption, volatile organic compound production, exopolysaccharide production, modifications to plant physiological processes (photosynthesis, transpiration, and stomatal conductance), and molecular alterations to alter various biochemical and physiological processes. Salt tolerance and endurance mechanism in MI plants ensures plant growth by improving nutrient uptake and maintaining ionic homeostasis, promoting superior water use efficiency and osmoprotection, enhancing photosynthetic efficiency, preserving cell ultrastructure, and reinforcing antioxidant metabolism. Molecular research in MI plants under salt stress conditions has found variations in the expression profiles of genes such as HKT1, NHX, and SOS1 (ion transporters), PIPs and TIPs (aquaporins), RBCS, RBCL (RuBisCo subunits), Lipoxygenase2 [jasmonic acid (JA) signaling], ABA (abscisic acid)-responsive gene, and APX, CAT, and POD (involved in antioxidant defense). Proteomic analysis in arbuscular mycorrhizal fungi-inoculated plants revealed upregulated expression of signal transduction proteins, including Ca²⁺ transporter ATPase, calcium-dependent protein kinase, calmodulin, and energy-related proteins (NADH dehydrogenase, iron-sulfur protein NADH dehydrogenase, cytochrome C oxidase, and ATP synthase). Future research should focus on the role of stress hormones, such as JA, salicylic acid, and brassinosteroids, in salt-stressed MI plants and how MI affects the cell wall, secondary metabolism, and signal transduction in host plants.

Arbuscular mycorrhiza influences carbon-use efficiency and grain yield of wheat grown under pre- and post-anthesis salinity stress

Soil salinity severely affects and constrains crop production worldwide. Salinity causes osmotic and ionic stress, inhibiting gas exchange and photosynthesis, ultimately impairing plant growth and development. Arbuscular mycorrhiza (AM) have been shown to maintain light and carbon use efficiency under stress, possibly providing a tool to improve salinity tolerance of the host plants. Thus, it was hypothesized that AM will contribute to improved growth and yield under stress conditions. Wheat plants (Triticum aestivum L.) were grown with (AMF+) or without (AMF-) arbuscular mycorrhizal fungi (AMF) inoculation. Plants were subjected to salinity stress (200 mm NaCl) either at pre- or post-anthesis or at both stages. Growth and yield components, leaf chlorophyll content as well as gas exchange parameters and AMF colonization were analysed. AM plants exhibited a higher rate of net photosynthesis and stomatal conductance and lower intrinsic water use efficiency. Furthermore, AM wheat plants subjected to salinity stress at both pre-anthesis and post-anthesis maintained higher grain yield than non-AM salinity-stressed plants. These results suggest that AMF inoculation mitigates the negative effects of salinity stress by influencing carbon use efficiency and maintaining higher grain yield under stress.

Photosynthetic Traits and Nitrogen Uptake in Crops: Which Is the Role of Arbuscular Mycorrhizal Fungi?

Arbuscular mycorrhizal (AM) fungi are root symbionts that provide mineral nutrients to the host plant in exchange for carbon compounds. AM fungi positively affect several aspects of plant life, improving nutrition and leading to a better growth, stress tolerance, and disease resistance and they interact with most crop plants such as cereals, horticultural species, and fruit trees. For this reason, they receive expanding attention for the potential use in sustainable and climate-smart agriculture context. Although several positive effects have been reported on photosynthetic traits in host plants, showing improved performances under abiotic stresses such as drought, salinity and extreme temperature, the involved mechanisms are still to be fully discovered. In this review, some controversy aspects related to AM symbiosis and photosynthesis performances will be discussed, with a specific focus on nitrogen acquisition-mediated by AM fungi.

