Plants regularly face adverse growth conditions, such as drought, salinity, chilling, freezing, and high temperatures. These stresses can delay growth and development, reduce productivity, and, in extreme cases, cause plant death. Plant stress responses are dynamic and involve complex cross-talk between different regulatory levels, including adjustment of metabolism and gene expression for physiological and morphological adaptation. In this review, information about metabolic regulation in response to drought, extreme temperature, and salinity stress is summarized and the signalling events involved in mediating stress-induced metabolic changes are presented.

Drought, salt, and temperature stress-induced metabolic rearrangements and regulatory networks

A positive temperature coefficient is the term which has been used to indicate that an increase in solubility occurs as the temperature is raised, whereas a negative coefficient indicates a decrease in solubility with rise in temperature.

Effect of Temperature

Plant responses to different stresses are highly complex and involve changes at the transcriptome, cellular, and physiological levels. Recent evidence shows that plants respond to multiple stresses differently from how they do to individual stresses, activating a specific programme of gene expression relating to the exact environmental conditions encountered. Rather than being additive, the presence of an abiotic stress can have the effect of reducing or enhancing susceptibility to a biotic pest or pathogen, and vice versa. This interaction between biotic and abiotic stresses is orchestrated by hormone signalling pathways that may induce or antagonize one another, in particular that of abscisic acid. Specificity in multiple stress responses is further controlled by a range of molecular mechanisms that act together in a complex regulatory network. Transcription factors, kinase cascades, and reactive oxygen species are key components of this cross-talk, as are heat shock factors and small RNAs. This review aims to characterize the interaction between biotic and abiotic stress responses at a molecular level, focusing on regulatory mechanisms important to both pathways. Identifying master regulators that connect both biotic and abiotic stress response pathways is fundamental in providing opportunities for developing broad-spectrum stress-tolerant crop plants.

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

Tolerance of plants to abiotic stressors such as drought and salinity is triggered by complex multicomponent signaling pathways to restore cellular homeostasis and promote survival. Major plant transcription factor families such as bZIP, NAC, AP2/ERF, and MYB orchestrate regulatory networks underlying abiotic stress tolerance. Sucrose non-fermenting 1-related protein kinase 2 and mitogen-activated protein kinase pathways contribute to initiation of stress adaptive downstream responses and promote plant growth and development. As a convergent point of multiple abiotic cues, cellular effects of environmental stresses are not only imbalances of ionic and osmotic homeostasis but also impaired photosynthesis, cellular energy depletion, and redox imbalances. Recent evidence of regulatory systems that link sensing and signaling of environmental conditions and the intracellular redox status have shed light on interfaces of stress and energy signaling. ROS (reactive oxygen species) cause severe cellular damage by peroxidation and de-esterification of membrane-lipids, however, current models also define a pivotal signaling function of ROS in triggering tolerance against stress. Recent research advances suggest and support a regulatory role of ROS in the cross talks of stress triggered hormonal signaling such as the abscisic acid pathway and endogenously induced redox and metabolite signals. Here, we discuss and review the versatile molecular convergence in the abiotic stress responsive signaling networks in the context of ROS and lipid-derived signals and the specific role of stomatal signaling.

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Tolerance to drought and salt stress in plants: Unraveling the signaling networks.

