Amelioration of short-term drought stress during different growth stages in Brassica juncea by rhizobacteria mediated maintenance of ROS homeostasis

Effects on plant physiology in response to inoculation of growth-promoting bacteria: systematic review

Changes in physiological mechanisms resulting from the association of plant growth-promoting bacteria as well as the responses generated to stressful factors are of interest for sustainable agriculture. Based on this, the objective of this study was to gather insights from recent years (2012-2022) on the impacts on plant physiology of the use of inoculants from plant growth-promoting bacteria. To do this, the search for articles was done in three different databases, Science Direct, Springer Nature and Google Scholar, using the following descriptors: plant growth promoting bacteria, plant hormones, biological control, photosynthesis and abiotic stress. After selection, the included articles were systematized in the Excel program. Pearson Correlation and Principal Component Analysis were used for comparative analysis of physiological variables. 81 articles were included in the review, where a beneficial association was observed in 45 plant species distributed in 13 Orders and 13 Families, with emphasis on the Families Poaceae, Fabaceae, Solanaceae and Brassicaceae. 47 genera and 98 bacterial species were verified, where Bacillus and Pseudomonas represented 52% of the verified strains, with emphasis on Bacillus subtilis and Pseudomonas fluorescens. The main applications were growth promotion, productivity, control of biotic stress and abiotic stress. Positive regulation of photosynthesis was observed, modulating the gene expression of photosynthetic apparatus proteins, pigments, antioxidant production, increased hormonal and nutritional production, osmolyte content, antimicrobial production and decreased lipid peroxidation. Based on this review, it was possible to understand the multifaceted role of plant growth-promoting bacteria in contributing to the better direction of technology in agriculture.

Harnessing microbes as sun cream against high light stress

Plants rely on solar energy for growth through photosynthesis, yet excessive light intensity can induce physiological damage. Despite the considerable harm, inadequate attention has been directed toward understanding how plant-associated microorganisms mitigate this stress, and the impact of high light intensity on plant microbial communities remains underexplored. Through this Viewpoint, we aim to highlight the potential of microbial communities to enhance plant resilience and understand how light stress can shape plant microbiome. A full understanding of these dynamics is essential to design strategies that take advantage of microbial assistance to plants under light stress and to effectively manage the impact of changing light conditions on plant-microbe interactions.

Lemnaceae as Novel Crop Candidates for CO2 Sequestration and Additional Applications

Atmospheric carbon dioxide (CO2) is projected to be twice as high as the pre-industrial level by 2050. This review briefly highlights key responses of terrestrial plants to elevated CO2 and compares these with the responses of aquatic floating plants of the family Lemnaceae (duckweeds). Duckweeds are efficient at removing CO2 from the atmosphere, which we discuss in the context of their exceptionally high growth rates and capacity for starch storage in green tissue. In contrast to cultivation of terrestrial crops, duckweeds do not contribute to CO2 release from soils. We briefly review how this potential for contributions to stabilizing atmospheric CO2 levels is paired with multiple additional applications and services of duckweeds. These additional roles include wastewater phytoremediation, feedstock for biofuel production, and superior nutritional quality (for humans and livestock), while requiring minimal space and input of light and fertilizer. We, furthermore, elaborate on other environmental factors, such as nutrient availability, light supply, and the presence of a microbiome, that impact the response of duckweed to elevated CO2. Under a combination of elevated CO2 with low nutrient availability and moderate light supply, duckweeds' microbiome helps maintain CO2 sequestration and relative growth rate. When incident light intensity increases (in the presence of elevated CO2), the microbiome minimizes negative feedback on photosynthesis from increased sugar accumulation. In addition, duckweed shows a clear propensity for absorption of ammonium over nitrate, accepting ammonium from their endogenous N2-fixing Rhizobium symbionts, and production of large amounts of vegetative storage protein. Finally, cultivation of duckweed could be further optimized using hydroponic vertical farms where nutrients and water are recirculated, saving both resources, space, and energy to produce high-value products.