Changes in Photo-Protective Energy Dissipation of Photosystem II in Response to Beneficial Bacteria Consortium in Durum Wheat under Drought and Salinity Stresses

The present research aimed at evaluating the harmless dissipation of excess excitation energy by durum wheat (Triticum durum Desf.) leaves in response to the application of a bacterial consortium consisting of four plant growth-promoting bacteria (PGPB). Three pot experiments were carried out under non-stress, drought (at 40% field capacity), and salinity (150 mM NaCl) conditions. The results showed that drought and salinity affected photo-protective energy dissipation of photosystem II (PSII) increasing the rate of non-photochemical chlorophyll fluorescence quenching (NPQ (non-photochemical quenching) and qCN (complete non-photochemical quenching)), as well as decreasing the total quenching of chlorophyll fluorescence (qTQ), total quenching of variable chlorophyll fluorescence (qTV) and the ratio of the quantum yield of actual PSII photochemistry, in light-adapted state to the quantum yield of the constitutive non-regulatory NPQ (PQ rate). Our results also indicated that the PGPB inoculants can mitigate the adverse impacts of stresses on leaves, especially the saline one, in comparison with the non-fertilized (control) treatment, by increasing the fraction of light absorbed by the PSII antenna, PQ ratio, qTQ, and qTV. In the light of findings, our beneficial bacterial strains showed the potential in reducing reliance on traditional chemical fertilizers, in particular in saline soil, by improving the grain yield and regulating the amount of excitation energy.

Arbuscular mycorrhizal fungus inocula from coastal sand dunes arrest olive cutting growth under salinity stress

Cultivation of olive trees covers large coastal areas of land in Mediterranean regions, many of them characterized by low soil fertility and exposed to salinity and seasonal drought. In this frame, we developed mixed community inocula of arbuscular mycorrhizal fungi (AMF) derived from the extreme, seasonally arid environments of six Mediterranean sand dunes and evaluated their effects, in the form of community inocula, on rooted semi-woody olive tree cuttings (Olea europaea cv. Koroneiki). The plantlets were grown in the greenhouse for 10 months under 50 mM and 100 mM concentrations of NaCl, successively applied to induce osmotic stress. Inoculation had a positive effect on plant growth and nutrient uptake. However, the three best-performing inocula in early colonization and in plant growth enhancement also resulted in high plant sensitivity to high salinity, which was not observed for the other three inocula. This was expressed by decreased nutrient uptake and drastically lower plant growth, plant photosynthesis, and stomatal conductance (generally an over 50% reduction compared to no salinity application). Amplicon sequencing analysis of the olive plants under salinity stress showed that the AMF communities in the roots were clearly differentiated by inoculation treatment. We could not, however, consistently associate the plant responses observed under high salinity with specific shared AMF community membership or assembly attributes. The observed physiological overreaction to osmotic stress may be an adaptation trait, potentially brought about by host selection coupled to abiotic environmental filtering, in the harsh conditions from which the AMF inocula were derived. The overreaction may, however, be undesirable if conveyed to allochthonous plants at an agronomic level.

Ericoid mycorrhizal fungi enhance salt tolerance in ericaceous plants

To examine the effects of ericoid mycorrhizal (ERM) fungi on salt tolerance of ericaceous plants, we inoculated roots of velvetleaf blueberry (Vaccinium myrtilloides), Labrador tea (Rhododendron groenlandicum), and lingonberry (Vaccinium vitis-idaea) with ericoid mycorrhizal fungi Oidiodendron maius and Meliniomyces variabilis. Plants were subjected to 0 (NaCl control) and 30 mM NaCl treatments, and plant dry weights, gas exchange, and leaf chlorophyll concentrations were compared in inoculated and non-inoculated plants. M. variabilis increased root dry weights in all three species of NaCl-treated plants, and O. maius enhanced root dry weights of lingonberry plants treated with NaCl. Both fungal species were especially effective in enhancing root and shoot dry weights in control (0 mM NaCl) and NaCl-treated lingonberry seedlings. Leaf chlorophyll concentrations were enhanced by fungal inoculation in all three plant species, and this effect persisted under salt stress in Labrador tea and lingonberry. Salt treatment drastically reduced transpiration rates (E) and lowered net photosynthesis (Pn) to the negative values in all three species of non-inoculated plants, and this effect was partly or almost completely reversed by the inoculation with O. maius and M. variabilis. Fungal inoculation was especially effective in reducing NaCl effects on Pn in lingonberry. Oidiodendron maius and M. variabilis were also equally effective in reversing NaCl-induced declines of E in velvetleaf blueberry and lingonberry. However, in Labrador tea, O. maius reversed the decline of E in NaCl-treated plants less compared with M. variabilis resulting in high photosynthetic water use efficiency values. The results support the hypothesis that, similarly to arbuscular mycorrhizal and ectomycorrhizal associations, ERM association increases salt tolerance of plants.

