Drought: A context-dependent damper and aggravator of plant diseases.

Drought dynamically influences the interactions between plants and pathogens, thereby affecting disease outbreaks. Understanding the intricate mechanistic aspects of the multiscale interactions among plants, pathogens, and the environment-known as the disease triangle-is paramount for enhancing the climate resilience of crop plants. In this review, we systematically compile and comprehensively analyse current knowledge on the influence of drought on the severity of plant diseases. We emphasise that studying these stresses in isolation is not sufficient to predict how plants respond to combined stress from both drought and pathogens. The impact of drought and pathogens on plants is complex and multifaceted, encompassing the activation of antagonistic signalling cascades in response to stress factors. The nature, intensity, and temporality of drought and pathogen stress occurrence significantly influence the outcome of diseases. We delineate the drought-sensitive nodes of plant immunity and highlight the emerging points of crosstalk between drought and defence signalling under combined stress. The limited mechanistic understanding of these interactions is acknowledged as a key research gap in this area. The information synthesised herein will be crucial for crafting strategies for the accurate prediction and mitigation of future crop disease risks, particularly in the context of a changing climate.

XLG2 and CORI3 function additively to regulate plant defense against the necrotrophic pathogen Sclerotinia sclerotiorum.

The membrane-bound heterotrimeric G-proteins in plants play a crucial role in defending against a broad range of pathogens. This study emphasizes the significance of Extra-large Gα protein 2 (XLG2), a plant-specific G-protein, in mediating the plant response to Sclerotinia sclerotiorum, which infects over 600 plant species worldwide. Our analysis of Arabidopsis G-protein mutants showed that loss of XLG2 function increased susceptibility to S. sclerotiorum, accompanied by compromised accumulation of jasmonic acid (JA) during pathogen infection. Overexpression of the XLG2 gene in xlg2 mutant plants resulted in higher resistance and increased JA accumulation during S. sclerotiorum infection. Co-immunoprecipitation (co-IP) analysis on S. sclerotiorum infected Col-0 samples, using two different approaches, identified 201 XLG2-interacting proteins. The identified JA-biosynthetic and JA-responsive proteins had compromised transcript expression in the xlg2 mutant during pathogen infection. XLG2 was found to interact physically with a JA-responsive protein, Coronatine induced 1 (CORI3) in Co-IP, and confirmed using split firefly luciferase complementation and bimolecular fluorescent complementation assays. Additionally, genetic analysis revealed an additive effect of XLG2 and CORI3 on resistance against S. sclerotiorum, JA accumulation, and expression of the defense marker genes. Overall, our study reveals two independent pathways involving XLG2 and CORI3 in contributing resistance against S. sclerotiorum.

When two negatives make a positive: the favorable impact of the combination of abiotic stress and pathogen infection on plants.

Combined abiotic and biotic stresses modify plant defense signaling, leading to either the activation or suppression of defense responses. Although the majority of combined abiotic and biotic stresses reduce plant fitness, certain abiotic stresses reduce the severity of pathogen infection in plants. Remarkably, certain pathogens also improve the tolerance of some plants to a few abiotic stresses. While considerable research focuses on the detrimental impact of combined stresses on plants, the upside of combined stress remains hidden. This review succinctly discusses the interactions between abiotic stresses and pathogen infection that benefit plant fitness. Various factors that govern the positive influence of combined abiotic stress and pathogen infection on plant performance are also discussed. In addition, we provide a brief overview of the role of pathogens, mainly viruses, in improving plant responses to abiotic stresses. We further highlight the critical nodes in defense signaling that guide plant responses during abiotic stress towards enhanced resistance to pathogens. Studies on antagonistic interactions between abiotic and biotic stressors can uncover candidates in host plant defense that may shield plants from combined stresses.

Stress combinations and their interactions in plants database: a one-stop resource on combined stress responses in plants.