Phenylalanine supply alleviates the drought stress in mustard (Brassica campestris) by modulating plant growth, photosynthesis, and antioxidant defense system

Mustard (Brassica campestris L.) is a major oilseed crop that plays a crucial role in agriculture. Nevertheless, a number of abiotic factors, drought in particular, significantly reduce its production. Phenylalanine (PA) is a significant and efficacious amino acid in alleviating the adverse impacts of abiotic stressors, such as drought. Thus, the current experiment aimed to evaluate the effects of PA application (0 and 100 mg/L) on brassica varieties i.e., Faisal (V1) and Rachna (V2) under drought stress (50% field capacity). Drought stress reduced the shoot length (18 and 17%), root length (12.1 and 12.3%), total chlorophyll contents (47 and 45%), and biological yield (21 and 26%) of both varieties (V1 and V2), respectively. Foliar application of PA helped overcome drought-induced losses and enhanced shoot length (20 and 21%), total chlorophyll contents (46 and 58%), and biological yield (19 and 22%), whereas reducing the oxidative activities of H2O2 (18 and 19%), MDA concentration (21 and 24%), and electrolyte leakage (19 and 21%) in both varieties (V1 and V2). Antioxidant activities, i.e., CAT, SOD, and POD, were further enhanced under PA treatment by 25, 11, and 14% in V1 and 31, 17, and 24% in V2. Overall findings suggest that exogenous PA treatment reduced the drought-induced oxidative damage and improved the yield, and ionic contents of mustard plants grown in pots. It should be emphasized, however, that studies examining the impacts of PA on open-field-grown brassica crops are still in their early stages, thus more work is needed in this area.

Influence of Thermotolerant Rhizobacteria Bacillus spp. on Biochemical Attributes and Antioxidant Status of Mustard Under High Temperature Stress

Due to global warming, increasing incidences of higher-than-normal temperatures have been observed, which adversely affect seed germination, crop growth, and productivity. Several reports are available on the effect of inoculation with rhizobacteria on plant growth and biochemical attributes; however, information on their influence on seed germination and plant stress levels is lacking. In the present study, under heat stress, we studied the effect of three thermotolerant rhizobacterial strains on mustard seed germination, seedling vigor, and plant growth. Effect of inoculation with the rhizobacterial strains on the plant stress levels, biochemical attributes and antioxidant activity was also determined. Under heat stress, inoculation with the rhizobacterial strains improved seed germination and seedling fresh weight and plumule length; while only Bacillus licheniformis SSA 61 inoculated plants showed better radicle length. There was a concomitant decrease in the plant ethylene levels in the inoculated treatments. Inoculated plants showed higher shoot fresh weight, however, Bacillus sp. MRD-17 inoculated plants only improved root growth. There was significant increase in most of the plant biochemical parameters and activities of antioxidant enzymes superoxide dismutase, catalase, and ascorbate peroxidase. Significant reduction in proline and total sugar content was noted in the inoculated treatments; while increase in the amino acid and phenolics content was observed. A further increase in the antioxidant enzyme activity was recorded in most of the inoculated treatments compared with no stress. Thus, our study indicated that thermotolerant rhizobacterial strains reduced plant stress levels; enhanced seed germination, seedling vigor, plant biomass, and thermotolerance of mustard.

Co-inoculation of biochar and arbuscular mycorrhizae for growth promotion and nutrient fortification in soybean under drought conditions

Drought is significant abiotic stress that affects the development and yield of many crops. The present study is to investigate the effect of arbuscular mycorrhizal fungi (AMF) and biochar on root morphological traits, growth, and physiological traits in soybean under water stress. Impact of AMF and biochar on development and root morphological traits in soybean and AMF spores number and the soil enzymes' activities were studied under drought conditions. After 40 days, plant growth parameters were measured. Drought stress negatively affected soybean growth, root parameters, physiological traits, microbial biomass, and soil enzyme activities. Biochar and AMF individually increase significantly plant growth (plant height, root dry weight, and nodule number), root parameters such as root diameter, root surface area, total root length, root volume, and projected area, total chlorophyll content, and nitrogen content in soybean over to control in water stress. In drought conditions, dual applications of AMF and biochar significantly enhanced shoot and root growth parameters, total chlorophyll, and nitrogen contents in soybean than control. Combined with biochar and AMF positively affects AMF spores number, microbial biomass, and soil enzyme activities in water stress conditions. In drought stress, dual applications of biochar and AMF increase microbial biomass by 28.3%, AMF spores number by 52.0%, alkaline phosphomonoesterase by 45.9%, dehydrogenase by 46.5%, and fluorescein diacetate by 52.2%, activities. The combined application of biochar and AMF enhance growth, root parameters in soybean and soil enzyme activities, and water stress tolerance. Dual applications with biochar and AMF benefit soybean cultivation under water stress conditions.