Effects of single and multiple species inocula of arbuscular mycorrhizal fungi on the salinity tolerance of a Bangladeshi rice (Oryza sativa L.) cultivar

Soil salinization due to sea level rise and groundwater irrigation has become an important agronomic problem in many parts of the world. Symbiosis between crop species and arbuscular mycorrhizal fungi (AMF) may alleviate salt stress-induced detrimental effects on crop growth and yield, for example, through helping the host plant to selectively absorb potassium while avoiding uptake of excessive sodium. Here, we performed a greenhouse experiment to evaluate growth, grain yield, and salt tolerance of a Bangladeshi rice cultivar under three levels of salt stress (0, 75, and 120 mM) after inoculation with three different AMF species from three different genera (Funnelliformis mosseae (BEG12), Acaulospora laevis (BEG13), and Gigaspora margarita (BEG34)), singly and in combination. We found that under salt stress, AMF inoculation enhanced total chlorophyll concentration, shoot K⁺/Na⁺ ratio, and lowered shoot Na⁺/root Na⁺ ratio, accompanied by increased root biomass, spikelet fertility, and grain yield compared with the non-inoculated control plants. Specifically, we found that the combination of BEG13 and BEG34 increased rice yield by 125 and 143% as compared with the non-inoculated controls, at the 75 and 120mM salt levels, respectively. In general, the low AMF diversity treatments (one species or a combination of two AMF species) were found to be the most effective in mediating salt stress tolerance for the majority of the measured crop performance variables. Overall, our results indicate that specific AMF species can promote the salt tolerance and productivity of rice, likely by increasing photosynthetic efficiency and restricting Na⁺ uptake and transport from root to shoot in AMF-inoculated plants.

Increased arbuscular mycorrhizal fungal colonization reduces yield loss of rice (Oryza sativa L.) under drought

Drought reduces the availability of soil water and the mobility of nutrients, thereby limiting the growth and productivity of rice. Under drought, arbuscular mycorrhizal fungi (AMF) increase P uptake and sustain rice growth. However, we lack knowledge of how the AMF symbiosis contributes to drought tolerance of rice. In the greenhouse, we investigated mechanisms of AMF symbiosis that confer drought tolerance, such as enhanced nutrient uptake, stomatal conductance, chlorophyll fluorescence, and hormonal balance (abscisic acid (ABA) and indole acetic acid (IAA)). Two greenhouse pot experiments comprised three factors in a full factorial design with two AMF treatments (low- and high-AMF colonization), two water treatments (well-watered and drought), and three rice varieties. Soil water potential was maintained at 0 kPa in the well-watered treatment. In the drought treatment, we reduced soil water potential to - 40 kPa in experiment 1 (Expt 1) and to - 80 kPa in experiment 2 (Expt 2). Drought reduced shoot and root dry biomass and grain yield of rice in both experiments. The reduction of grain yield was less with higher AMF colonization. Plants with higher AMF colonization showed higher leaf P concentrations than plants with lower colonization in Expt 1, but not in Expt 2. Plants with higher AMF colonization exhibited higher stomatal conductance and chlorophyll fluorescence than plants with lower colonization, especially under drought. Drought increased the levels of ABA and IAA, and AMF colonization also resulted in higher levels of IAA. The results suggest both nutrient-driven and plant hormone-driven pathways through which AMF confer drought tolerance to rice.

