Response Mechanism of Plants to Drought Stress

The combined effect of water deficit stress and TiO2 nanoparticles on cell membrane and antioxidant enzymes in Helianthus annuus L

Nanotechnology has become one of the several approaches attempting to ameliorate the severe effect of drought on plant's production and to increase the plants tolerance against water deficit for the water economy. In this research, the effect of foliar application of TiO₂, nanoparticles or ordinary TiO₂, on Helianthus annuus subjected to different levels of water deficit was studied. Cell membrane injury increased by increasing the level of water deficit and TiO₂ concentration, and both types of TiO₂ affected the leaves in analogous manner. Ord-TiO₂ increased H₂O₂ generation by 67-240% and lipid peroxidation by 4-67% in leaves. These increases were more than that induced by Nano-TiO₂ and the effect was concentration dependent. Proline significantly increased in leaves by water deficit stress, reaching at 25% field capacity (FC) to more than fivefold compared to that in plants grown on full FC. Spraying plants with water significantly decreased the activities of enzymes in the water deficit stressed roots. The water deficit stress exerted the highest magnitude of effect on the changes of cell membrane injury, MDA, proline content, and activities of CAT and GPX. Nano-TiO₂ was having the highest effect on contents of H₂O₂ and GPX activity. In roots, the level of water deficit causes highest effect on enzyme activities, but TiO₂ influenced more on the changes of MDA and H₂O₂ contents. GPX activity increased by 283% in leaves of plants treated with 50 and 150 ppm Nano-TiO₂, while increased by 170% in those treated with Ord-TiO₂, but APX and CAT activities increased by 17-197%, in average, with Ord-TiO₂. This study concluded that Nano-TiO₂ didn't ameliorate the effects of drought stress on H. annuus but additively increased the stress, so its use in nano-phytotechnology mustn't be expanded without extensive studies.

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The online version contains supplementary material available at 10.1007/s12298-022-01153-z.

Foliar application of zinc oxide nanoparticles: An effective strategy to mitigate drought stress in cucumber seedling by modulating antioxidant defense system and osmolytes accumulation

Drought is a major environmental threat that affects plant growth and productivity. Strategies to mitigate the detrimental impacts of drought stress on plants are under scrutiny. Nanotechnology is considered an effective tool in resolving a wide range of environmental issues by offering novel and pragmatic solutions. A pot experiment was performed to determine the efficacy of zinc oxide nanoparticles (ZnO NPs) as a foliar application (25 mg L^-1 and 100 mg L^-1) on the growth performance of cucumber subjected to drought stress. Applied ZnO NPs under normal conditions resulted in significant growth and biomass enhancement while reducing drought-induced decline. Photosynthetic pigments, photosynthesis, and PSII activity enhanced due to ZnO NPs application, attaining maximal values at 100 mg L^-1 of ZnO NPs. Drought stress restricted growth and biomass buildup in cucumber seedlings by stimulating oxidative stress, which was manifested to excessive buildup of reactive oxygen species (ROS) and peroxidation, thereby decreasing membrane functioning. Plants exposed to ZnO NPs exhibited a reduction in ROS accumulation and lipid peroxidation. The substantial reduction in oxidative damage was manifested with the enhancement of enzymatic and non-enzymatic antioxidant components. The phenol and mineral contents were reduced due to drought stress. In addition, the content of proline, glycine betaine, free amino acids, and sugars increased due to ZnO NPs under normal and drought conditions. Furthermore, the drought-induced decline in the content of phenol and mineral nutrients was mitigated by ZnO NPs foliar application. These findings reveal that exogenous ZnO NPs application may be a pragmatic option in dealing with the drought stress of cucumber seedlings.

Remodeling of Cell Wall Components in Root Nodules and Flower Abscission Zone under Drought in Yellow Lupine

We recently showed that yellow lupine is highly sensitive to soil water deficits since this stressor disrupts nodule structure and functioning, and at the same time triggers flower separation through abscission zone (AZ) activation in the upper part of the plant. Both processes require specific transformations including cell wall remodeling. However, knowledge about the involvement of particular cell wall elements in nodulation and abscission in agronomically important, nitrogen-fixing crops, especially under stressful conditions, is still scarce. Here, we used immuno-fluorescence techniques to visualize dynamic changes in cell wall compounds taking place in the root nodules and flower AZ of Lupinus luteus following drought. The reaction of nodules and the flower AZ to drought includes the upregulation of extensins, galactans, arabinans, xylogalacturonan, and xyloglucans. Additionally, modifications in the localization of high- and low-methylated homogalacturonans and arabinogalactan proteins were detected in nodules. Collectively, we determined for the first time the drought-associated modification of cell wall components responsible for their remodeling in root nodules and the flower AZ of L. luteus. The involvement of these particular molecules and their possible interaction in response to stress is also deeply discussed herein.

