The interaction of plant biotic and abiotic stresses: from genes to the field

Loss of function of a DMR6 ortholog in tomato confers broad-spectrum disease resistance

Plant diseases are among the major causes of crop yield losses around the world. To confer disease resistance, conventional breeding relies on the deployment of single resistance (R) genes. However, this strategy has been easily overcome by constantly evolving pathogens. Disabling susceptibility (S) genes is a promising alternative to R genes in breeding programs, as it usually offers durable and broad-spectrum disease resistance. In Arabidopsis, the S gene DMR6 (AtDMR6) encodes an enzyme identified as a susceptibility factor to bacterial and oomycete pathogens. Here, we present a model-to-crop translational work in which we characterize two AtDMR6 orthologs in tomato, SlDMR6-1 and SlDMR6-2. We show that SlDMR6-1, but not SlDMR6-2, is up-regulated by pathogen infection. In agreement, Sldmr6-1 mutants display enhanced resistance against different classes of pathogens, such as bacteria, oomycete, and fungi. Notably, disease resistance correlates with increased salicylic acid (SA) levels and transcriptional activation of immune responses. Furthermore, we demonstrate that SlDMR6-1 and SlDMR6-2 display SA-5 hydroxylase activity, thus contributing to the elucidation of the enzymatic function of DMR6. We then propose that SlDMR6 duplication in tomato resulted in subsequent subfunctionalization, in which SlDMR6-2 specialized in balancing SA levels in flowers/fruits, while SlDMR6-1 conserved the ability to fine-tune SA levels during pathogen infection of the plant vegetative tissues. Overall, this work not only corroborates a mechanism underlying SA homeostasis in plants, but also presents a promising strategy for engineering broad-spectrum and durable disease resistance in crops.

Ionomic Approaches for Discovery of Novel Stress-Resilient Genes in Plants

Plants, being sessile, face an array of biotic and abiotic stresses in their lifespan that endanger their survival. Hence, optimized uptake of mineral nutrients creates potential new routes for enhancing plant health and stress resilience. Recently, minerals (both essential and non-essential) have been identified as key players in plant stress biology, owing to their multifaceted functions. However, a realistic understanding of the relationship between different ions and stresses is lacking. In this context, ionomics will provide new platforms for not only understanding the function of the plant ionome during stresses but also identifying the genes and regulatory pathways related to mineral accumulation, transportation, and involvement in different molecular mechanisms under normal or stress conditions. This article provides a general overview of ionomics and the integration of high-throughput ionomic approaches with other "omics" tools. Integrated omics analysis is highly suitable for identification of the genes for various traits that confer biotic and abiotic stress tolerance. Moreover, ionomics advances being used to identify loci using qualitative trait loci and genome-wide association analysis of element uptake and transport within plant tissues, as well as genetic variation within species, are discussed. Furthermore, recent developments in ionomics for the discovery of stress-tolerant genes in plants have also been addressed; these can be used to produce more robust crops with a high nutritional value for sustainable agriculture.

Physiological and metabolic changes in two Himalayan medicinal herbs under drought, heat and combined stresses

Valeriana jatamansi Jones and Hedychium spicatum Ham-ex-Smith are important medicinal herbs of the Himalayan region, which are highly demanded by pharmaceutical industries. Climatic variability especially increasing temperature and water deficit affects the growth and productivity of these species. In addition, increased temperature and water deficit may trigger the biosynthesis of medicinally important bioactive metabolites, which influence the quality of raw plant material and finished products. Therefore, V. jatamansi and H. spicatum plants were undertaken and subjected to different levels of drought (no irrigation), heat (35 °C), and combined stresses for investigating their physiological and metabolic responses. Both the treatments (individually and in combination) reduced relative water content, photosynthesis, carboxylation efficiency, chlorophyll content, while increased intracellular CO₂, malondialdehyde and H₂O₂ content in both the species. Transpiration and stomatal conductance increased under heat and reduced under drought stress as compared to control. Water use efficiency was found to be increased under drought, while reduced under heat stress. Protein, proline, carotenoid content and antioxidant enzymes activities (superoxide dismutase, peroxidise, catalase) initially increased and thereafter decreased during late stages of stress. Exposure of plants to combined stress was more detrimental than individual stress. In V. jatamansi, exposure to drought stress significantly (p < 0.05) increased valerenic acid content in all plant parts (1.0-6.9 fold) with maximum increase after 20 days of exposure, while under heat stress, valerenic acid content increased (1.0-1.2 fold) in belowground part of V. jatamansi, and decreased (1.1-1.3 fold) in aerial part as compared to control. In H. spicatum, exposure of individual heat stress for 25-30 days and combined stress for 5-15 days significantly (p < 0.05) increased linalool content to 6.2-6.5 fold and 8.3-19.6 fold, respectively, as compared to control. Higher accumulation of bioactive compounds after exposure to mild stress provides encouraging prospects for enhancing pharmaceutical properties of these Himalayan herbs.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01027-w.

Root zone warming represses foliar diseases in tomato by inducing systemic immunity

Plants employ systemic-induced resistance as part of their defence arsenal against pathogens. In recent years, the application of mild heating has been found to induce resistance against several pathogens. In the present study, we investigated the effect of root zone warming (RZW) in promoting tomato's resistance against the necrotrophic fungus Botrytis cinerea (Bc), the hemibiotrophic bacterium Xanthomonas campestris pv. vesicatoria (Xcv) and the biotrophic fungus Oidium neolycopersici (On). We demonstrate that RZW enhances tomato's resistance to Bc, On and Xcv through a process that is dependent on salicylic acid and ethylene. RZW induced tomato immunity, resulting in increased defence gene expression, reactive oxygen species (ROS) and ethylene output when plants were challenged, even in the absence of pathogens. Overall, the results provide novel insights into the underlying mechanisms of warming-induced immune responses against phytopathogens with different lifestyles in tomato.

Electrical Stimulation Enhances Plant Defense Response in Grapevine through Salicylic Acid-Dependent Defense Pathway

Concern over environmental pollution generated by chemical fungicides has led to the introduction of alternative pest management strategies to chemical fungicide application. One of those strategies is the induction of plant defense response by an abiotic elicitor. In the present study, field-grown grapevines were subjected to electrical stimulation using a solar panel from two weeks before flowering to harvest in the 2016 and 2020 growing seasons. In both years, electrical stimulation decreased the incidence of gray mold and/or ripe rot on bunches and downy mildew on leaves of the field-grown grapevine. Transcription of a gene encoding β-1,3-glucanase but not class IV chitinase in leaves of potted grapevine seedlings was upregulated 20 days after electrical stimulation, suggesting that electrical stimulation acts as an abiotic elicitor of plant defense response to fungal diseases. The gene expression of PR1 but not PDF1.2 was upregulated in Arabidopsis plants subjected to electrical stimulation. On the other hand, PR1 gene expression was not induced in salicylic acid (SA)-insensitive Arabidopsis mutant npr1-5 subjected to electrical stimulation. Taken together, electrical stimulation is responsible for plant defense response through the SA-dependent defense pathway. These findings would help us develop a novel and innovative practical technique that uses electrical stimulation in integrated pest management.

Plant virus evolution under strong drought conditions results in a transition from parasitism to mutualism

Environmental conditions are an important factor driving pathogens' evolution. Here, we explore the effects of drought stress in plant virus evolution. We evolved turnip mosaic potyvirus in well-watered and drought conditions in Arabidopsis thaliana accessions that differ in their response to virus infection. Virus adaptation occurred in all accessions independently of watering status. Drought-evolved viruses conferred a significantly higher drought tolerance to infected plants. By contrast, nonsignificant increases in tolerance were observed in plants infected with viruses evolved under standard watering. The magnitude of this effect was dependent on the plant accessions. Differences in tolerance were correlated to alterations in the expression of host genes, some involved in regulation of the circadian clock, as well as in deep changes in the balance of phytohormones regulating defense and growth signaling pathways. Our results show that viruses can promote host survival in situations of abiotic stress, with the magnitude of such benefit being a selectable trait.