Proteomic analysis and interactions network in leaves of mycorrhizal and nonmycorrhizal sorghum plants under water deficit

For understanding the water deficit stress mechanism in sorghum, we conducted a physiological and proteomic analysis in the leaves of Sorghum bicolor L. Moench (a drought tolerant crop model) of non-colonized and colonized plants with a consortium of arbuscular mycorrhizal fungi. Physiological results indicate that mycorrhizal fungi association enhances growth and photosynthesis in plants, under normal and water deficit conditions. 2D-electrophoresis profiles revealed 51 differentially accumulated proteins in response to water deficit, of which HPLC/MS successfully identified 49. Bioinformatics analysis of protein–protein interactions revealed the participation of different metabolic pathways in nonmycorrhizal compared to mycorrhizal sorghum plants under water deficit. In noninoculated plants, the altered proteins are related to protein synthesis and folding (50S ribosomal protein L1, 30S ribosomal protein S10, Nascent polypeptide-associated complex subunit alpha), coupled with multiple signal transduction pathways, guanine nucleotide-binding beta subunit (Rack1) and peptidyl-prolyl-cis-trans isomerase (ROC4). In contrast, in mycorrhizal plants, proteins related to energy metabolism (ATP synthase-24kDa, ATP synthase β), carbon metabolism (malate dehydrogenase, triosephosphate isomerase, sucrose-phosphatase), oxidative phosphorylation (mitochondrial-processing peptidase) and sulfur metabolism (thiosulfate/3-mercaptopyruvate sulfurtransferase) were found. Our results provide a set of proteins of different metabolic pathways involved in water deficit produced by sorghum plants alone or associated with a consortium of arbuscular mycorrhizal fungi isolated from the tropical rain forest Los Tuxtlas Veracruz, México.

Ethylene insensitive mutation improves Arabidopsis plant tolerance to NO2 exposure

Ethylene signaling was addressed, for the first time, in plant responses to nitrogen dioxide (NO₂) by comparatively analyzing the performance of Arabidopsis ethylene insensitive 2 (ein2-1) with wild-type (WT) plants. Following NO₂ fumigation, severe leaf wilting and chlorosis occurred in WT plants, but much less symptoms were observed in ein2-1. The activities of superoxide dismutase (SOD), peroxidase (PRX) and catalase (CAT) were 39%, 92%, and 11% higher, respectively, in ein2-1 than in WT following NO₂ exposure. Although glutathione contents and the ratio of its reduced form (GSH) to oxidized form (GSSG) were decreased by NO₂, an obviously alleviated degree was detected in ein2-1 relative to WT. Correspondingly, the contents of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA), and electrolyte leakage were 25%, 24%, and 29% lower, respectively, in ein2-1 than in WT. The difference of oxidative stress between two tested genotypes was also revealed by the leaf staining regarding the production and distribution of H₂O₂, superoxide anion (O₂˙-), and cell death. The genes involved in antioxidation or oxidation-reduction processes mostly presented a stronger expression in ein2-1 than in WT under NO₂ stress. The photosynthesis-related parameters including chlorophyll and soluble sugar contents, net photosynthetic rate (Pn), and ribulose bisphosphate carboxylase/oxygenase (Rubisco) activity and gene expression, and chlorophyll fluorescence parameters were affected, generally, to a lesser degree in ein2-1 than in WT following NO₂ fumigation. The enzymatic activities and gene expressions of invertases mostly displayed a higher level in ein2-1 relative to WT following NO₂ fumigation. For example, the activities of cytoplasmic, cell wall and vacuolar invertases were 76%, 26%, and 26% higher, respectively, in ein2-1 than in WT. Together, these data suggest that ethylene signal insensitivity efficiently improves plant tolerance to NO₂ exposure, and the possible mechanisms might be correlated with leaf antioxidative defense, photosynthesis-related processes, and sucrose metabolisms.