We have developed a compendium and interactive platform, named Stress Combinations and their Interactions in Plants Database (SCIPDb; http://www.nipgr.ac.in/scipdb.php), which offers information on morpho-physio-biochemical (phenome) and molecular (transcriptome and metabolome) responses of plants to different stress combinations. SCIPDb is a plant stress informatics hub for data mining on phenome, transcriptome, trait-gene ontology, and data-driven research for advancing mechanistic understanding of combined stress biology. We analyzed global phenome data from 939 studies to delineate the effects of various stress combinations on yield in major crops and found that yield was substantially affected under abiotic-abiotic stresses. Transcriptome datasets from 36 studies hosted in SCIPDb identified novel genes, whose roles have not been earlier established in combined stress. Integretome analysis under combined drought-heat stress pinpointed carbohydrate, amino acid, and energy metabolism pathways as the crucial metabolic, proteomic, and transcriptional components in plant tolerance to combined stress. These examples illustrate the application of SCIPDb in identifying novel genes and pathways involved in combined stress tolerance. Further, we showed the application of this database in identifying novel candidate genes and pathways for combined drought and pathogen stress tolerance. To our knowledge, SCIPDb is the only publicly available platform offering combined stress-specific omics big data visualization tools, such as an interactive scrollbar, stress matrix, radial tree, global distribution map, meta-phenome analysis, search, BLAST, transcript expression pattern table, Manhattan plot, and co-expression network. These tools facilitate a better understanding of the mechanisms underlying plant responses to combined stresses.

MicroRNA397 regulates tolerance to drought and fungal infection by regulating lignin deposition in chickpea root.

Plants deposit lignin in the secondary cell wall as a common response to drought and pathogen attacks. Cell wall localised multicopper oxidase family enzymes LACCASES (LACs) catalyse the formation of monolignol radicals and facilitate lignin formation. We show an upregulation of the expression of several LAC genes and a downregulation of microRNA397 (CamiR397) in response to natural drought in chickpea roots. CamiR397 was found to target LAC4 and LAC17L out of twenty annotated LACs in chickpea. CamiR397 and its target genes are expressed in the root. Overexpression of CamiR397 reduced expression of LAC4 and LAC17L and lignin deposition in chickpea root xylem causing reduction in xylem wall thickness. Downregulation of CamiR397 activity by expressing a short tandem target mimic (STTM397) construct increased root lignin deposition in chickpea. CamiR397-overexpressing and STTM397 chickpea lines showed sensitivity and tolerance, respectively, towards natural drought. Infection with a fungal pathogen Macrophomina phaseolina, responsible for dry root rot (DRR) disease in chickpea, induced local lignin deposition and LAC gene expression. CamiR397-overexpressing and STTM397 chickpea lines showed more sensitivity and tolerance, respectively, to DRR. Our results demonstrated the regulatory role of CamiR397 in root lignification during drought and DRR in an agriculturally important crop chickpea.

AtSWEET11 and AtSWEET12 transporters function in tandem to modulate sugar flux in plants.

The sugar will eventually be exported transporter (SWEET) members in Arabidopsis, AtSWEET11 and AtSWEET12 are the important sucrose efflux transporters that act synergistically to perform distinct physiological roles. These two transporters are involved in apoplasmic phloem loading, seed filling, and sugar level alteration at the site of pathogen infection. Here, we performed the structural analysis of the sucrose binding pocket of AtSWEET11 and AtSWEET12 using molecular docking followed by rigorous molecular dynamics (MD) simulations. We observed that the sucrose molecule binds inside the central cavity and in the middle of the transmembrane (TM) region of AtSWEET11 and AtSWEET12, that allows the alternate access to the sucrose molecule from either side of the membrane during transport. Both AtSWEET11 and AtSWEET12, shares the similar amino acid residues that interact with sucrose molecule. Further, to achieve more insights on the role of these two transporters in other plant species, we did the phylogenetic and the in-silico analyses of AtSWEET11 and AtSWEET12 orthologs from 39 economically important plants. We reported the extensive information on the gene structure, protein domain and cis-acting regulatory elements of AtSWEET11 and AtSWEET12 orthologs from different plants. The cis-elements analysis indicates the involvement of AtSWEET11 and AtSWEET12 orthologs in plant development and also during abiotic and biotic stresses. Both in silico and in planta expression analysis indicated AtSWEET11 and AtSWEET12 are well-expressed in the Arabidopsis leaf tissues. However, the orthologs of AtSWEET11 and AtSWEET12 showed the differential expression pattern with high or no transcript expression in the leaf tissues of different plants. Overall, these results offer the new insights into the functions and regulation of AtSWEET11 and AtSWEET12 orthologs from different plant species. This might be helpful in conducting the future studies to understand the role of these two crucial transporters in Arabidopsis and other crop plants.

Comparative transcriptome profiling reveals differential defense responses among Alternaria brassicicola resistant Sinapis alba and susceptible Brassica rapa.