Biochemical and Molecular Aspects of Phosphorus Limitation in Diatoms and Their Relationship with Biomolecule Accumulation

Simple Summary

Phosphorus (P) is a key nutrient involved in the transfer of energy and the synthesis of several cellular components. It has been reported that P limitation in diatoms induces the synthesis of biomolecules and the accumulation of storage compounds, such as pigments, carbohydrates and lipids, with diverse biological activities, which can be used in diverse biotechnological applications. However, the molecular and biochemical mechanisms related to how diatoms cope with P deficiency are not clear, and research into this has been limited to a few species. The integration of results obtained from omics sciences could provide a broad understanding of the response of diatoms to P limitation, and the information obtained could help to solve challenges such as biomass production, by-products yield and genetic improvement of strains.

Abstract

Diatoms are the most abundant group of phytoplankton, and their success lies in their significant adaptation ability to stress conditions, such as nutrient limitation. Phosphorus (P) is a key nutrient involved in the transfer of energy and the synthesis of several cellular components. Molecular and biochemical mechanisms related to how diatoms cope with P deficiency are not clear, and research into this has been limited to a few species. Among the molecular responses that have been reported in diatoms cultured under P deficient conditions is the upregulation of genes encoding enzymes related to the transport, assimilation, remobilization and recycling of this nutrient. Regarding biochemical responses, due to the reduction of the requirements for carbon structures for the synthesis of proteins and phospholipids, more CO₂ is fixed than is consumed by the Calvin cycle. To deal with this excess, diatoms redirect the carbon flow toward the synthesis of storage compounds such as triacylglycerides and carbohydrates, which are excreted as extracellular polymeric substances. This review aimed to gather all current knowledge regarding the biochemical and molecular mechanisms of diatoms related to managing P deficiency in order to provide a wider insight into and understanding of their responses, as well as the metabolic pathways affected by the limitation of this nutrient.

Temperature-Inducible Transgenic EDS1 and PAD4 in Arabidopsis Confer an Enhanced Disease Resistance at Elevated Temperature

Temperature is one of the most important environmental factors greatly affecting plant disease development. High temperature favors outbreaks of many plant diseases, which threaten food security and turn to be a big issue along with climate change and global warming. Here, we found that concurrent constitutive expression of the key immune regulators EDS1 and PAD4 in Arabidopsis significantly enhanced resistance to virulent bacterial pathogen Pseudomonas syringae pv. tomato at elevated temperature; however, autoimmunity-related growth retardation was also observed on these plants at a normal temperature. To balance this growth-defense trade-off, we generated transgenic plants dual expressing EDS1 and PAD4 genes under the control of a thermo-sensitive promoter from the HSP70 gene, whose expression is highly induced at an elevated temperature. Unlike constitutive overexpression lines, the proHSP70-EP transgenic lines exhibited enhanced resistance to bacterial pathogens at an elevated temperature without growth defects at normal condition. Thus, this study provides a potential strategy for genetic manipulation of plants to deal with the simultaneous abiotic and biotic stresses.

Exploration of Epigenetics for Improvement of Drought and Other Stress Resistance in Crops: A Review

Crop plants often have challenges of biotic and abiotic stresses, and they adapt sophisticated ways to acclimate and cope with these through the expression of specific genes. Changes in chromatin, histone, and DNA mostly serve the purpose of combating challenges and ensuring the survival of plants in stressful environments. Epigenetic changes, due to environmental stress, enable plants to remember a past stress event in order to deal with such challenges in the future. This heritable memory, called "plant stress memory", enables plants to respond against stresses in a better and efficient way, not only for the current plant in prevailing situations but also for future generations. Development of stress resistance in plants for increasing the yield potential and stability has always been a traditional objective of breeders for crop improvement through integrated breeding approaches. The application of epigenetics for improvements in complex traits in tetraploid and some other field crops has been unclear. An improved understanding of epigenetics and stress memory applications will contribute to the development of strategies to incorporate them into breeding for complex agronomic traits. The insight in the application of novel plant breeding techniques (NPBTs) has opened a new plethora of options among plant scientists to develop germplasms for stress tolerance. This review summarizes and discusses plant stress memory at the intergenerational and transgenerational levels, mechanisms involved in stress memory, exploitation of induced and natural epigenetic changes, and genome editing technologies with their future possible applications, in the breeding of crops for abiotic stress tolerance to increase the yield for zero hunger goals achievement on a sustainable basis in the changing climatic era.

Flooding and Herbivory Interact to Alter Volatile Organic Compound Emissions in Two Maize Hybrids

Flooding is a major plant abiotic stress factor that is frequently experienced by plants simultaneously with other biotic stresses, including herbivory. How plant volatile emissions, which mediate interactions with a wide range of organisms, are influenced by flooding and by multiple co-occurring stress factors remains largely unexplored. Using Spodoptera frugiperda (Lepidoptera: Noctuidae) (fall armyworm) as the insect pest and two maize (Zea mays, L. Poaceae) hybrids differentially marketed for conventional and organic production, we assessed the effects of flooding, herbivory, and both stress factors on the composition of blends of emitted volatiles. Headspace volatiles were collected from all treatment combinations seven days after flooding. We documented metrics indicative of biomass allocation to determine the effects of individual and combined stressors on plant growth. We also evaluated relationships between volatile emissions and indicators of soil chemical characteristics as influenced by treatment factors. Flooding and herbivory induced the emission of volatile organic compounds (VOCs) in similar ways on both maize hybrids, but the interaction of both stress factors produced significantly larger quantities of emitted volatiles. Thirty-eight volatile compounds were identified, including green leaf volatiles, monoterpenes, an aldehyde, a benzoate ester, sesquiterpenes, a diterpene alcohol, and alkane hydrocarbons. The hybrid marketed for organic production was a stronger VOC emitter. As expected, plant biomass was detrimentally affected by flooding. Soil chemical properties were less responsive to the treatment factors. Taken together, the results suggest that flooding stress and the interactions of flooding and insect attack can shape the emission of plant volatiles and further influence insect-plant interactions.

Exogenous salicylic acid alleviates the negative impacts on production components, biomass and gas exchange in tomato plants under water deficit improving redox status and anatomical responses

Salicylic acid (SA) is an interesting messenger in plant metabolism that modulates multiple pathways, including the antioxidant defence pathway, and stimulates anatomical structures essential to carbon dioxide fixation during the photosynthetic process. The aim of this research was to determine whether pre-treatment with exogenous SA can alleviate the deleterious effects induced by water deficit on production components, biomass and gas exchange, measuring reactive oxygen species, antioxidant enzymes, variables connected to photosynthetic machinery, anatomical responses, and agro-morphological traits in tomato plants under water deficit. The experiment used a factorial design with four treatments, including two water conditions (control and water deficit) and two salicylic acid concentrations (0 and 0.1 mM salicylic acid). Water deficit negatively impacted the biomass and fruit number of tomato plants. Pre-treatment using 0.1 mM SA in plants submitted to water restriction induced increments in fruit number, weight, and biomass. These results were related to the protective role triggered by this substance, stimulating superoxide dismutase (27.07%), catalase (17.81%), ascorbate peroxidase (50.52%), and peroxidase (10.81%) as well as reducing the cell damage (malondialdehyde and electrolyte leakage) caused by superoxide and hydrogen peroxide. Simultaneously, application of SA improved the net photosynthetic rate (84.55%) and water-use efficiency (65.00%) of stressed plants in which these factors are connected to anatomical benefits, as verified by stomatal density, palisade and spongy parenchyma, combined with improved performance linked to photosystem II.

Ozone responses in Arabidopsis: beyond stomatal conductance

Tropospheric ozone (O3) is a major air pollutant that decreases yield of important crops worldwide. Despite long-lasting research of its negative effects on plants, there are many gaps in our knowledge on how plants respond to O3. In this study, we used natural variation in the model plant Arabidopsis (Arabidopsis thaliana) to characterize molecular and physiological mechanisms underlying O3 sensitivity. A key parameter in models for O3 damage is stomatal uptake. Here we show that the extent of O3 damage in the sensitive Arabidopsis accession Shahdara (Sha) does not correspond with O3 uptake, pointing toward stomata-independent mechanisms for the development of O3 damage. We compared tolerant (Col-0) versus sensitive accessions (Sha, Cvi-0) in assays related to photosynthesis, cell death, antioxidants, and transcriptional regulation. Acute O3 exposure increased cell death, development of lesions in the leaves, and decreased photosynthesis in sensitive accessions. In both Sha and Cvi-0, O3-induced lesions were associated with decreased maximal chlorophyll fluorescence and low quantum yield of electron transfer from Photosystem II to plastoquinone. However, O3-induced repression of photosynthesis in these two O3-sensitive accessions developed in different ways. We demonstrate that O3 sensitivity in Arabidopsis is influenced by genetic diversity given that Sha and Cvi-0 developed accession-specific transcriptional responses to O3. Our findings advance the understanding of plant responses to O3 and set a framework for future studies to characterize molecular and physiological mechanisms allowing plants to respond to high O3 levels in the atmosphere as a result of high air pollution and climate change.