Alleviation of Salt Stress in Upland Rice (Oryza sativa L. ssp. indica cv. Leum Pua) Using Arbuscular Mycorrhizal Fungi Inoculation

Arbuscular mycorrhizal fungi (AMF) symbionts not only promote the growth of host plant but also alleviate abiotic stresses. This study aimed to investigate the putative role of AMF in salt stress regulation of upland pigmented rice cv. Leum Pua (LP) comparing with Pokkali salt tolerant (positive check). In general, LP is a variety of glutinous rice that contains anthocyanin pigment in the black pericarp, due to which it possesses high antioxidant activities compared to non-pigmented rice. Pot experiment was conducted to evaluate the impact of inoculated AMF, Glomus etunicatum (GE), Glomus geosporum (GG), and Glomus mosseae (GM) strains, in the LP plantlets subjected to 0 (control) or 150 mM NaCl (salt stress) for 2 weeks in comparison with Pokkali (a salt tolerant rice cultivar), which was maintained as a positive check. Root colonization percentage under NaCl conditions ranged from 23 to 30%. Na⁺ content in the flag leaf tissues was increased to 18-35 mg g^-1 DW after exposure to 150 mM NaCl for 14 days in both inoculated and un-inoculated LP plants, whereas Na:K ratio was very low in cv. Pokkali. Interestingly, sucrose content in the flag leaf tissues of un-inoculated LP plants under salt stress was increased significantly by 50 folds over the control as an indicator of salt stress response, whereas it was unchanged in all AMF treatments. Fructose and free proline in GE inoculated plants under salt stress were accumulated over control by 5.75 and 13.59 folds, respectively, for osmotic adjustment of the cell, thereby maintaining the structure and functions of chlorophyll pigments, F_v/F_m, Φ_PSII, and stomatal function. Shoot height, flag leaf length, number of panicles, panicle length, panicle weight, and 100-grain weight in GE inoculated plants of cv. LP under salt stress were maintained similar to cv. Pokkali. Interestingly, cyanidin-3-glucoside (C3G) and peonidin-3-glucoside (P3G) in the pericarp of cv. LP were regulated by GE inoculation under salt stress conditions. In summary, AMF-inoculation in rice crop is a successful alternative approach to reduce salt toxicity, maintain the yield attributes, and regulate anthocyanins enrichment in the pericarp of grains.

Soil phenanthrene phytoremediation capacity in bacteria-assisted Spartina densiflora

Polycyclic aromatic hydrocarbons (PAH) have become a threat for the conservation of wetlands worldwide. The halophyte Spartina densiflora has shown to be potentially useful for soil phenanthrene phytoremediation, but no studies on bacteria-assisted hydrocarbon phytoremediation have been carried out with this halophyte. In this work, three phenanthrene-degrading endophytic bacteria were isolated from S. densiflora tissues and used for plant inoculation. Bacterial bioaugmentation treatments slightly improved S. densiflora growth, photosynthetic and fluorescence parameters. But endophyte-inoculated S. densiflora showed lower soil phenanthrene dissipation rates than non-inoculated S. densiflora (30% below) or even bulk soil (23% less). Our work demonstrates that endophytic inoculation on S. densiflora under greenhouse conditions with the selected PAH-degrading strains did not significantly increase inherent phenanthrene soil dissipation capacity of the halophyte. It would therefore be advisable to provide effective follow-up of bacterial colonization, survival and metabolic activity during phenanthrene soil phytoremediation.

Mycorrhizal frequency, physiological parameters, and yield of strawberry plants inoculated with endomycorrhizal fungi and rhizosphere bacteria

Due to the impoverishment of agricultural and horticultural soils and replant diseases, there is a need to use bioproducts and beneficial microorganisms in order to improve the quality of soils and growth substrates. For this reason, research was undertaken to assess the impact of arbuscular mycorrhizal fungi and rhizosphere bacteria on changes in soil microbiology, the degree of colonization of plant roots by mycorrhizal fungi, selected physiological parameters, and fruit quality and yield of the strawberry cultivar "Rumba." The plants were inoculated with the mycorrhizal preparation Mykoflor (Rhizophagus irregularis, Funneliformis mosseae, Claroideoglomus etunicatum), MYC 800 (Rhizophagus intraradices), and the bacterial preparation Rhizocell C (Bacillus amyloliquefaciens IT45). The applied preparations increased the total number of bacteria and fungi in the soil and mycorrhizal frequency in the roots of the strawberry plants. They increased the chlorophyll "a" and total chlorophyll concentrations in the leaves as well as the rate of transpiration and CO₂ concentration in the intercellular spaces in the leaves. The plants treated with Rhizocell C and MYC 800 exhibited a higher CO₂ assimilation rate than control plants. The biopreparations increased chlorophyll fluorescence parameters such as maximum fluorescence (F_M) and the maximum potential photochemical reaction efficiency in PS II (F_V/F_M). The influence of the species of rhizosphere bacteria and mycorrhizal fungi used in the experiment on the physiological traits of strawberry plants contributed, especially in the second year of the study, to increase the yield and mean weight of strawberry fruit.

