Mechanistic assessment of tolerance to iron deficiency mediated by Trichoderma harzianum in soybean roots

Plant growth-promoting microorganisms: New insights and the way forward

In the context of global challenges such as climate change, soil degradation, and food security, understanding the modes of action of Plant Growth-Promoting Microorganisms (PGPMs), their formulation, and their application is crucial and can be more focused in future research projects. This editorial paper aims to elucidate diverse modes of action employed by different types of PGPMs, including nitrogen fixation, phosphorus solubilization, production or regulation of phytohormones, and plant protection against environmental and biotic stresses as demonstrated and discussed in the Special Issue entitled "Plant Growth-Promoting Microorganisms: new insights and the way forward".

Trichoderma afroharzianum T22 Induces Rhizobia and Flavonoid-Driven Symbiosis to Promote Tolerance to Alkaline Stress in Garden Pea

Soil alkalinity is a limiting factor for crops, yet the role of beneficial fungi in mitigating this abiotic stress in garden pea is understudied. In this study, Trichoderma afroharzianum T22 colonised the roots of garden pea cultivars exposed to soil alkalinity in a host-specific manner. In alkaline-exposed Sugar Snap, T22 improved growth parameters, consistent with increased tissue mineral content, particularly Fe and Mn, as well as enhanced rhizosphere siderophore levels. The split-root assay demonstrated that the beneficial effects of T22 on alkaline stress mitigation are the result of a whole-plant association rather than localised root-specific effects. RNA-seq analysis showed 575 and 818 differentially expressed genes upregulated and downregulated in the roots inoculated with T22 under alkaline conditions. The upregulated genes were mostly involved in the flavonoid biosynthetic pathway (monooxygenase activity, ammonia-lyase activity, 4-coumarate-CoA ligase), along with genes related to mineral transport and redox homoeostasis. Further, a flavonoid precursor restored plant health even in the absence of T22, confirming the role of microbial symbiosis in mitigating alkaline stress. Interestingly, T22 restored the abundance of rhizobia, particularly Rhizobium leguminosarum and Rhizobium indicum, along with the induction of NifA, NifD, and NifH in nodules, suggesting a connection between T22 and rhizobia under soil alkalinity. Further, the elevated rhizosphere siderophore, root flavonoid, expression of PsCoA (4-coumarate-CoA ligase) as well as the relative abundance of TaAOX1 and R. leguminosarum diminished when T22 was substituted with exogenous Fe. This suggests that exogenous Fe eliminates the need for microbiome-driven mineral mobilisation, while T22-mediated alkaline stress mitigation depends on flavonoid-driven symbiosis and R. leguminosarum abundance. It was further supported by the positive interaction of T22 on R. leguminosarum growth in alkaline media. Thus, the beneficial effect of T22 on rhizobia likely stems from their interactions, not solely from the improved mineral status, particularly Fe, in plants. This study provides the first mechanistic insights into T22 interactions with host and rhizobia, advancing microbiome strategies to alleviate soil alkalinity in peas and other legumes.

Plants breathing under pressure: mechanistic insights into soil compaction-induced physiological, molecular and biochemical responses in plants

MAIN CONCLUSION

This review highlights the molecular, biochemical and physiological responses of plants under soil compaction and presents suitable strategies for optimizing soil compaction for sustainable and intelligent plant production. Soil compaction (SC) increases the mechanical impedance of agricultural crops, which restricts plant growth, root elongation, and productivity. Therefore, exploring the impacts of SC-induced alterations in plants and developing optimization strategies are crucial for sustainable agricultural production and ensuring global food security. However, the regulation of molecular, biochemical and physiological responses to SC in plants has not yet been well explored. Here, we conducted a thorough analysis of the relevant literature regarding the primary factors behind SC in agricultural soils, mechanistic insights into SC-mediated molecular and physiological alterations in plants, the impact of SC on plant productivity, and SC-minimization strategies for eco-friendly and intelligent agricultural production. The existing information suggests that plant roots sense SC-induced changes in soil properties, including decreased soil water content, hypoxia, nutrient deficiency and mechanical stimuli, through altering the expression of membrane-located ion channel- or stimulus receptor-related genes, such as MSLs, MCA1, and AHK. After signal transduction, the synthesis and transport of several plant hormones, mainly ABA, ethylene and auxin, change and restrict root deepening but promote root thickening. In addition, the changes in plant hormones in combination with decreased water availability and decreased root hydraulic conductance induced by SC affect aboveground physiological responses, such as decreasing leaf hydraulic conductance, promoting stomatal closure and inhibiting plant photosynthesis. Comprehensive physiological insights into SC in plants and SC optimization strategies could be useful to soil biologists and plant eco-physiologists seeking to improve soil management and sustainable agricultural plant production to promote global food security.