Transcription-associated metabolomic adjustments in maize occur during combined drought and cold stress

Although simultaneous drought and cold stress occurs, especially in northwestern and eastern regions of China, and is an important factor limiting agricultural productivity, there are few studies focusing on plant responses to a combination of drought and cold stress. Here, by partially overlapping drought and cold stresses, we characterized the acclimation of maize (Zea mays B73) to these two stresses using physiological measurements, as well as comparative transcriptomics combined with metabolomics and hormonal analyses during the stress treatments and recovery stages. The combined drought and cold stress and drought stress alone were accompanied by a decline in photosynthetic capacity and enhanced transcriptional response, and subsequent recovery of these following removal from stress, whereas cold stress alone was accompanied by irreversible damage to photosynthetic capacity and chloroplast structure. The stress combination induced transcription-associated metabolomic alterations, in which raffinose, trehalose-6-phosphate, and proline accumulated, and monosaccharide abundance increased. Concomitantly, the increased abscisic acid (ABA) content and upregulated ABA signaling pathway may have provided the transcriptional regulation for the metabolic changes. In a parallel experiment, ABA treatments prior to exposure of the plants to cold stress primed the plants to survive the cold stress, thus confirming a key role for the endogenous ABA activated by the drought pretreatment in acclimation of the plants to cold. We present a model showing that the plant response to the combined stress is multi-faceted and reveal an ABA-dependent maize acclimation mechanism to the stress combination.

Sulfur Deprivation Modulates Salicylic Acid Responses via Nonexpressor of Pathogenesis-Related Gene 1 in Arabidopsis thaliana

Mineral nutrients are essential for plant growth and reproduction, yet only a few studies connect the nutritional status to plant innate immunity. The backbone of plant defense response is mainly controlled by two major hormones: salicylic acid (SA) and jasmonic acid (JA). This study investigated changes in the macronutrient concentration (deficiency/excess of nitrogen, phosphorus, potassium, magnesium, and sulfur) on the expression of PR1, a well-characterized marker in the SA-pathway, and PDF1.2 and LOX2 for the JA-pathway, analyzing plants carrying the promoter of each gene fused to GUS as a reporter. After histochemical GUS assays, we determined that PR1 gene was strongly activated in response to sulfur (S) deficiency. Using RT-PCR, we observed that the induction of PR1 depended on the function of Non-expressor of Pathogenesis-Related gene 1 (NPR1) and SA accumulation, as PR1 was not expressed in npr1-1 mutant and NahG plants under S-deprived conditions. Plants treated with different S-concentrations showed that total S-deprivation was required to induce SA-mediated defense responses. Additionally, bioassays revealed that S-deprived plants, induced resistance to the hemibiotrophic pathogen Pseudomonas syringae pv. DC3000 and increase susceptibility to the necrotrophic Botrytis cinerea. In conclusion, we observed a relationship between S and SA/JA-dependent defense mechanisms in Arabidopsis.

Heat Stress Pre-Exposure May Differentially Modulate Plant Defense to Powdery Mildew in a Resistant and Susceptible Barley Genotype

Heat stress negatively affects barley production and under elevated temperatures defense responses to powdery mildew (Blumeria graminis f. sp. hordei, Bgh) are altered. Previous research has analyzed the effects of short-term (30 s to 2 h) heat stress, however, few data are available on the influence of long-term exposure to heat on powdery mildew infections. We simultaneously assessed the effects of short and long term heat pre-exposure on resistance/susceptibility of barley to Bgh, evaluating powdery mildew infection by analyzing symptoms and Bgh biomass with RT-qPCR in barley plants pre-exposed to high temperatures (28 and 35 °C from 30 s to 5 days). Plant defense gene expression after heat stress pre-exposure and inoculation was also monitored. Our results show that prolonged heat stress (24, 48 and 120 h) further enhanced Bgh susceptibility in a susceptible barley line (MvHV118-17), while a resistant line (MvHV07-17) retained its pathogen resistance. Furthermore, prolonged heat stress significantly repressed the expression of several defense-related genes (BAX inhibitor-1, Pathogenesis related-1b and Respiratory burst oxidase homologue F2) in both resistant and susceptible barley lines. Remarkably, heat-suppressed defense gene expression returned to normal levels only in MvHV07-17, a possible reason why this barley line retains Bgh resistance even at high temperatures.

A Novel DUF569 Gene Is a Positive Regulator of the Drought Stress Response in Arabidopsis

In the last two decades, global environmental change has increased abiotic stress on plants and severely affected crops. For example, drought stress is a serious abiotic stress that rapidly and substantially alters the morphological, physiological, and molecular responses of plants. In Arabidopsis, several drought-responsive genes have been identified; however, the underlying molecular mechanism of drought tolerance in plants remains largely unclear. Here, we report that the “domain of unknown function” novel gene DUF569 (AT1G69890) positively regulates drought stress in Arabidopsis. The Arabidopsis loss-of-function mutant atduf569 showed significant sensitivity to drought stress, i.e., severe wilting at the rosette-leaf stage after water was withheld for 3 days. Importantly, the mutant plant did not recover after rewatering, unlike wild-type (WT) plants. In addition, atduf569 plants showed significantly lower abscisic acid accumulation under optimal and drought-stress conditions, as well as significantly higher electrolyte leakage when compared with WT Col-0 plants. Spectrophotometric analyses also indicated a significantly lower accumulation of polyphenols, flavonoids, carotenoids, and chlorophylls in atduf569 mutant plants. Overall, our results suggest that novel DUF569 is a positive regulator of the response to drought in Arabidopsis.

Comprehensive molecular dissection of TIFY Transcription factors reveal their dynamic responses to biotic and abiotic stress in wheat (Triticum aestivum L.)

The plant specific TIFY (previously known as ZIM) transcription factor (TF) family plays crucial roles in cross talk between Jasmonic Acid and other phytohormones like gibberellins, salicylic acid, abscisic acid, auxin, and ethylene signaling pathways. Wheat yield is severely affected by rust diseases and many abiotic stresses, where different phytohormone signaling pathways are involved. TIFYs have been studied in many plants yet reports describing their molecular structure and function in wheat are lacking. In the present study, we have identified 23 novel TIFY genes in wheat genome using in silico approaches. The identified proteins were characterized based on their conserved domains and phylogenetically classified into nine subfamilies. Chromosomal localization of the identified TIFY genes showed arbitrary distribution. Forty cis-acting elements including phytohormone, stress and light receptive elements were detected in the upstream regions of TIFY genes. Seventeen wheat microRNAs targeted the identified wheat TIFY genes. Gene ontological studies revealed their major contribution in defense response and phytohormone signaling. Secondary structure of TIFY proteins displayed the characteristic alpha-alpha-beta fold. Synteny analyses indicated all wheat TIFY genes had orthologous sequences in sorghum, rice, maize, barley and Brachypodium indicating presence of similar TIFY domains in monocot plants. Six TIFY genes had been cloned from wheat genomic and cDNA. Sequence characterization revealed similar characteristics as the in silico identified novel TIFY genes. Tertiary structures predicted the active sites in these proteins to play critical roles in DNA binding. Expression profiling of TIFY genes showed their contribution during incompatible and compatible leaf rust infestation. TIFY genes were also highly expressed during the initial hours of phytohormone induced stress. This study furnishes fundamental information on characterization and putative functions of TIFY genes in wheat.