Modulation of drought adversities in Vicia faba by the application of plant growth promoting rhizobacteria and biochar

Drought is the greatest threat to world food security, seen as the catalyst for the great famines of the past. Given that the world's water supply is limited, it is likely that future demand of food for increasing population will further exacerbate the drought effects. Therefore, the present study was aimed to investigate the effect of biochar and plant growth promoting rhizobacteria (PGPR) Sphingobacterium pakistanensis (NCCP246) and Cellulomonas pakistanensis (NCCP11) on agronomic and physiological attributes of Vicia faba two varieties Desi (V1) and Pulista (V2) under induced drought stress. The seeds were sown in earthen pots filled with 3 kg sand and soil (1:2), and biochar (0 and 5% w/w) in triplicate arranged in complete randomized design. Analysis of biochar possessed 0.49 g cm^-3 bulk density, 9.6 pH; 5.4 cmol kg^-1 cation exchange capacity, 3.64% organic carbon and EC 6.7 ds/m. Agronomic attributes including seed LAI, LAR, SVI, %PHSI and RWC were improved by 30.4-180.4%, 14.37-47.20%, 37.64-50.91%, 18.21-30.80, and 35.82-54.34% in both varieties by the co-application of biochar and PGPR. Stomatal physiology and epidermal vigor was successfully improved by the application of PGPR and biochar as analyzed by scanning electron microscopy (SEM). Photosynthetic pigments, flavonoids, phenols, proline and glycine betaine were amplified by 58.33-173.8%, 50.59-130.33%, 46.58-86.62%, 46.66-109.30%, 35.74-56.10%, and 21.96-77.22% in both varieties by the co-application of biochar and PGPR. So, the present work concluded that, combined application of biochar and PGPR could be an effective strategy to alleviate the adversities of drought in V. faba growing in drastic ecosystems.

Sphingomonas sp. Hbc-6 alters physiological metabolism and recruits beneficial rhizosphere bacteria to improve plant growth and drought tolerance

Drought poses a serious threat to plant growth. Plant growth-promoting bacteria (PGPB) have great potential to improve plant nutrition, yield, and drought tolerance. Sphingomonas is an important microbiota genus that is extensively distributed in the plant or rhizosphere. However, the knowledge of its plant growth-promoting function in dry regions is extremely limited. In this study, we investigated the effects of PGPB Sphingomonas sp. Hbc-6 on maize under normal conditions and drought stress. We found that Hbc-6 increased the biomass of maize under normal conditions and drought stress. For instance, the root fresh weight and shoot dry weight of inoculated maize increased by 39.1% and 34.8% respectively compared with non-inoculated plant, while they increased by 61.3% and 96.3% respectively under drought conditions. Hbc-6 also promoted seed germination, maintained stomatal morphology and increased chlorophyll content so as to enhance photosynthesis of plants. Hbc-6 increased antioxidant enzyme (catalase, superoxide, peroxidase) activities and osmoregulation substances (proline, soluble sugar) and up-regulated the level of beneficial metabolites (resveratrol, etc.). Moreover, Hbc-6 reshaped the maize rhizosphere bacterial community, increased its richness and diversity, and made the rhizosphere bacterial community more complex to resist stress; Hbc-6 could also recruit more potentially rhizosphere beneficial bacteria which might promote plant growth together with Hbc-6 both under normal and drought stress. In short, Hbc-6 increased maize biomass and drought tolerance through the above ways. Our findings lay a foundation for exploring the complex mechanisms of interactions between Sphingomonas and plants, and it is important that Sphingomonas sp. Hbc-6 can be used as a potential biofertilizer in agricultural production, which will assist finding new solutions for improving the growth and yield of crops in arid areas.