Alternaria blight is a devastating disease that causes significant crop losses in oilseed Brassicas every year. Adoption of conventional breeding to generate disease-resistant varieties has so far been unsuccessful due to the lack of suitable resistant source germplasms of cultivated Brassica spp. A thorough understanding of the molecular basis of resistance, as well as the identification of defense-related genes involved in resistance responses in closely related wild germplasms, would substantially aid in disease management. In the current study, a comparative transcriptome profiling was performed using Illumina based RNA-seq to detect differentially expressed genes (DEGs) specifically modulated in response to Alternaria brassicicola infection in resistant Sinapis alba, a close relative of Brassicas, and the highly susceptible Brassica rapa. The analysis revealed that, at 48 hpi (hours post inoculation), 3396 genes were upregulated and 23239 were downregulated, whereas at 72 hpi, 4023 genes were upregulated and 21116 were downregulated. Furthermore, a large number of defense response genes were detected to be specifically regulated as a result of Alternaria infection. The transcriptome data was validated using qPCR-based expression profiling for selected defense-related DEGs, that revealed significantly higher fold change in gene expression in S. alba when compared to B. rapa. Expression of most of the selected genes was elevated across all the time points under study with significantly higher expression towards the later time point of 72 hpi in the resistant germplasm. S. alba activates a stronger defense response reaction against the disease by deploying an array of genes and transcription factors involved in a wide range of biological processes such as pathogen recognition, signal transduction, cell wall modification, antioxidation, transcription regulation, etc. Overall, the study provides new insights on resistance of S. alba against A. brassicicola, which will aid in devising strategies for breeding resistant varieties of oilseed Brassica.

AtSWEET11 and AtSWEET12: the twin traders of sucrose.

AtSWEET11 and AtSWEET12 are central players in phloem loading and long-distance sucrose translocation. During drought stress, these transporters enhance sucrose transport from shoot to root, increasing root proliferation. Chen et al. have now unravelled novel aspects of sucrose transport regulation, occurring via AtSWEET11 and AtSWEET12 phosphorylation and oligomerisation.

Combined Drought and Heat Stress Influences the Root Water Relation and Determine the Dry Root Rot Disease Development Under Field Conditions: A Study Using Contrasting Chickpea Genotypes.

Abiotic stressors such as drought and heat predispose chickpea plants to pathogens of key importance leading to significant crop loss under field conditions. In this study, we have investigated the influence of drought and high temperature on the incidence and severity of dry root rot disease (caused by Macrophomina phaseolina) in chickpea, under extensive on- and off-season field trials and greenhouse conditions. We explored the association between drought tolerance and dry root rot resistance in two chickpea genotypes, ICC 4958 and JG 62, with contrasting resistance to dry root rot. In addition, we extensively analyzed various patho-morphological and root architecture traits altered by combined stresses under field and greenhouse conditions in these genotypes. We further observed the role of edaphic factors in dry root rot incidence under field conditions. Altogether, our results suggest a strong negative correlation between the plant water relations and dry root rot severity in chickpeas, indicating an association between drought tolerance and dry root rot resistance. Additionally, the significant role of heat stress in altering the dynamics of dry root rot and the importance of combinatorial screening of chickpea germplasm for dry root rot resistance, drought, and heat stress have been revealed.

Functional role of formate dehydrogenase 1 (FDH1) for host and nonhost disease resistance against bacterial pathogens.

Nonhost disease resistance is the most common type of plant defense mechanism against potential pathogens. In the present study, the metabolic enzyme formate dehydrogenase 1 (FDH1) was identified to associate with nonhost disease resistance in Nicotiana benthamiana and Arabidopsis thaliana. In Arabidopsis, AtFDH1 was highly upregulated in response to both host and nonhost bacterial pathogens. The Atfdh1 mutants were compromised in nonhost resistance, basal resistance, and gene-for-gene resistance. The expression patterns of salicylic acid (SA) and jasmonic acid (JA) marker genes after pathogen infections in Atfdh1 mutant indicated that both SA and JA are involved in the FDH1-mediated plant defense response to both host and nonhost bacterial pathogens. Previous studies reported that FDH1 localizes to mitochondria, or both mitochondria and chloroplasts. Our results showed that the AtFDH1 mainly localized to mitochondria, and the expression level of FDH1 was drastically increased upon infection with host or nonhost pathogens. Furthermore, we identified the potential co-localization of mitochondria expressing FDH1 with chloroplasts after the infection with nonhost pathogens in Arabidopsis. This finding suggests the possible role of FDH1 in mitochondria and chloroplasts during defense responses against bacterial pathogens in plants.