Diversifying maize genomic selection models

Genomic selection (GS) is one of the most powerful tools available for maize breeding. Its use of genome-wide marker data to estimate breeding values translates to increased genetic gains with fewer breeding cycles. In this review, we cover the history of GS and highlight particular milestones during its adaptation to maize breeding. We discuss how GS can be applied to developing superior maize inbreds and hybrids. Additionally, we characterize refinements in GS models that could enable the encapsulation of non-additive genetic effects, genotype by environment interactions, and multiple levels of the biological hierarchy, all of which could ultimately result in more accurate predictions of breeding values. Finally, we suggest the stages in a maize breeding program where it would be beneficial to apply GS. Given the current sophistication of high-throughput phenotypic, genotypic, and other -omic level data currently available to the maize community, now is the time to explore the implications of their incorporation into GS models and thus ensure that genetic gains are being achieved as quickly and efficiently as possible.

5-azacytidine pre-treatment alters DNA methylation levels and induces genes responsive to salt stress in kenaf (Hibiscus cannabinus L.)

Soil salinization is becoming a major threat to the sustainable development of global agriculture. Kenaf is an industrial fiber crop with high tolerance to salt stress and could be used for soil phytoremediation. However, the molecular mechanism of kenaf salt tolerance remains largely unknown. DNA methylation is an important epigenetic modifications phenomena and plays a key role in gene expression regulation under abiotic stress condition. In the present study, the kenaf seedlings were pre-treated or not with 50 μM 5-azacytidine (5-azaC, a DNA methylation inhibitor) and then subjected to different concentrations of NaCl. Results showed that the biomass and antioxidant activities (superoxide dismutase, peroxidase and catalase) of kenaf seedlings pre-treated with 5-azaC were significantly increased, while the contents of superoxide anion (O₂^-) and malondialdehyde (MDA) were decreased, indicating that 5-azaC pre-treatment could significantly alleviate salt stress injury. Furthermore, the methylation-sensitive amplified polymorphism (MSAP) analysis revealed that DNA methylation level of keanf seedlings pre-treated with 5-azaC significantly decreased. The expression of seven differentially methylated genes responsing to salt stress was significantly changed from real-time fluorescent quantitative (qRT-PCR) analysis. Finally, knocked-down of the l-ascorbate oxidase (L-AAO) gene by virus-induced gene silencing (VIGS) resulted in increased sensitivity of kenaf seedlings under salt stress. Overall, it was suggested that 5-azaC pre-treatment can significantly improve salt tolerance in kenaf by decreasing ROS content, raising anti-oxidant activities, and regulating DNA methylation and expression of stress-responsive genes.

Metabolomics, a Powerful Tool for Understanding Plant Abiotic Stress

Metabolomics is a technology that generates large amounts of data and contributes to obtaining wide and integral explanations of the biochemical state of a living organism. Plants are continuously affected by abiotic stresses such as water scarcity, high temperatures and high salinity, and metabolomics has the potential for elucidating the response-to-stress mechanisms and develop resistance strategies in affected cultivars. This review describes the characteristics of each of the stages of metabolomic studies in plants and the role of metabolomics in the characterization of the response of various plant species to abiotic stresses.

Membrane dynamics during individual and combined abiotic stresses in plants and tools to study the same

The plasma membrane (PM) is possibly the most diverse biological membrane of plant cells; it separates and guards the cell against its external environment. It has an extremely complex structure comprising a mosaic of lipids and proteins. The PM lipids are responsible for maintaining fluidity, permeability and integrity of the membrane and also influence the functioning of membrane proteins. However, the PM is the primary target of environmental stress, which affects its composition, conformation and properties, thereby disturbing the cellular homeostasis. Maintenance of integrity and fluidity of the PM is a prerequisite for ensuring the survival of plants during adverse environmental conditions. The ability of plants to remodel membrane lipid and protein composition plays a crucial role in adaptation towards varying abiotic environmental cues, including high or low temperature, drought, salinity and heavy metals stress. The dynamic changes in lipid composition affect the functioning of membrane transporters and ultimately regulate the physical properties of the membrane. Plant membrane-transport systems play a significant role in stress adaptation by cooperating with the membrane lipidome to maintain the membrane integrity under stressful conditions. The present review provides a holistic view of stress responses and adaptations in plants, especially the changes in the lipidome and proteome of PM under individual or combined abiotic stresses, which cause alterations in the activity of membrane transporters and modifies the fluidity of the PM. The tools to study the varying lipidome and proteome of the PM are also discussed.

Physiological and Gene Expression Changes of Clematis crassifolia and Clematis cadmia in Response to Heat Stress

Clematis is a superior perennial ornamental vine known for varied colors and shapes of its flowers. Clematis crassifolia is sensitive to high temperature, whereas Clematis cadmia has a certain temperature adaptability. Here we analyzed the potential regulatory mechanisms of C. crassifolia and C. cadmia in response to heat stress by studying the photosynthesis, antioxidant parameters, amino acids, and gene expression patterns under three temperature treatments. Heat stress caused the fading of leaves; decreased net photosynthetic rate, stomatal conductance, superoxide dismutase, and catalase activity; increased 13 kinds of amino acids content; and up-regulated the expression of seven genes, including C194329_G3, C194434_G1, and C188817_g1, etc., in C. crassifolia plants. Under the treatments of heat stress, the leaf tips of C. cadmia were wilted, and the net photosynthetic rate and soluble protein content decreased, with the increase of 12 amino acids content and the expression of c194329_g3, c194434_g1, and c195983_g1. Our results showed that C. crassifolia and C. cadmia had different physiological and molecular response mechanisms to heat stress during the ecological adaptation.

A Transcriptomic Approach to Understanding the Combined Impacts of Supra-Optimal Temperatures and CO2 Revealed Different Responses in the Polyploid Coffea arabica and Its Diploid Progenitor C. canephora

Understanding the effect of extreme temperatures and elevated air (CO₂) is crucial for mitigating the impacts of the coffee industry. In this work, leaf transcriptomic changes were evaluated in the diploid C. canephora and its polyploid C. arabica, grown at 25 °C and at two supra-optimal temperatures (37 °C, 42 °C), under ambient (aCO₂) or elevated air CO₂ (eCO₂). Both species expressed fewer genes as temperature rose, although a high number of differentially expressed genes (DEGs) were observed, especially at 42 °C. An enrichment analysis revealed that the two species reacted differently to the high temperatures but with an overall up-regulation of the photosynthetic machinery until 37 °C. Although eCO₂ helped to release stress, 42 °C had a severe impact on both species. A total of 667 photosynthetic and biochemical related-DEGs were altered with high temperatures and eCO₂, which may be used as key probe genes in future studies. This was mostly felt in C. arabica, where genes related to ribulose-bisphosphate carboxylase (RuBisCO) activity, chlorophyll a-b binding, and the reaction centres of photosystems I and II were down-regulated, especially under 42°C, regardless of CO₂. Transcriptomic changes showed that both species were strongly affected by the highest temperature, although they can endure higher temperatures (37 °C) than previously assumed.

Halophyte Plants and Their Residues as Feedstock for Biogas Production—Chances and Challenges

The importance of green technologies is steadily growing. Salt-tolerant plants have been proposed as energy crops for cultivation on saline lands. Halophytes such as Salicornia europaea, Tripolium pannonicum, Crithmum maritimum and Chenopodium quinoa, among many other species, can be cultivated in saline lands, in coastal areas or for treating saline wastewater, and the biomass might be used for biogas production as an integrated process of biorefining. However, halophytes have different salt tolerance mechanisms, including compartmentalization of salt in the vacuole, leading to an increase of sodium in the plant tissues. The sodium content of halophytes may have an adverse effect on the anaerobic digestion process, which needs adjustments to achieve stable and efficient conversion of the halophytes into biogas. This review gives an overview of the specificities of halophytes that needs to be accounted for using their biomass as feedstocks for biogas plants in order to expand renewable energy production. First, the different physiological mechanisms of halophytes to grow under saline conditions are described, which lead to the characteristic composition of the halophyte biomass, which may influence the biogas production. Next, possible mechanisms to avoid negative effects on the anaerobic digestion process are described, with an overview of full-scale applications. Taking all these aspects into account, halophyte plants have a great potential for biogas and methane production with yields similar to those produced by other energy crops and the simultaneous benefit of utilization of saline soils.

Exogenous 1',4'-trans-Diol-ABA Induces Stress Tolerance by Affecting the Level of Gene Expression in Tobacco (Nicotiana tabacum L.)