Salt-Induced Damage is Alleviated by Short-Term Pre-Cold Treatment in Bermudagrass (Cynodon dactylon)

Excess salinity is a major environmental stress that limits growth and development of plants. Improving salt stress tolerance of plants is important in order to enhance land utilization and crop yield. Cold priming has been reported to trigger the protective processes in plants that increase their stress tolerance. Bermudagrass (Cynodon dactylon) is one of the most widely used turfgrass species around the world. However, the effect of cold priming on salt tolerance of bermudagrass is largely unknown. In the present study, wild bermudagrass was pre-treated with 4 °C for 6 h before 150 mM NaCl treatment for one week. The results showed that the cell membrane stability, ion homeostasis and photosynthesis process which are usually negatively affected by salt stress in bermudagrass were alleviated by short-term pre-cold treatment. Additionally, the gene expression profile also corresponded to the change of physiological indexes in bermudagrass. The results suggest that cold priming plays a positive role in improving salt stress tolerance of bermudagrass.

Comparative transcriptome analysis of the garden asparagus (Asparagus officinalis L.) reveals the molecular mechanism for growth with arbuscular mycorrhizal fungi under salinity stress

Soil salinity is one of the most abiotic stress factors that severely affects the growth and development of many plants, which can ultimately threaten crop yield. Arbuscular mycorrhiza fungi (AMF) has been proven to be effective in mitigating salinity stress by symbiosis in many crops. Asparagus officinalis are perennial plants grown in saline-alkaline soil, however, limited information on their molecular mechanisms has restricted efficient application of AMF to garden asparagus under salinity stress. In this study, we conducted a transcriptome analysis on the leaves of garden asparagus to identify gene expression under salinity stress. Seedlings were grown in 4 treatments, including non-inoculated AMF using distilled water (NI), inoculated AMF using distilled water (AMF), non-inoculated with salinity stress (NI + S), and inoculated with salinity stress (AMF + S). A total of 6019 novel genes were obtained based on the reference-guided assembly of the garden asparagus transcriptome. Results revealed that 455 differentially expressed genes (DEGs) were identified when comparing NI + S to AMF + S. However, among the up-regulated DEGs, 41 DEGs were down-regulated, while 242 DEGs had no differences in their expression levels when comparing NI to NI + S. These DEGs' expression patterns may be key induced by AMF under salinity stress. Additionally, the GO and KEGG enrichment analyses of 455 DEGs revealed that these genes mainly participate in the improvement of the internal environment in plant cells, nitrogen metabolic-related processes, and possible photoprotection mechanisms. These findings provide insight into enhanced salinity stress adaptation by AMF inoculation, as well as salt-tolerant candidate genes for further functional analyses.