Light Quality Plays a Crucial Role in Regulating Germination, Photosynthetic Efficiency, Plant Development, Reactive Oxygen Species Production, Antioxidant Enzyme Activity, and Nutrient Acquisition in Alfalfa

Light is a vital regulator of photosynthesis, energy production, plant growth, and morphogenesis. Although these key physiological processes are well understood, the effects of light quality on the pigment content, oxidative stress, reactive oxygen species (ROS) production, antioxidant defense systems, and biomass yield of plants remain largely unexplored. In this study, we applied different light-emitting diode (LED) treatments, including white light, red light, blue light, and a red+blue (1:1) light combination, to evaluate the traits mentioned above in alfalfa (Medicago sativa L.). Fluorescence staining showed that red light significantly triggered the oxidative stress indicators compared to blue and white light, while the combined red and blue light treatment significantly reduced the ROS (O2•-, H2O2) intensity in alfalfa seedlings. Interestingly, the combined light treatment significantly boosted the seed germination rate (%), maximum photochemical quantum yield of PSII (Fv/Fm), leaf greenness (SPAD score), photosynthetic pigment levels (chlorophyll a, chlorophyll b, and carotenoids), and plant biomass yield in alfalfa seedlings. The red and/or combined (red+blue) light treatments significantly regulated antioxidant enzymes (SOD, CAT, APX, and GR) and the expression of genes related to the ascorbate-glutathione (AsA-GSH) pathway, including monodehydroascorbate reductase (MsMDHAR), dehydroascorbate reductase (MsDHAR), ascorbate peroxidase (MsAPX), and glutathione reductase (MsGR). These results indicate that light quality is crucial for regulating the morphological, physiological, and molecular traits linked to alfalfa improvement. These findings suggest a new approach to enhancing the adaptation, as well as the morphological and agronomic yield, of alfalfa and forage legumes through light-quality-mediated improvement.

Local signal from Trichoderma afroharzianum T22 induces host transcriptome and endophytic microbiome leading to growth promotion in sorghum

Trichoderma, a highly abundant soil fungus, may benefit plants, yet it remains understudied in sorghum (Sorghum bicolor). In this study, sorghum plants were grown for 5 weeks in pots of soil with or without inoculation of T. afroharzianum T22. Inoculation with T. afroharzianum T22 significantly increased growth parameters and nutrient levels, demonstrating its beneficial role in sorghum. A split-root assay demonstrated that T. afroharzianum T22 is essential in both compartments of the pot for promoting plant growth, suggesting that local signals from this fungus drive symbiotic benefits in sorghum. RNA-seq analysis revealed that inoculation with T. afroharzianum T22 induced genes responsible for mineral transport (such as nitrate and aquaporin transporters), auxin response, sugar assimilation (hexokinase), and disease resistance (thaumatin) in sorghum roots. Microbial community analysis further unveiled the positive role of T. afroharzianum T22 in enriching Penicillium and Streptomyces while reducing disease-causing Fusarium in the roots. The microbial consortium, consisting of enriched microbiomes from bacterial and fungal communities, showed disrupted morphological features in plants inoculated with T. afroharzianum T22 in the absence of Streptomyces griseus. However, this disruption was not observed in the absence of Penicillium chrysogenum. These results indicate that S. griseus acts as a helper microbe in close association with T. afroharzianum T22 in the sorghum endosphere. This study provides the first comprehensive explanation of how T. afroharzianum T22 modulates host molecular determinants and endophytic helper microbes, thereby collectively promoting sorghum growth. These findings may facilitate the formulation of synthetic microbial inoculants dominated by T. afroharzianum T22 to enhance growth and stress resilience in sorghum and similar crops.

Dual RNA sequencing during Trichoderma harzianum-Phytophthora capsici interaction reveals multiple biological processes involved in the inhibition and highlights the cell wall as a potential target

BACKGROUND

Phytophthora capsici is a destructive oomycete pathogen, causing huge economic losses for agricultural production. The genus Trichoderma represents one of the most extensively researched categories of biocontrol agents, encompassing a diverse array of effective strains. The commercial biocontrol agent Trichoderma harzianum strain T-22 exhibits pronounced biocontrol effects against many plant pathogens, but its activity against P. capsici is not known.