1',4'-trans-diol-ABA is a key precursor of the biosynthesis of abscisic acid (ABA) biosynthesis in fungi. We successfully obtained the pure compound from a mutant of Botrytis cinerea and explored its function and possible mechanism on plants by spraying 2 mg/L 1',4'-trans-diol-ABA on tobacco leaves. Our results showed that this compound enhanced the drought tolerance of tobacco seedlings. A comparative transcriptome analysis showed that a large number of genes responded to the compound, exhibiting 1523 genes that were differentially expressed at 12 h, which increased to 1993 at 24 h and 3074 at 48 h, respectively. The enrichment analysis demonstrated that the differentially expressed genes (DEGs) were primarily enriched in pathways related to hormones and resistance. The DEGs of transcription factors were generally up-regulated and included the bHLH, bZIP, ERF, MYB, NAC, WRKY and HSF families. Moreover, the levels of expression of PYL/PYR, PP2C, SnRK2, and ABF at the ABA signaling pathway responded positively to exogenous 1',4'-trans-diol-ABA. Among them, seven ABF transcripts that were detected were significantly up-regulated. In addition, the genes involved in salicylic acid, ethylene and jasmonic acid pathways, reactive oxygen species scavenging system, and other resistance related genes were primarily induced by 1',4'-trans-diol-ABA. These findings indicated that treatment with 1',4'-trans-diol-ABA could improve tolerance to plant abiotic stress and potential biotic resistance by regulating gene expression, similar to the effects of exogenous ABA.

Recent progress in understanding salinity tolerance in plants: Story of Na+/K+ balance and beyond

High salt concentrations in the growing medium can severely affect the growth and development of plants. It is imperative to understand the different components of salt-tolerant network in plants in order to produce the salt-tolerant cultivars. High-affinity potassium transporter- and myelocytomatosis proteins have been shown to play a critical role for salinity tolerance through exclusion of sodium (Na⁺) ions from sensitive shoot tissues in plants. Numerous genes, that limit the uptake of salts from soil and their transport throughout the plant body, adjust the ionic and osmotic balance of cells in roots and shoots. In the present review, we have tried to provide a comprehensive report of major research advances on different mechanisms regulating plant tolerance to salinity stress at proteomics, metabolomics, genomics and transcriptomics levels. Along with the role of ionic homeostasis, a major focus was given on other salinity tolerance mechanisms in plants including osmoregulation and osmo-protection, cell wall remodeling and integrity, and plant antioxidative defense. Major proteins and genes expressed under salt-stressed conditions and their role in enhancing salinity tolerance in plants are discussed as well. Moreover, this manuscript identifies and highlights the key questions on plant salinity tolerance that remain to be discussed in the future.

Towards doubling fibre yield for cotton in the semiarid agricultural area by increasing tolerance to drought, heat and salinity simultaneously

Abiotic stresses such as extreme temperatures, water-deficit and salinity negatively affect plant growth and development, and cause significant yield losses. It was previously shown that co-overexpression of the Arabidopsis vacuolar pyrophosphatase gene AVP1 and the rice SUMO E3 ligase gene OsSIZ1 in Arabidopsis significantly increased tolerance to multiple abiotic stresses and led to increased seed yield for plants grown under single or multiple abiotic stress conditions. It was hypothesized that there might be synergistic effects between AVP1 overexpression and OsSIZ1 overexpression, which could lead to substantially increased yields if these two genes are co-overexpressed in real crops. To test this hypothesis, AVP1 and OsSIZ1 were co-overexpressed in cotton, and the impact of OsSIZ1/AVP1 co-overexpression on cotton's performance under normal growth and multiple stress conditions were analysed. It was found that OsSIZ1/AVP1 co-overexpressing plants performed significantly better than AVP1-overexpressing, OsSIZ1-overexpressing and wild-type cotton plants under single, as well as under multiple stress conditions in laboratory and field conditions. Two field studies showed that OsSIZ1/AVP1 co-overexpressing plants produced 133% and 81% more fibre than wild-type cotton in the dryland conditions of West Texas. This research illustrates that co-overexpression of AVP1 and OsSIZ1 is a viable strategy for engineering abiotic stress-tolerant crops and could substantially improve crop yields in low input or marginal environments, providing a solution for food security for countries in arid and semiarid regions of the world.

Mass spectrometry-based forest tree metabolomics

Research in forest tree species has advanced slowly when compared with other agricultural crops and model organisms, mainly due to the long-life cycles, large genome sizes, and lack of genomic tools. Additionally, trees are complex matrices, and the presence of interferents (e.g., oleoresins and cellulose) challenges the analysis of tree tissues with mass spectrometry (MS)-based analytical platforms. In this review, advances in MS-based forest tree metabolomics are discussed. Given their economic and ecological significance, particular focus is given to Pinus, Quercus, and Eucalyptus forest tree species to better understand their metabolite responses to abiotic and biotic stresses in the current climate change scenario. Furthermore, MS-based metabolomics technologies produce large and complex datasets that require expertize to adequately manage, process, analyze, and store the data in dedicated repositories. To ensure that the full potential of forest tree metabolomics data are translated into new knowledge, these data should comply with the FAIR principles (i.e., Findable, Accessible, Interoperable, and Re-usable). It is essential that adequate standards are implemented to annotate metadata from forest tree metabolomics studies as is already required by many science and governmental agencies and some major scientific publishers. © 2019 John Wiley & Sons Ltd. Mass Spec Rev 40:126-157, 2021.

Induction of tocopherol biosynthesis through heat shock treatment in Arabidopsis

Plants have developed various self-defense systems to survive many types of unfavorable conditions. Heat shock (HS) treatment, an abiotic stress, activates salicylic acid (SA) biosynthesis to enhance resistance to biotic stresses in some plant species. Since SA is produced from the shikimate pathway, other related metabolic pathways were expected to be upregulated by HS treatment. We speculated that tocopherol biosynthesis utilizing chorismic acid would be activated by HS treatment. In Arabidopsis, expression analysis of tocopherol biosynthetic genes, HPPD, VTE2, VTE3, VTE1, and VTE4, in combination with measurement of metabolites, indicated that HS treatment enhanced the biosynthesis and accumulation of tocopherols. Analyses using an SA biosynthesis-deficient mutant indicated that the upregulation of tocopherol biosynthesis was independent of the SA-mediated signaling pathway.

Drought and Pathogen Effects on Survival, Leaf Physiology, Oxidative Damage, and Defense in Two Middle Eastern Oak Species

The charcoal disease agents, Biscogniauxia mediterranea and Obolarina persica are two latent, ascomycetous oak pathogens in the Middle Eastern Zagros forests, where they have devastating effects, particularly during drought. Under greenhouse conditions, we investigated the effects of the two charcoal disease agents individually and in combination with drought on survival, growth, foliar gas-exchange, pigment content, oxidative stress and the antioxidant response of Quercus infectoria and Q. libani, two of the dominant tree species in this region. Commonly, the strongest negative effects emerged in the drought–pathogen interaction treatments. Q. infectoria showed less severe lesions, higher survival, more growth, and less leaf loss than Q. libani under combined biotic and abiotic stress. In both oak species, the combination of pathogen infection and drought resulted in more than 50% reduction in foliar gas-exchange parameters with partial recovery over time in Q. infectoria suggesting a superior defense system. Indeed, enhanced foliar anthocyanin, total soluble protein and glutathione concentrations imply an upregulation of the antioxidant defense system in Q. infectoria under stress while none of these parameters showed a significant treatment response in Q. libani. Consequently, Q. infectoria foliage showed no significant increase in superoxide, lower lipoxygenase activity, and less electrolyte leakage compared to the highly elevated levels seen in Q. libani indicating oxidative damage. Our findings indicate greater drought tolerance and pathogen resilience in Q. infectoria compared to Q. libani. Under future climate scenarios, we therefore expect changes in forest community structure driven by a decline in Q. libani and closely associated organisms.