The arbuscular mycorrhizal symbiosis regulates aquaporins activity and improves root cell water permeability in maize plants subjected to water stress

Studies have suggested that increased root hydraulic conductivity in mycorrhizal roots could be the result of increased cell-to-cell water flux via aquaporins. This study aimed to elucidate if the key effect of the regulation of maize aquaporins by the arbuscular mycorrhizal (AM) symbiosis is the enhancement of root cell water transport capacity. Thus, water permeability coefficient (P_f ) and cell hydraulic conductivity (L_pc ) were measured in root protoplast and intact cortex cells of AM and non-AM plants subjected or not to water stress. Results showed that cells from droughted-AM roots maintained P_f and L_pc values of nonstressed plants, whereas in non-AM roots, these values declined drastically as a consequence of water deficit. Interestingly, the phosphorylation status of PIP2 aquaporins increased in AM plants subjected to water deficit, and P_f values higher than 12 μm s^-1 were found only in protoplasts from AM roots, revealing the higher water permeability of AM root cells. In parallel, the AM symbiosis increased stomatal conductance, net photosynthesis, and related parameters, showing a higher photosynthetic capacity in these plants. This study demonstrates a better performance of AM root cells in water transport under water deficit, which is connected to the shoot physiological performance in terms of photosynthetic capacity.

The foliar application of a mixture of semisynthetic chitosan derivatives induces tolerance to water deficit in maize, improving the antioxidant system and increasing photosynthesis and grain yield

Research has shown that chitosan induces plant stress tolerance and protection, but few studies have explored chemical modifications of chitosan and their effects on plants under water stress. Chitosan and its derivatives were applied (isolated or in mixture) to maize hybrids sensitive to water deficit under greenhouse conditions through foliar spraying at the pre-flowering stage. After the application, water deficit was induced for 15 days. Analyses of leaves and biochemical gas exchange in the ear leaf were performed on the first and fifteenth days of the stress period. Production attributes were also analysed at the end of the experiment. In general, the application of the two chitosan derivatives or their mixture potentiated the activities of the antioxidant enzymes superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase and guaiacol peroxidase at the beginning of the stress period, in addition to reducing lipid peroxidation (malonaldehyde content) and increasing gas exchange and proline contents at the end of the stress period. The derivatives also increased the content of phenolic compounds and the activity of enzymes involved in their production (phenylalanine ammonia lyase and tyrosine ammonia lyase). Dehydroascorbate reductase and compounds such as total soluble sugars, total amino acids, starch, grain yield and harvest index increased for both the derivatives and chitosan. However, the mixture of derivatives was the treatment that led to the higher increase in grain yield and harvest index compared to the other treatments. The application of semisynthetic molecules derived from chitosan yielded greater leaf gas exchange and a higher incidence of the biochemical conditions that relieve plant stress.

Inoculation With Piriformospora indica Is More Efficient in Wild-Type Rice Than in Transgenic Rice Over-Expressing the Vacuolar H+-PPase

Achieving food security in a context of environmental sustainability is one of the main challenges of the XXI century. Two competing strategies to achieve this goal are the use of genetically modified plants and the use of plant growth promoting microorganisms (PGPMs). However, few studies assess the response of genetically modified plants to PGPMs. The aim of this study was to compare the response of over-expressing the vacuolar H+-PPase (AVP) and wild-type rice types to the endophytic fungus; Piriformospora indica. Oryza sativa plants (WT and AVP) were inoculated with P. indica and 30 days later, morphological, ecophysiological and bioenergetic parameters, and nutrient content were assessed. AVP and WT plant heights were strongly influenced by inoculation with P. indica, which also promoted increases in fresh and dry matter of shoot in both genotypes. This may be related with the stimulatory effect of P. indica on ecophysiological parameters, especially photosynthetic rate, stomatal conductance, intrinsic water use efficiency and carboxylation efficiency. However, there were differences between the genotypes concerning the physiological mechanisms leading to biomass increment. In WT plants, inoculation with P. indica stimulated all H+ pumps. However, in inoculated AVP plants, H+-PPase was stimulated, but P- and V-ATPases were inhibited. Fungal inoculation enhanced nutrient uptake in both shoots and roots of WT and AVP plants, compared to uninoculated plants; but among inoculated genotypes, the nutrient uptake was lower in AVP than in WT plants. These results clearly demonstrate that the symbiosis between P. indica and AVP plants did not benefit those plants, which may be related to the inefficient colonization of this fungus on the transgenic plants, demonstrating an incompatibility of this symbiosis, which needs to be further studied.