RESULTS

T. harzianum T-22 significantly inhibited the growth of P. capsici mycelia and the culture filtrate of T-22 induced lysis of P. capsici zoospores. Electron microscopic analyses indicated that T-22 significantly modulated the ultrastructural composition of P. capsici, with a severe impact on the cell wall integrity. Dual RNA sequencing revealed multiple biological processes involved in the inhibition during the interaction between these two microorganisms. In particular, a marked upregulation of genes was identified in T. harzianum that are implicated in cell wall degradation or disruption. Concurrently, the presence of T. harzianum appeared to potentiate the susceptibility of P. capsici to cell wall biosynthesis inhibitors such as mandipropamid and dimethomorph. Further investigations showed that mandipropamid and dimethomorph could strongly inhibit the growth and development of P. capsici but had no impact on T. harzianum even at high concentrations, demonstrating the feasibility of combining T. harzianum and these cell wall synthesis inhibitors to combat P. capsici.

CONCLUSION

These findings provided enhanced insights into the biocontrol mechanisms against P. capsici with T. harzianum and evidenced compatibility between specific biological and chemical control strategies. © 2024 Society of Chemical Industry.

Molecular insights into the mutualism that induces iron deficiency tolerance in sorghum inoculated with Trichoderma harzianum

Iron (Fe) deficiency is a common mineral stress in plants, including sorghum. Although the soil fungus Trichoderma harzianum has been shown to mitigate Fe deficiency in some circumstances, neither the range nor mechanism(s) of this process are well understood. In this study, high pH-induced Fe deficiency in sorghum cultivated in pots with natural field soil exhibited a significant decrease in biomass, photosynthetic rate, transpiration rate, stomatal conductance, water use efficiency, and Fe-uptake in both the root and shoot. However, the establishment of T. harzianum colonization in roots of Fe-deprived sorghum showed significant improvements in morpho-physiological traits, Fe levels, and redox status. Molecular detection of the fungal ThAOX1 (L-aminoacid oxidase) gene showed the highest colonization of T. harzianum in the root tips of Fe-deficient sorghum, a location thus targeted for further analysis. Expression studies by RNA-seq and qPCR in sorghum root tips revealed a significant upregulation of several genes associated with Fe uptake (SbTOM2), auxin synthesis (SbSAURX15), nicotianamine synthase 3 (SbNAS3), and a phytosiderophore transporter (SbYS1). Also induced was the siderophore synthesis gene (ThSIT1) in T. harzianum, a result supported by biochemical evidence for elevated siderophore and IAA (indole acetic acid) levels in roots. Given the high affinity of fungal siderophore to chelate insoluble Fe3+ ions, it is likely that elevated siderophore released by T. harzianum led to Fe(III)-siderophore complexes in the rhizosphere that were then transported into roots by the induced SbYS1 (yellow-stripe 1) transporter. In addition, the observed induction of several plant peroxidase genes and ABA (abscisic acid) under Fe deficiency after inoculation with T. harzianum may have helped induce tolerance to Fe-deficiency-induced oxidative stress and adaptive responses. This is the first mechanistic explanation for T. harzianum's role in helping alleviate Fe deficiency in sorghum and suggests that biofertilizers using T. harzianum will improve Fe availability to crops in high pH environments.

Mechanisms for plant growth promotion activated by Trichoderma in natural and managed terrestrial ecosystems

Trichoderma spp. are free-living fungi present in virtually all terrestrial ecosystems. These soil fungi can stimulate plant growth and increase plant nutrient acquisition of macro- and micronutrients and water uptake. Generally, plant growth promotion by Trichoderma is a consequence of the activity of potent fungal signaling metabolites diffused in soil with hormone-like activity, including indolic compounds as indole-3-acetic acid (IAA) produced at concentrations ranging from 14 to 234 μg l-1, and volatile organic compounds such as sesquiterpene isoprenoids (C15), 6-pentyl-2H-pyran-2-one (6-PP) and ethylene (ET) produced at levels from 10 to 120 ng over a period of six days, which in turn, might impact plant endogenous signaling mechanisms orchestrated by plant hormones. Plant growth stimulation occurs without the need of physical contact between both organisms and/or during root colonization. When associated with plants Trichoderma may cause significant biochemical changes in plant content of carbohydrates, amino acids, organic acids and lipids, as detected in Arabidopsis thaliana, maize (Zea mays), tomato (Lycopersicon esculentum) and barley (Hordeum vulgare), which may improve the plant health status during the complete life cycle. Trichoderma-induced plant beneficial effects such as mechanisms of defense and growth are likely to be inherited to the next generations. Depending on the environmental conditions perceived by the fungus during its interaction with plants, Trichoderma can reprogram and/or activate molecular mechanisms commonly modulated by IAA, ET and abscisic acid (ABA) to induce an adaptative physiological response to abiotic stress, including drought, salinity, or environmental pollution. This review, provides a state of the art overview focused on the canonical mechanisms of these beneficial fungi involved in plant growth promotion traits under different environmental scenarios and shows new insights on Trichoderma metabolites from different chemical classes that can modulate specific plant growth aspects. Also, we suggest new research directions on Trichoderma spp. and their secondary metabolites with biological activity on plant growth.