Insights into the Transcriptional Regulation of Branching Hormonal Signaling Pathways Genes under Drought Stress in Arabidopsis

A large number of hormonal biosynthetic or signaling pathways genes controlling shoot branching are widely known for their roles in regulating plant growth and development, operating in synergetic or antagonistic manner. However, their involvement in abiotic stress response mechanism remains unexplored. Initially, we performed an in silico analysis to identify potential transcription binding sites for the basic leucine zipper 62 transcription factor (bZIP62 TF) in the target branching related genes. The results revealed the presence of cis-regulatory elements specific to two bZIP TFs, AtbZIP18 and AtbZIP69, rather than AtbZIP62. Interestingly, these bZIP TFs were previously proposed to be negatively regulated by the AtbZIP62 TF under salinity in Arabidopsis. Therefore, we investigated the transcriptional regulation of more axillary branching (MAX, strigolactone), PIN-FORMED (PINs, auxin carriers), gibberellic acid (GA)-biosynthetic genes as well as isopentenyltransferase (IPT, cytokinin biosynthesis pathway) genes in response to drought stress in Arabidopsis Col-0 wild type. In addition, in the perspective of exploring the transcriptional interplay of the selected genes with the AtbZIP62, we measured their expression by qPCR in the atbzip62 (lacking the AtbZIP62 gene) background under the same conditions. Our findings revealed that the expression of AtMAX2, AtMAX3, and AtMAX4 was differentially regulated by drought stress between the atbzip62 and Col-0 wild type, but not AtMAX1. Similarly, the transcripts accumulation of AtPIN3 and AtPIN7 (known as auxin efflux carriers), and that of the AtAXR1 showed similar regulation patterns in atbzip62. However, AtPIN1 expression was downregulated in Col-0, but no change was observed in atbzip62. Furthermore, AtIPT5 and AtIPT7 exhibited a differential transcripts accumulation pattern in atbzip62 and Col-0 wild type (WT). In the same way, the expression of the GA biosynthetic genes AtGA2ox1 and AtGA20ox2, and that of AtRGA1 were differentially regulated in atbzip62 compared to the Col-0. Meanwhile, AtGA2ox1 showed a similar expression pattern with Col-0. Therefore, all results suggest PIN, MAX, IPT, and GA-biosynthetic genes, which are differentially regulated by AtbZIP62 transcription factor, as emerging candidate genes that could be involved in drought stress response mechanism in Arabidopsis.

Progress in understanding salt stress response in plants using biotechnological tools

Salinization is a worldwide environmental problem, which is negatively impacting crop yield and thus posing a threat to the world's food security. Considering the rising threat of salinity, it is need of time, to understand the salt tolerant mechanism in plants and find avenues for the development of salinity resistant plants. Several plants tolerate salinity in a different manner, thereby halophytes and glycophytes evolved altered mechanisms to counter the stress. Therefore, in this review article, physiological, metabolic, and molecular aspects of the plant adaptation to salt stress have been discussed. The conventional breeding techniques for developing salt tolerant plants has not been much successful, due to its multigenic trait. The inflow of data from plant sequencing projects and annotation of genes led to the identification of many putative genes having a role in salt stress. The bioinformatics tools provided preliminary information and were helpful for making salt stress-specific databases. The microRNA identification and characterization led to unraveling the finer intricacies of the network. The transgenic approach finally paved a way for overexpressing some important genes viz. DREB, MYB, COMT, SOS, PKE, NHX, etc. conferred salt stress tolerance. In this review, we tried to show the effect of salinity on plants, considering ion homeostasis, antioxidant defense response, proteins involved, possible utilization of transgenic plants, and bioinformatics for coping with this stress factor. An overview of previous studies related to salt stress is presented in order to assist researchers in providing a potential solution for this increasing environmental threat.

LC-MS based plant metabolic profiles of thirteen grassland species grown in diverse neighbourhoods

In plants, secondary metabolite profiles provide a unique opportunity to explore seasonal variation and responses to the environment. These include both abiotic and biotic factors. In field experiments, such stress factors occur in combination. This variation alters the plant metabolic profiles in yet uninvestigated ways. This data set contains trait and mass spectrometry data of thirteen grassland species collected at four time points in the growing season in 2017. We collected above-ground vegetative material of seven grass and six herb species that were grown in plant communities with different levels of diversity in the Jena Experiment. For each sample, we recorded visible traits and acquired shoot metabolic profiles on a UPLC-ESI-Qq-TOF-MS. We performed the raw data pre-processing in Galaxy-W4M and prepared the data for statistical analysis in R by applying missing data imputation, batch correction, and validity checks on the features. This comprehensive data set provides the opportunity to investigate environmental dynamics across diverse neighbourhoods that are reflected in the metabolomic profile.

Differential development times of galls induced by Leptocybe invasa (Hymenoptera: Eulophidae) reveal differences in susceptibility between two Eucalyptus clones

BACKGROUND

The significance of morphological responses of hosts on susceptibility against gall-inducing insects is relatively unknown, especially in planted forests. Here, we investigate the temporal morphological responses (gall development) induced by the invasive gall wasp Leptocybe invasa and the subsequent insect development in two Eucalyptus clones.

RESULTS

Our results identified a novel stage of gall development, not previously reported, termed here 'brownish ring'. In both hosts similar gall development stages were observed. Although L. invasa oviposited in both clones, comparison of external morphological traits of galls over time revealed a differential response in the number of galls between clones. Comparison of the developmental time of each gall and insect stage between clones suggests that plant defense mechanisms against L. invasa are activated shortly after oviposition by the wasp, yet before gall formation.

CONCLUSION

Gall number is an important parameter that should be used to measure host susceptibility among Eucalyptus clones. To the best of our knowledge, this is the first study showing differential morphological responses induced by a galling insect, even before gall formation, revealing differences in susceptibility between different plant hosts. These findings provide insight into the use of early stages of gall formation by L. invasa to prevent invasion and establishment of this pest.

Responses to Drought Stress Modulate the Susceptibility to Plasmopara viticola in Vitis vinifera Self-Rooted Cuttings

Climate change will increase the occurrence of plants being simultaneously subjected to drought and pathogen stress. Drought can alter the way in which plants respond to pathogens. This research addresses how grapevine responds to the concurrent challenge of drought stress and Plasmopara viticola, the causal agent of downy mildew, and how one stress affects the other. Self-rooted cuttings of the drought-tolerant grapevine cultivar Xynisteri and the drought-sensitive cultivar Chardonnay were exposed to full or deficit irrigation (40% of full irrigation) and artificially inoculated with P. viticola in vitro or in planta. Leaves were sampled at an early infection stage to determine the influence of the single and combined stresses on oxidative parameters, chlorophyll, and phytohormones. Under full irrigation, Xynisteri was more susceptible to P. viticola than the drought-sensitive cultivar Chardonnay. Drought stress increased the susceptibility of grapevine leaves inoculated in vitro, but both cultivars showed resistance against P. viticola when inoculated in planta. Abscisic acid, rather than jasmonic acid and salicylic acid, seemed to play a prominent role in this resistance. The irrigation-dependent susceptibility observed in this study indicates that the practices used to mitigate the effects of climate change may have a profound impact on plant pathogens.

Dissecting the molecular responses of lentil to individual and combined drought and heat stresses by comparative transcriptomic analysis

Lentil cultivation could be challenged by combined heat and drought stress in semi-arid regions. We used RNA-seq approach to profile transcriptome changes of Lens culinaris exposed to individual and combined heat and drought stresses. It was determined that most of the differentially expressed genes observed in response to combined stress, could not be identified by analysis of transcriptome exposed to corresponding individual stresses. Interestingly, this study results revealed that the expression of ribosome generation and protein biosynthesis and starch degradation pathways related genes were uniquely up-regulated under the combined stress. Although multiple genes related to antioxidant activity were up-regulated in response to all stresses, variation in types and expression levels of these genes under the combined stress were higher than that of individual stresses. Using this comparative approach, for the first time, we reported up-regulation of several TF, CDPK, CYP, and antioxidant genes in response to combined stress in plants.