Mitigation of Salinity Stress in Plants by Arbuscular Mycorrhizal Symbiosis: Current Understanding and New Challenges

Modern agriculture is facing twin challenge of ensuring global food security and executing it in a sustainable manner. However, the rapidly expanding salinity stress in cultivable areas poses a major peril to crop yield. Among various biotechnological techniques being used to reduce the negative effects of salinity, the use of arbuscular mycorrhizal fungi (AMF) is considered to be an efficient approach for bio-amelioration of salinity stress. AMF deploy an array of biochemical and physiological mechanisms that act in a concerted manner to provide more salinity tolerance to the host plant. Some of the well-known mechanisms include improved nutrient uptake and maintenance of ionic homeostasis, superior water use efficiency and osmoprotection, enhanced photosynthetic efficiency, preservation of cell ultrastructure, and reinforced antioxidant metabolism. Molecular studies in past one decade have further elucidated the processes involved in amelioration of salt stress in mycorrhizal plants. The participating AMF induce expression of genes involved in Na+ extrusion to the soil solution, K+ acquisition (by phloem loading and unloading) and release into the xylem, therefore maintaining favorable Na+:K+ ratio. Colonization by AMF differentially affects expression of plasma membrane and tonoplast aquaporins (PIPs and TIPs), which consequently improves water status of the plant. Formation of AM (arbuscular mycorrhiza) surges the capacity of plant to mend photosystem-II (PSII) and boosts quantum efficiency of PSII under salt stress conditions by mounting the transcript levels of chloroplast genes encoding antenna proteins involved in transfer of excitation energy. Furthermore, AM-induced interplay of phytohormones, including strigolactones, abscisic acid, gibberellic acid, salicylic acid, and jasmonic acid have also been associated with the salt tolerance mechanism. This review comprehensively covers major research advances on physiological, biochemical, and molecular mechanisms implicated in AM-induced salt stress tolerance in plants. The review identifies the challenges involved in the application of AM in alleviation of salt stress in plants in order to improve crop productivity.

Symbiotic Root-Endophytic Soil Microbes Improve Crop Productivity and Provide Environmental Benefits

Plants should not be regarded as entities unto themselves, but as the visible part of plant-microbe complexes which are best understood as "holobiomes." Some microorganisms when given the opportunity to inhabit plant roots become root symbionts. Such root colonization by symbiotic microbes can raise crop yields by promoting the growth of both shoots and roots, by enhancing uptake, fixation, and/or more efficient use of nutrients, by improving plants' resistance to pests, diseases, and abiotic stresses that include drought, salt, and other environmental conditions, and by enhancing plants' capacity for photosynthesis. We refer plant-microbe associations with these capabilities that have been purposefully established as enhanced plant holobiomes (EPHs). Here, we consider four groups of phylogenetically distinct and distant symbiotic endophytes: (1) Rhizobiaceae bacteria; (2) plant-obligate arbuscular mycorrhizal fungi (AMF); (3) selected endophytic strains of fungi in the genus Trichoderma; and (4) fungi in the Sebicales order, specifically Piriformospora indica. Although these exhibit quite different "lifestyles" when inhabiting plants, all induce beneficial systemic changes in plants' gene expression that are surprisingly similar. For example, all induce gene expression that produces proteins which detoxify reactive oxygen species (ROS). ROS are increased by environmental stresses on plants or by overexcitation of photosynthetic pigments. Gene overexpression results in a cellular environment where ROS levels are controlled and made more compatible with plants' metabolic processes. EPHs also frequently exhibit increased rates of photosynthesis that contribute to greater plant growth and other capabilities. Soil organic matter (SOM) is augmented when plant root growth is increased and roots remain in the soil. The combination of enhanced photosynthesis, increasing sequestration of CO2 from the air, and elevation of SOM removes C from the atmosphere and stores it in the soil. Reductions in global greenhouse gas levels can be accelerated by incentives for carbon farming and carbon cap-and-trade programs that reward such climate-friendly agriculture. The development and spread of EPHs as part of such initiatives has potential both to enhance farm productivity and incomes and to decelerate global warming.