A spotlight on non-host resistance to plant viruses.

Plant viruses encounter a range of host defenses including non-host resistance (NHR), leading to the arrest of virus replication and movement in plants. Viruses have limited host ranges, and adaptation to a new host is an atypical phenomenon. The entire genotypes of plant species which are imperceptive to every single isolate of a genetically variable virus species are described as non-hosts. NHR is the non-specific resistance manifested by an innately immune non-host due to pre-existing and inducible defense responses, which cannot be evaded by yet-to-be adapted plant viruses. NHR-to-plant viruses are widespread, but the phenotypic variation is often not detectable within plant species. Therefore, molecular and genetic mechanisms of NHR need to be systematically studied to enable exploitation in crop protection. This article comprehensively describes the possible mechanisms of NHR against plant viruses. Also, the previous definition of NHR to plant viruses is insufficient, and the main aim of this article is to sensitize plant pathologists to the existence of NHR to plant viruses and to highlight the need for immediate and elaborate research in this area.

Drought Stress Exacerbates Fungal Colonization and Endodermal Invasion and Dampens Defense Responses to Increase Dry Root Rot in Chickpea.

Drought plays a central role in increasing the incidence and severity of dry root rot (DRR) disease in chickpea. This is an economically devastating disease, compromising chickpea yields particularly severely in recent years due to erratic rainfall patterns. Macrophomina phaseolina (formerly Rhizoctonia bataticola) is the causal agent of DRR disease in the chickpea plant. The infection pattern in chickpea roots under well-watered conditions and drought stress are poorly understood at present. This study provides detailed disease symptomatology and the characteristics of DRR fungus at morphological and molecular levels. Using microscopy techniques, the infection pattern of DRR fungus in susceptible chickpea roots was investigated under well-watered and drought-stress conditions. Our observations suggested that drought stress intensifies the progression of already ongoing infection by weakening the endodermal barrier and overall defense. Transcriptomic analysis suggested that the plant's innate immune defense program is downregulated in infected roots when subjected to drought stress. Furthermore, genes involved in hormonal regulation are differentially expressed under drought stress. These findings provide hints in terms of potential chickpea genes to target in crop improvement programs to develop climate-change-resilient cultivars.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Recent Advances in Plant Gene Silencing Methods.

With the increasing understanding of fundamentals of gene silencing pathways in plants, various tools and techniques for downregulating the expression of a target gene have been developed across multiple plant species. This chapter provides an insight into the molecular mechanisms of gene silencing and highlights the advancements in various gene silencing approaches. The prominent aspects of different gene silencing methods, their advantages and disadvantages have been discussed. A succinct discussion on the newly emerged microRNA-based technologies like microRNA-induced gene silencing (MIGS) and microRNA-mediated virus-induced gene silencing (MIR-VIGS) are also presented. We have also discussed the gene-editing system like CRISPR-Cas. The prominent bottlenecks in gene silencing methods are the off-target effects and lack of universal applicability. However, the tremendous growth in understanding of this field reflects the potentials for improvements in the currently available approaches and the development of new widely applicable methods for easy, fast, and efficient functional characterization of plant genes.

High-Throughput Analysis of Gene Function under Multiple Abiotic Stresses Using Leaf Disks from Silenced Plants.

The high throughputness and affordability of "omics" technologies is leading to the identification of a large number of abiotic stress genes, with many of them responsive to multiple stresses. In vivo functional characterization of these genes under multiple stresses is challenging but essential to develop resilient crops for the changing climate. Here we describe a high-throughput Virus-Induced Gene Silencing-based methodology for functional analysis of genes under multiple abiotic stresses using leaf disks. Leaves with maximal silencing, which is localized to only a few leaves and to a short period, can be effectively used for multiple stress imposition and stress affect quantification.

Drought attenuates plant defence against bacterial pathogens by suppressing the expression of CBP60g/SARD1 during combined stress.