Bacterial Root Endophytes: Characterization of Their Competence and Plant Growth Promotion in Soybean (Glycine max (L.) Merr.) under Drought Stress

Recently, the application of endophytes in the alleviation of different types of stresses has received considerable attention, but their role in drought stress alleviation and growth promotion in soybean is not well-stated. In this study, twenty bacterial endophytes were isolated from soybean root tissues and screened for plant growth-promoting (PGP) traits, biocontrol potential, and drought stress alleviation. Out of them, 80% showed PGP traits, and 20% showed antagonistic activity against Fusarium oxysporum (ITCC 2389), Macrophomina phaseolina (ITCC 1800), and Alternaria alternata (ITCC 3467), and only three of them showed drought tolerance up to 15% (-0.3 MPa). Results indicated that drought-tolerant PGP endophytic bacteria enhanced soybean seedling growth under drought stress conditions. Morphological, biochemical, and molecular characterization (16S rRNA) revealed that these three bacterial isolates, AKAD A1-1, AKAD A1-2, and AKAD A1-16, closely resemble Bacillus cereus (GenBank accession No. MN079048), Pseudomonas otitidis (MW301101), and Pseudomonas sp. (MN079074), respectively. We observed that the soybean seedlings were grown in well-watered and drought-stressed soil showed the adverse effect of drought stress on morphological (stem length, root length, plant fresh and dry weight) as well as on biochemical parameters (a decline of photosynthetic pigments, membrane damage, etc.). However, soybean seedlings inoculated with these endophytes have improved the biomass significantly (p ≤ 0.05) under normal as well as in drought stress conditions over control treatments by influencing several biochemical changes. Among these three endophytes, AKAD A1-16 performed better than AKAD A1-2 and AKAD A1-1, which was further validated by the ability to produce the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase in the following order: AKAD A1-16 > AKAD A1-2 > AKAD A1-1. Scanning electron microscopy images also showed a bacterial presence inside the roots of soybean seedlings. These findings supported the application of bacterial root endophytes as a potential tool to mitigate the effect of drought as well as of fungal diseases on the early seedling growth of soybean.

Development of a robust hydroponic method for screening of sunflower (Helianthus annuus L.) accessions for tolerance to heat and osmotic stress

Hydroponic systems are known to provide a platform for uniform growth conditions until the reproductive stage. However, many plant species, including sunflower, show poor growth and survivability under conventional hydroponic systems due to poor nutrient availability, hypoxia and algal contamination. Thus, we tested various hydroponic systems to select a hydroponic system suitable for screening of sunflower germplasm. Sunflower accessions showed better growth and leaf gas exchange in newly-designed over conventional hydroponic systems. Selected hydroponic systems were further engaged in sunflower accession screening under heat and osmotic stress in a two-pan system (210 cm × 60 cm). Heat stress treatment was applied by growing sunflower germplasm at 42 °C and osmotic stress by adding polyethylene glycol 8000 which decreased the osmotic potential to - 0.6 MPa. There was significant variability among the sunflower accessions for their ability to survive under stress. Accessions such as C-2721 (43%), C-291 (46%) and D-14 (43%) had lower cell membrane injury percentage under osmotic stress and high seedling survivability (60‒80%) under heat stress when compared with susceptible accessions. Moreover, resistant accessions exhibited greater cuticular waxes and root length but lower transpiration losses. The newly designed hydroponic platform proved reliable for the selection of resistant sunflower accessions. Selected parental lines were validated by assessing their hybrids under field trials across two seasons under water and temperature stress during the reproductive phase (autumn). Hybrid H3 obtained by crossing drought and heat resistant parents had the highest seed yield and water use efficiency.

Predictability of Biotic Stress Structures Plant Defence Evolution

To achieve ecological and reproductive success, plants need to mitigate a multitude of stressors. The stressors encountered by plants are highly dynamic but typically vary predictably due to seasonality or correlations among stressors. As plants face physiological and ecological constraints in responses to stress, it can be beneficial for plants to evolve the ability to incorporate predictable patterns of stress in their life histories. Here, we discuss how plants predict adverse conditions, which plant strategies integrate predictability of biotic stress, and how such strategies can evolve. We propose that plants commonly optimise responses to correlated sequences or combinations of herbivores and pathogens, and that the predictability of these patterns is a key factor governing plant strategies in dynamic environments.

Accumulation and translocation of food chain in soil-mulberry (Morus alba L.)-silkworm (Bombyx mori) under single and combined stress of lead and cadmium

In recent years, heavy metal pollution has caused immeasurable harm to the environment. As an emerging technology, phytoremediation technology has gained a place in the treatment of heavy metal pollution with its unique advantages. This study analyzes the toxic effects of mulberry (Morus alba) seeds, seedling growth and silkworm under heavy metal stress of lead (Pb) and cadmium (Cd), and explore the accumulation and migration of Pb and Cd in the soil-mulberry tree-silkworm system. The main results were as follows: (1) Seed germination and potted seedling experiments were conducted under heavy metal Pb and Cd stresses, and it was found that Pb and Cd had inhibitory effects on mulberry seed germination, growth and photosynthesis of mulberry seedlings, and as the concentration of heavy metals increased, the stronger the inhibitory effect. Moreover, Pb and Cd have a synergistic effect under compound stress. (2) The accumulation and transfer rules of Pb and Cd ions in mulberry were different. The content of Pb in mulberry was root > leaf > stem and the content of Cd was root > stem > leaf. The combined stress promoted the transfer of Pb and Cd from the underground part to the aerial portion of mulberry. (3) The silkworm feeds on mulberry leaves contaminated with heavy metals in this experiment and found that: with the increase of silkworm feeding, the heavy metal content in the silkworm body increased significantly, but the content remained in the silkworm body was less, most of it was excreted with silkworm excrement. Combined stress has no significant effect on the detoxification mechanism of silkworm. It is indispensable to think of the synergistic effect of heavy metals on plants germination when seeds are used for phytoremediation.

Effect of elevated ozone and carbon dioxide interaction on growth, yield, nutrient content and wilt disease severity in chickpea grown in Northern India

Wilt caused by Fusarium oxysporum, sp. Ciceris (FOC) is an important disease causing losses up to 10% in chickpea yield. Experiments were conducted growing chickpea in free air ozone and carbon dioxide enrichment rings under four treatments of elevated ozone (O3) (EO:60 ± 10 ppb), elevated carbon dioxide (CO2) (ECO2:550 ± 25 ppm), combination of elevated CO2 and O3 (EO + ECO2) and ambient control for quantifying the effect on growth, yield, biochemical and nutrient content of chickpea. For studying the impact on wilt disease, chickpea was grown additionally in pots with soil containing FOC in these rings. The incidence of Fusarium wilt reduced significantly (p < 0.01) under EO as compared to ambient and ECO2. The activities of pathogenesis-related proteins chitinase and β-1,3- glucanase, involved in plant defense mechanism were enhanced under EO. The aboveground biomass and pod weight declined by 18.7 and 15.8% respectively in uninnoculated soils under EO, whereas, in FOC inoculated soil (diseased plants), the decline under EO was much less at 8.6 and 9.9% as compared to the ambient. Under EO, the activity of super oxide dismutase increased significantly (p < 0.5, 40%) as compared to catalase (12.5%) and peroxidase (17.5%) without any significant increase under EO + ECO2. The proline accumulation was significantly (p < 0.01) higher in EO as compared to EO + ECO2, and ECO2. The seed yield declined under EO due to significant reduction (p < 0.01) in the number of unproductive pods and seed weight. No change in the protein, total soluble sugars, calcium and phosphorus content was observed in any of the treatments, however, a significant decrease in potassium (K) content was observed under EO + ECO2. Elevated CO2 (554ppm) countered the impacts of 21.1 and 14.4 ppm h (AOT 40) O3 exposure on the seed yield and nutrient content (except K) in the EO + CO2 treatment and reduced the severity of wilt disease in the two years' study.

Multiple levels of crosstalk in hormone networks regulating plant defense

Plant hormones are essential for regulating the interactions between plants and their complex biotic and abiotic environments. Each hormone initiates a specific molecular pathway and these different hormone pathways are integrated in a complex network of synergistic, antagonistic and additive interactions. This inter-pathway communication is called hormone crosstalk. By influencing the immune network topology, hormone crosstalk is essential for tailoring plant responses to diverse microbes and insects in diverse environmental and internal contexts. Crosstalk provides robustness to the immune system but also drives specificity of induced defense responses against the plethora of biotic interactors. Recent advances in dry-lab and wet-lab techniques have greatly enhanced our understanding of the broad-scale effects of hormone crosstalk on immune network functioning and have revealed underlying principles of crosstalk mechanisms. Molecular studies have demonstrated that hormone crosstalk is modulated at multiple levels of regulation, such as by affecting protein stability, gene transcription and hormone homeostasis. These new insights into hormone crosstalk regulation of plant defense are reviewed here, with a focus on crosstalk acting on the jasmonic acid pathway in Arabidopsis thaliana, highlighting the transcription factors MYC2 and ORA59 as major targets for modulation by other hormones.