Hydrogen peroxide is involved in drought stress long-distance signaling controlling early stomatal closure in tomato plants

Abstract It has long been hypothesized that hydrogen peroxide (H2O2) may play an essential role in root-to-shoot long-distance signaling during drought conditions. Thus, to better understand the involvement of H2O2 in drought signaling, two experiments were carried out using tomato plants. In the first experiment, a split-root scheme was used, while in the second experiment, the tomato plants were grown in a single pot and subjected to drought stress. In both experiments, H2O2 and catalase were applied together with irrigation. Control plants continued to be irrigated according to the water loss. In the split-root experiment, it was verified that the application of H2O2 to roots induced a clear reduction in plant transpiration compared to untreated or catalase-treated plants. In the second experiment, we observed that H2O2-treated plants exhibited similar transpiration when compared to untreated and catalase-treated plants under drought stress. Similarly, no difference in water use efficiency was observed. Thus, we conclude that the increase in H2O2 in the root system can act as a long-distance signal leading to reduced transpiration even when there is no water limitation in the shoot. But it has little effect when there is a reduction in the shoot water potential.

Subcellular Localization of Seed-Expressed LEA_4 Proteins Reveals Liquid-Liquid Phase Separation for LEA9 and for LEA48 Homo- and LEA42-LEA48 Heterodimers

LEA proteins are involved in plant stress tolerance. In Arabidopsis, the LEA_4 Pfam group is the biggest group with the majority of its members being expressed in dry seeds. To assess subcellular localization in vivo, we investigated 11 seed-expressed LEA_4 proteins in embryos dissected from dry seeds expressing LEA_4 fusion proteins under its native promoters with the Venus fluorescent protein (proLEA_4::LEA_4:Venus). LEA_4 proteins were shown to be localized in the endoplasmic reticulum, nucleus, mitochondria, and plastids. LEA9, in addition to the nucleus, was also found in cytoplasmic condensates in dry seeds dependent on cellular hydration level. Most investigated LEA_4 proteins were detected in 4-d-old seedlings. In addition, we assessed bioinformatic tools for predicting subcellular localization and promoter motifs of 11 seed-expressed LEA_4 proteins. Ratiometric bimolecular fluorescence complementation assays showed that LEA7, LEA29, and LEA48 form homodimers while heterodimers were formed between LEA7-LEA29 and LEA42-LEA48 in tobacco leaves. Interestingly, LEA48 homodimers and LEA42-LEA48 heterodimers formed droplets structures with liquid-like behavior. These structures, along with LEA9 cytoplasmic condensates, may have been formed through liquid-liquid phase separation. These findings suggest possible important roles of LLPS for LEA protein functions.

R2R3-MYB Transcription Factor PlMYB108 Confers Drought Tolerance in Herbaceous Peony (Paeonia lactiflora Pall.)

The MYB transcription factor (TF) is crucial for plant growth, development, and response to abiotic stress, but it is rarely reported in the herbaceous peony (Paeonia lactiflora Pall.). Here, an MYB TF gene was isolated, and based on our prior mRNA data from P. lactiflora samples, it was treated with drought stress (DS). Its complete cDNA structure was 1314 bp, which encoded 291 amino acids (aa). Furthermore, using sequence alignment analysis, we demonstrated that PlMYB108 was an R2R3-MYB TF. We also revealed that PlMYB108 was primarily localized in the nucleus. Its levels rose during DS, and it was positively correlated with drought tolerance (DT) in P. lactiflora. In addition, when PlMYB108 was overexpressed in tobacco plants, the flavonoid content, antioxidant enzyme activities, and photosynthesis were markedly elevated. Hence, the transgenic plants had stronger DT with a higher leaf water content and lower H₂O₂ accumulation compared to the wild-type (WT) plants. Based on these results, PlMYB108 is a vital gene that serves to increase flavonoid accumulation, reactive oxygen species (ROS), scavenging capacity, and photosynthesis to confer DT. The results would provide a genetic resource for molecular breeding to enhance plant DT.

Impact of Trichoderma asperellum on Chilling and Drought Stress in Tomato (Solanum lycopersicum)