In nature, plants are frequently exposed to drought and bacterial pathogens simultaneously. However, information on how the drought and defence pathways interact and orchestrate global transcriptional regulation is limited. Here, we show that moderate drought stress enhances the susceptibility of Arabidopsis thaliana to Pseudomonas syringae pv. tomato DC3000. Using transcriptome meta-analysis, we found that drought and bacterial stress antagonistically modulate a large set of genes predominantly involved in salicylic acid (SA) and abscisic acid (ABA) signalling networks. We identified that the levels of SA and ABA are dynamically regulated during the course of stress. Importantly, under combined stress, drought through the ABA pathway downregulates the induction of Calmodulin-binding Protein 60 g (CBP60g) and Systemic Acquired Resistance Deficient 1 (SARD1), two transcription factors crucial for SA production upon bacterial infection. We also identified an important role of NPR1-LIKE PROTEIN 3 and 4 (NPR3/4) transcriptional repressors in the drought-mediated negative regulation of CBP60g/SARD1 expression. Using a genetic approach, we show that CBP60g/SARD1 expression is the key determinant of plant defence against bacterial pathogens under combined stress. Thus, these transcription factors act as critical nodes for the crosstalk between drought and bacterial stress signalling under combined stress in plants.

Dry Root Rot of Chickpea: A Disease Favored by Drought.

Chickpea is an essential crop for protein nutrition and is grown around the world in rain-fed conditions. However, chickpea cultivation is under threat due to emerging diseases favored by drought stress. Dry root rot (DRR), an economically devastating disease, is an example. Chickpea-specific strains of a necrotic fungal phytopathogen, Macrophomina phaseolina, cause DRR. Microsclerotia of this fungus, which are capable of withstanding harsh environmental conditions, serve as primary inoculum. Initial symptoms are scattered necrotic spots in roots, progressing to rotting and withering lateral roots, accompanied by prematurely dried, straw-colored foliage. The recent rise in global temperature and worsening of drought spells have aggravated DRR outbreaks in chickpea. To date, DRR epidemiology has not been clarified in detail. Also, the literature lacks clarity on M. phaseolina taxonomy, morphology, disease progression, and diagnosis. In this article, research progress on patterns of DRR occurrence in the field and belowground and aboveground symptoms are clarified. In addition, the current understanding of taxonomy and management practices is elaborated. We also summarize knowledge of the impact of drought and high temperature on DRR severity. Furthermore, we provide future perspectives on the importance of host resistance, quantitative trait loci identification, and genotype screening for the identification of resistant genotypes. The article proposes new research priorities and a corresponding plan for the mitigation of DRR.

The ins and outs of SWEETs in plants: Current understanding of the basics and their prospects in crop improvement.

Sugar will eventually be exported transporters (SWEETs), a newly discovered class of sugar transporters, play a significant role in sugar efflux processes across various kingdoms of life. In fact, SWEETs have a long evolutionary path from prokaryotes to higher plants. In plants, they are involved in developmental processes, including nectar secretion, pollen nutrition, and seed filling. While the role of SWEETs has been well studied in biotic stresses, particularly their manipulation by pathogens for sugar acquisition, they have also been linked to many abiotic stresses. Although the phylogenetic relationships and solved structures of SWEETs in different plants have been revealed, their regulation remains unexplored. The current review deals with all the exciting discoveries around SWEETs, including their classification and diversity, and bridges the gaps in their evolutionary story, from bacterial semiSWEETs to eukaryotic SWEETs. We also critically examine SWEETs at genomic, transcriptomic, and proteomic levels, as evinced by recently published examples from grain, millet, and horticultural crops. In addition, we highlight the possibilities of utilizing SWEETs in applications such as bioethanol production and disease diagnostic markers. We attempt to elucidate and unify findings related to the yet unsolved puzzle of SWEET regulation in plants to improve crop production and protection for sustainable agriculture.

A sick plot-based protocol for dry root rot disease assessment in field-grown chickpea plants.

PREMISE: A comprehensive field-based screening protocol is lacking for dry root rot (DRR) disease in chickpea, which is caused by Macrophomina phaseolina (formerly referred to as Rhizoctonia bataticola). Here, we describe a protocol for establishing a sick plot for DRR to enable disease assessment of a large number of chickpea plants during the natural growing season. METHODS AND RESULTS: We used a chickpea plot with >30% DRR incidence, and enriched the inoculum by cultivating highly susceptible chickpea plant genotypes and incorporating infected plant material into the soil. The chickpea plants were then subjected to infection in developed sick plots with various levels of soil moisture under natural field conditions. CONCLUSIONS: Our protocol provides a robust way to impose M. phaseolina infection on chickpea plants under natural field conditions and to investigate plant responses to the infection at morphological, physiological, and molecular levels. This method can also be used to screen for other soil-borne diseases in a variety of plants.