Nucleoredoxin Gene TaNRX1 Positively Regulates Drought Tolerance in Transgenic Wheat (Triticum aestivum L.)

Drought is the main abiotic stress factor limiting the growth and yield of wheat (Triticum aestivum L.). Therefore, improving wheat tolerance to drought stress is essential for maintaining yield. Previous studies have reported on the important role of TaNRX1 in conferring drought stress tolerance. Therefore, to elucidate the regulation mechanism by which TaNRX1 confers drought resistance in wheat, we generated TaNRX1 overexpression (OE) and RNA interference (RNAi) wheat lines. The results showed that the tolerance of the OE lines to drought stress were significantly enhanced. The survival rate, leaf chlorophyll, proline, soluble sugar content, and activities of the antioxidant enzymes (catalase, superoxide dismutase, and peroxidase) of the OE lines were higher than those of the wild type (WT); however, the relative electrical conductivity and malondialdehyde, hydrogen peroxide, and superoxide anion levels of the OE lines were lower than those of the WT; the RNAi lines showed the opposite results. RNA-seq results showed that the common differentially expressed genes of TaNRX1 OE and RNAi lines, before and after drought stress, were mainly distributed in the plant-pathogen interaction, plant hormone signal transduction, phenylpropane biosynthesis, starch and sucrose metabolism, and carbon metabolism pathways and were related to the transcription factors, including WRKY, MYB, and bHLH families. This study suggests that TaNRX1 positively regulates drought stress tolerance in wheat.

Fight hard or die trying: when plants face pathogens under heat stress

In their natural environment, plants are exposed to biotic or abiotic stresses that occur sequentially or simultaneously. Plant responses to these stresses have been studied widely and have been well characterised in simplified systems involving single plant species facing individual stress. Temperature elevation is a major abiotic driver of climate change and scenarios have predicted an increase in the number and severity of epidemics. In this context, here we review the available data on the effect of heat stress on plant-pathogen interactions. Considering 45 studies performed on model or crop species, we discuss the possible implications of the optimum growth temperature of plant hosts and pathogens, mode of stress application and temperature variation on resistance modulations. Alarmingly, most identified resistances are altered under temperature elevation, regardless of the plant and pathogen species. Therefore, we have listed current knowledge on heat-dependent plant immune mechanisms and pathogen thermosensory processes, mainly studied in animals and human pathogens, that could help to understand the outcome of plant-pathogen interactions under elevated temperatures. Based on a general overview of the mechanisms involved in plant responses to pathogens, and integrating multiple interactions with the biotic environment, we provide recommendations to optimise plant disease resistance under heat stress and to identify thermotolerant resistance mechanisms.

Maize Responses Challenged by Drought, Elevated Daytime Temperature and Arthropod Herbivory Stresses: A Physiological, Biochemical and Molecular View

Maize (Zea mays L.) is one of the main cereals grown around the world. It is used for human and animal nutrition and also as biofuel. However, as a direct consequence of global climate change, increased abiotic and biotic stress events have been reported in different regions of the world, which have become a threat to world maize yields. Drought and heat are environmental stresses that influence the growth, development, and yield processes of maize crops. Plants have developed dynamic responses at the physiological, biochemical, and molecular levels that allow them to escape, avoid and/or tolerate unfavorable environmental conditions. Arthropod herbivory can generate resistance or tolerance responses in plants that are associated with inducible and constitutive defenses. Increases in the frequency and severity of abiotic stress events (drought and heat), as a consequence of climate change, can generate critical variations in plant-insect interactions. However, the behavior of herbivorous arthropods under drought scenarios is not well understood, and this kind of stress may have some positive and negative effects on arthropod populations. The simultaneous appearance of different environmental stresses and biotic factors results in very complex plant responses. In this review, recent information is provided on the physiological, biochemical, and molecular responses of plants to the combination of drought, heat stress, and the effect on some arthropod pests of interest in the maize crop.

Rice lectin protein r40c1 imparts drought tolerance by modulating S-adenosylmethionine synthase 2, stress-associated protein 8 and chromatin-associated proteins

Lectin proteins play an important role in biotic and abiotic stress responses in plants. Although the rice lectin protein Osr40c1 has been reported to be regulated by drought stress, the mechanism of its drought tolerance activity has not been studied so far. In this study, it is shown that expression of the Osr40c1 gene correlates with the drought tolerance potential of various rice cultivars. Transgenic rice plants overexpressing Osr40c1 were significantly more tolerant to drought stress than the wild-type plants. Furthermore, ectopic expression of the Osr40c1 gene in tobacco yielded a similar result. Interestingly, the protein displayed a nucleo-cytoplasmic localization and was found to interact with a number of drought-responsive proteins such as S-adenosylmethionine synthase 2 (OsSAM2), stress-associated protein 8 (OsSAP8), DNA-binding protein MNB1B (OsMNB1B), and histone 4 (OsH4). Silencing of each of these protein partners led to drought sensitivity in otherwise tolerant Osr40c1-expressing transgenic tobacco lines indicating that these partners were crucial for the Osr40c1-mediated drought tolerance in planta. Moreover, the association of Osr40c1 with these partners occurred specifically under drought stress forming a multi-protein complex. Together, our findings delineate a novel role of Osr40c1 in imparting drought tolerance by regulating OsMNB1B, OsSAM2, and OsH4 proteins, which presumably enables OsSAP8 to induce downstream gene expression.

Active and Passive Electro-Optical Sensors for Health Assessment in Food Crops

In agriculture, early detection of plant stresses is advantageous in preventing crop yield losses. Remote sensors are increasingly being utilized for crop health monitoring, offering non-destructive, spatialized detection and the quantification of plant diseases at various levels of measurement. Advances in sensor technologies have promoted the development of novel techniques for precision agriculture. As in situ techniques are surpassed by multispectral imaging, refinement of hyperspectral imaging and the promising emergence of light detection and ranging (LIDAR), remote sensing will define the future of biotic and abiotic plant stress detection, crop yield estimation and product quality. The added value of LIDAR-based systems stems from their greater flexibility in capturing data, high rate of data delivery and suitability for a high level of automation while overcoming the shortcomings of passive systems limited by atmospheric conditions, changes in light, viewing angle and canopy structure. In particular, a multi-sensor systems approach and associated data fusion techniques (i.e., blending LIDAR with existing electro-optical sensors) offer increased accuracy in plant disease detection by focusing on traditional optimal estimation and the adoption of artificial intelligence techniques for spatially and temporally distributed big data. When applied across different platforms (handheld, ground-based, airborne, ground/aerial robotic vehicles or satellites), these electro-optical sensors offer new avenues to predict and react to plant stress and disease. This review examines the key sensor characteristics, platform integration options and data analysis techniques recently proposed in the field of precision agriculture and highlights the key challenges and benefits of each concept towards informing future research in this very important and rapidly growing field.

Gene co-expression analysis reveals transcriptome divergence between wild and cultivated chickpea under drought stress

Ancestral adaptations in crop wild relatives can provide a genetic reservoir for crop improvement. Here we document physiological changes to mild and severe drought stress, and the associated transcriptome dynamics in both wild and cultivated chickpea. Over 60% of transcriptional changes were related to metabolism, indicating that metabolic plasticity is a core and conserved drought response. In addition, changes in RNA processing and protein turnover were predominant in the data, suggestive of broad restructuring of the chickpea proteome in response to drought. While 12% of the drought-responsive transcripts have similar dynamics in cultivated and wild accessions, numerous transcripts had expression patterns unique to particular genotypes, or that distinguished wild from cultivated genotypes and whose divergence may be a consequence of domestication. These and other comparisons provide a transcriptional correlate of previously described species' genetic diversity, with wild accessions well differentiated from each other and from cultivars, and cultivars essentially indistinguishable at the broad transcriptome level. We identified metabolic pathways such as phenylpropanoid metabolism, and biological processes such as stomatal development, which are differentially regulated across genotypes with potential consequences on drought tolerance. These data indicate that wild Cicer reticulatum may provide both conserved and divergent mechanisms as a resource in breeding for drought tolerance in cultivated chickpea.