The acceleration of climate change is necessitating the adoption of shifts in farming practices and technology to ensure the sustainability of agricultural production and food security. Because abiotic stresses such as drought and chilling represent major constraints on agricultural productivity worldwide, in this study, the mitigation of such stresses by the fungus Trichoderma asperellum HK703 was evaluated. The fungus was grown on whole grain oats, kaolin and vermiculite for 5 days and then the formulation was mixed with the potting soil to colonize the roots of the plants. The effect of the bioinoculant on tomato under drought or chilling was analyzed in tomato (Solanum lycopersicum) plants. Leaf, stem and root succulence, electrolyte leakage, the relative growth rate of plant height, stem thickness and leaf area, as well new leaf emergence and chlorophyll content were determined. The results showed that drought or chilling increased electrolyte leakage and reduced plant growth and development traits and chlorophyll (a,b) content. However, inoculation with T. asperellum eliminated or reduced most of the negative impacts of drought compared to the non-stressed plants, with the exception of chlorophyll b content. Furthermore, inoculation with T. asperellum improved some of the evaluated features in chilling stressed plants but had no effect on plant height or chlorophyll (a,b) content. The results of this study indicate that T. asperellum was more effective in alleviating drought than chilling stress in tomato plants.

Evaluation of the Morpho-Physiological, Biochemical and Molecular Responses of Contrasting Medicago truncatula Lines under Water Deficit Stress

Medicago truncatula is a forage crop of choice for farmers, and it is a model species for molecular research. The growth and development and subsequent yields are limited by water availability mainly in arid and semi-arid regions. Our study aims to evaluate the morpho-physiological, biochemical and molecular responses to water deficit stress in four lines (TN6.18, JA17, TN1.11 and A10) of M. truncatula. The results showed that the treatment factor explained the majority of the variation for the measured traits. It appeared that the line A10 was the most sensitive and therefore adversely affected by water deficit stress, which reduced its growth and yield parameters, whereas the tolerant line TN6.18 exhibited the highest root biomass production, a significantly higher increase in its total protein and soluble sugar contents, and lower levels of lipid peroxidation with greater cell membrane integrity. The expression analysis of the DREB1B gene using RT-qPCR revealed a tissue-differential expression in the four lines under osmotic stress, with a higher induction rate in roots of TN6.18 and JA17 than in A10 roots, suggesting a key role for DREB1B in water deficit tolerance in M. truncatula.

Metabolomics and Molecular Approaches Reveal Drought Stress Tolerance in Plants

Metabolic regulation is the key mechanism implicated in plants maintaining cell osmotic potential under drought stress. Understanding drought stress tolerance in plants will have a significant impact on food security in the face of increasingly harsh climatic conditions. Plant primary and secondary metabolites and metabolic genes are key factors in drought tolerance through their involvement in diverse metabolic pathways. Physio-biochemical and molecular strategies involved in plant tolerance mechanisms could be exploited to increase plant survival under drought stress. This review summarizes the most updated findings on primary and secondary metabolites involved in drought stress. We also examine the application of useful metabolic genes and their molecular responses to drought tolerance in plants and discuss possible strategies to help plants to counteract unfavorable drought periods.

Metabolomics Analysis Reveals Drought Responses of Trifoliate Orange by Arbuscular Mycorrhizal Fungi With a Focus on Terpenoid Profile

Soil water deficit seriously affects crop production, and soil arbuscular mycorrhizal fungi (AMF) enhance drought tolerance in crops by unclear mechanisms. Our study aimed to analyze changes in non-targeted metabolomics in roots of trifoliate orange (Poncirus trifoliata) seedlings under well-watered and soil drought after inoculation with Rhizophagus intraradices, with a focus on terpenoid profile. Root mycorrhizal fungal colonization varied from 70% under soil drought to 85% under soil well-watered, and shoot and root biomass was increased by AMF inoculation, independent of soil water regimes. A total of 643 secondary metabolites in roots were examined, and 210 and 105 differential metabolites were regulated by mycorrhizal fungi under normal water and drought stress, along with 88 and 17 metabolites being up-and down-regulated under drought conditions, respectively. KEGG annotation analysis of differential metabolites showed 38 and 36 metabolic pathways by mycorrhizal inoculation under normal water and drought stress conditions, respectively. Among them, 33 metabolic pathways for mycorrhization under drought stress included purine metabolism, pyrimidine metabolism, alanine, aspartate and glutamate metabolism, etc. We also identified 10 terpenoid substances, namely albiflorin, artemisinin (-)-camphor, capsanthin, β-caryophyllene, limonin, phytol, roseoside, sweroside, and α-terpineol. AMF colonization triggered the decline of almost all differential terpenoids, except for β-caryophyllene, which was up-regulated by mycorrhizas under drought, suggesting potential increase in volatile organic compounds to initiate plant defense responses. This study provided an overview of AMF-induced metabolites and metabolic pathways in plants under drought, focusing on the terpenoid profile.