The activation of iron deficiency responses of grapevine rootstocks is dependent to the availability of the nitrogen forms

BACKGROUND

In viticulture, iron (Fe) chlorosis is a common abiotic stress that impairs plant development and leads to yield and quality losses. Under low availability of the metal, the applied N form (nitrate and ammonium) can play a role in promoting or mitigating Fe deficiency stresses. However, the processes involved are not clear in grapevine. Therefore, the aim of this study was to investigate the response of two grapevine rootstocks to the interaction between N forms and Fe uptake. This process was evaluated in a hydroponic experiment using two ungrafted grapevine rootstocks Fercal (Vitis berlandieri x V. vinifera) tolerant to deficiency induced Fe chlorosis and Couderc 3309 (V. riparia x V. rupestris) susceptible to deficiency induced Fe chlorosis.

RESULTS

The results could differentiate Fe deficiency effects, N-forms effects, and rootstock effects. Interveinal chlorosis of young leaves appeared earlier on 3309 C from the second week of treatment with NO3-/NH4+ (1:0)/-Fe, while Fercal leaves showed less severe symptoms after four weeks of treatment, corresponding to decreased chlorophyll concentrations lowered by 75% in 3309 C and 57% in Fercal. Ferric chelate reductase (FCR) activity was by trend enhanced under Fe deficiency in Fercal with both N combinations, whereas 3309 C showed an increase in FCR activity under Fe deficiency only with NO3-/NH4+ (1:1) treatment. With the transcriptome analysis, Gene Ontology (GO) revealed multiple biological processes and molecular functions that were significantly regulated in grapevine rootstocks under Fe-deficient conditions, with more genes regulated in Fercal responses, especially when both forms of N were supplied. Furthermore, the expression of genes involved in the auxin and abscisic acid metabolic pathways was markedly increased by the equal supply of both forms of N under Fe deficiency conditions. In addition, changes in the expression of genes related to Fe uptake, regulation, and transport reflected the different responses of the two grapevine rootstocks to different N forms.

CONCLUSIONS

Results show a clear contribution of N forms to the response of the two grapevine rootstocks under Fe deficiency, highlighting the importance of providing both N forms (nitrate and ammonium) in an appropriate ratio in order to ease the rootstock responses to Fe deficiency.

Recovery of the soil fungal microbiome after steam disinfection to manage the plant pathogen Fusarium solani

Soil disinfection using high temperatures via steam is a promising approach to manage plant pathogens, pests, and weeds. Soil steaming is a viable option for growers who are moving away from dependence on chemical soil fumigants, especially in plant nursery or high tunnel environments. However, there are few studies that investigate how soil steaming causes substantial disturbance to the soil by killing both target pathogens and other soil biota. Steaming treatments also change the trajectory of the soil microbiome as it reassembles over time. Growers are interested in the health of soils after using steam-disinfection, especially if a virulent pathogen colonizes the soil and then flourishes in a situation where there are very few microbes to suppress its growth. Should recruitment of a virulent pathogen occur in the soil, this could have devasting effects on seed germination, seedling establishment and survival. Beneficial microbes are often used to prevent the colonization of plant pathogens, especially after a soil-steaming event. Here, we experimentally test how soil fungal communities assemble after steaming disinfection. We introduce to steam-treated soil Fusarium solani, an important fungal pathogen of soybean and Trichoderma harzianum, a known beneficial fungus used for soilborne pathogen suppression. Results show that F. solani significantly affects the relative abundance and diversity of the soil fungal microbiome, however, T. harzianum does not mitigate the amount of F. solani in the steam treated soil. Within the T. harzianum microbial addition, the soil fungal communities were similar to the control (steaming only). This result suggests inoculating the soil with T. harzianum does not drastically alter the assembly trajectory of the soil fungal microbiome. Other soil amendments such as a combination of Trichoderma spp. or other genera could suppress F. solani growth and shift soil microbiome composition and function post-steaming, however, more experimental research is needed.