Plant growth-promoting rhizobacteria Shewanella putrefaciens and Cronobacter dublinensis enhance drought tolerance of pearl millet by modulating hormones and stress-responsive genes

Drought is a major abiotic stress that affects crop productivity. Endophytic bacteria have been found to alleviate the adverse effects of drought on plants. In the present study, we evaluated the effects of two endophytic bacteria Shewanella putrefaciens strain MCL-1 and Cronobacter dublinensis strain MKS-1 on pearl millet (Pennisetum glaucum (L.) R. Br.) under drought stress conditions. Pearl millet plants were grown under three water levels: field capacity (FC), mild drought stress (MD), and severe drought stress (SD). The effects of inoculation on plant growth, physiological attributes, phytohormone content, and drought stress-responsive genes were assessed. The inoculation of pearl millet seeds with endophytes significantly improved shoot and root dry weight and root architecture of plants grown under FC and drought stress conditions. There was a significant increase in relative water content and proline accumulation in the inoculated plants. Among the phytohormones analyzed, the content of ABA and IAA was significantly higher in endophyte-treated plants under all moisture regimes than in uninoculated plants. C. dublinensis-inoculated plants had higher GA content than uninoculated plants under all moisture regimes. The expression level of genes involved in phytohormone biosynthesis (SbNCED, SbGA20oX, and SbYUC) and coding drought-responsive transcription factors (SbAP2, SbSNAC1 and PgDREB2A) was significantly higher under SD in endophyte-inoculated plants than in uninoculated plants. Thus, these endophytic bacteria presumably enhanced the tolerance of pearl millet to drought stress by modulating root growth, plant hormones, physiology and the expression of genes involved in drought tolerance.

Intersections: photosynthesis, abiotic stress, and the plant microbiome

Climate change impacts environmental conditions that affect photosynthesis. This review examines the effect of combinations of elevated atmospheric CO2, long photoperiods, and/or unfavorable nitrogen supply. Under moderate stress, perturbed plant source-sink ratio and redox state can be rebalanced but may result in reduced foliar protein content in C3 plants and a higher carbon-to-nitrogen ratio of plant biomass. More severe environmental conditions can trigger pronounced photosynthetic downregulation and impair growth. We comprehensively evaluate available evidence that microbial partners may be able to support plant productivity under challenging environmental conditions by providing (1) nutrients, (2) an additional carbohydrate sink, and (3) regulators of plant metabolism, especially plant redox state. In evaluating the latter mechanism, we note parallels to metabolic control in photosymbioses and microbial regulation of human redox biology.

Biochar and Arbuscular mycorrhizal fungi mediated enhanced drought tolerance in Okra (Abelmoschus esculentus) plant growth, root morphological traits and physiological properties

Drought is a major abiotic factor limiting plant growth and crop production. There is limited information on effect of interaction between biochar and Arbuscular mycorrhizal fungi (AMF) on okra growth, root morphological traits and soil enzyme activities under drought stress. We studied the influence of biochar and AMF on the growth of Okra (Abelmoschus esculentus) in pot experiments in a net house under drought condition. The results showed that the biochar treatment significantly increased plant growth (the plant height by 14.2%, root dry weight by 30.0%) and root morphological traits (projected area by 22.3% and root diameter by 22.7%) under drought stress. In drought stress, biochar treatment significantly enhanced the chlorophyll 'a' content by 32.7%, the AMF spore number by 22.8% and the microbial biomass as compared to the control. Plant growth parameters such as plant height, shoot and root dry weights significantly increased by AMF alone, by 16.6%, 21.0% and 40.0% respectively under drought condition. Other plant biometrics viz: the total root length, the root volume, the projected area and root diameter improved significantly with the application of AMF alone by 38.3%, 60.0%,16.8% and 15.9% respectively as compared with control. Compared to the control, AMF treatment alone significantly enhanced the total chlorophyll content by 36.6%, the AMF spore number by 39.0% and the microbial biomass by 29.0% under drought condition. However, the highest values of plant growth parameters (plant height, shoot dry weight, root dry weight) and root morphological traits (the total root length, root volume, projected area, root surface area) were observed in the combined treatment of biochar and AMF treatment viz: 31.9%, 34.2%, 60.0% and 68.6%, 66.6%, 45.5%, 41.8%, respectively compared to the control under drought stress. The nitrogen content, total chlorophyll content and microbial biomass increased over un-inoculated control. The soil enzymes; alkaline phosphatase, dehydrogenase and fluorescein diacetate enzyme activities significantly increased in the combined treatment by 55.8%, 68.7% and 69.5%, respectively as compared to the control under drought stress. We conclude that biochar and AMF together is potentially beneficial for cultivation of okra in drought stress conditions.