Abiotic stresses such as drought, cold, salt and heat cause reduction of plant growth and loss of crop yield worldwide. Reactive oxygen species (ROS) including hydrogen peroxide (H2O2), superoxide anions (O2•‾), hydroxyl radical (OH•) and singlet oxygen (1O2) are by-products of physiological metabolisms, and are precisely controlled by enzymatic and non-enzymatic antioxidant defense systems. ROS are significantly accumulated under abiotic stress conditions, which cause oxidative damage and eventually resulting in cell death. Recently, ROS have been also recognized as key players in the complex signaling network of plants stress responses. The involvement of ROS in signal transduction implies that there must be coordinated function of regulation networks to maintain ROS at non-toxic levels in a delicate balancing act between ROS production, involving ROS generating enzymes and the unavoidable production of ROS during basic cellular metabolism, and ROS-scavenging pathways. Increasing evidence showed that ROS play crucial roles in abiotic stress responses of crop plants for the activation of stress-response and defense pathways. More importantly, manipulating ROS levels provides an opportunity to enhance stress tolerances of crop plants under a variety of unfavorable environmental conditions. This review presents an overview of current knowledge about homeostasis regulation of ROS in crop plants. In particular, we summarize the essential proteins that are involved in abiotic stress tolerance of crop plants through ROS regulation. Finally, the challenges toward the improvement of abiotic stress tolerance through ROS regulation in crops are discussed.

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ROS Regulation During Abiotic Stress Responses in Crop Plants

Drought and high salinity are two major environmental factors that significantly limit the productivity of agricultural crops worldwide. WRKY transcription factors play essential roles in the adaptation of plants to abiotic stresses. However, WRKY genes involved in drought and salt tolerance in cotton (Gossypium hirsutum) are largely unknown. Here, a group IId WRKY gene, GhWRKY17, was isolated and characterized. GhWRKY17 was found to be induced after exposure to drought, salt, H2O2 and ABA. The constitutive expression of GhWRKY17 in Nicotiana benthamiana remarkably reduced plant tolerance to drought and salt stress, as determined through physiological analyses of the germination rate, root growth, survival rate, leaf water loss and Chl content. GhWRKY17 transgenic plants were observed to be more sensitive to ABA-mediated seed germination and root growth. However, overexpressing GhWRKY17 in N. benthamiana impaired ABA-induced stomatal closure. Furthermore, we found that GhWRKY17 modulated the increased sensitivity of plants to drought by reducing the level of ABA, and transcript levels of ABA-inducible genes, including AREB, DREB, NCED, ERD and LEA, were clearly repressed under drought and salt stress conditions. Consistent with the accumulation of reactive oxygen species (ROS), reduced proline contents and enzyme activities, elevated electrolyte leakage and malondialdehyde, and lower expression of ROS-scavenging genes, including APX, CAT and SOD, the GhWRKY17 transgenic plants exhibited reduced tolerance to oxidative stress compared with wild-type plants. These results therefore indicate that GhWRKY17 responds to drought and salt stress through ABA signaling and the regulation of cellular ROS production in plants.

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The Cotton WRKY Transcription Factor GhWRKY17 Functions in Drought and Salt Stress in Transgenic Nicotiana benthamiana Through ABA Signaling and the Modulation of Reactive Oxygen Species Production

WRKY transcription factors constitute a very large family of proteins in plants and participate in modulating plant biological processes, such as growth, development and stress responses. However, the exact roles of WRKY proteins are unclear, particularly in non-model plants. In this study, Gossypium hirsutum WRKY41 (GhWRKY41) was isolated and transformed into Nicotiana benthamiana. Our results showed that overexpression of GhWRKY41 enhanced the drought and salt stress tolerance of transgenic Nicotiana benthamiana. The transgenic plants exhibited lower malondialdehyde content and higher antioxidant enzyme activity, and the expression of antioxidant genes was upregulated in transgenic plants exposed to osmotic stress. A β-glucuronidase (GUS) staining assay showed that GhWRKY41 was highly expressed in the stomata when plants were exposed to osmotic stress, and plants overexpressing GhWRKY41 exhibited enhanced stomatal closure when they were exposed to osmotic stress. Taken together, our findings demonstrate that GhWRKY41 may enhance plant tolerance to stress by functioning as a positive regulator of stoma closure and by regulating reactive oxygen species (ROS) scavenging and the expression of antioxidant genes.

/pdf/the-cotton-wrky-gene-ghwrky41-positively-regulates-salt-and-58k3kom95e.pdf

The Cotton WRKY Gene GhWRKY41 Positively Regulates Salt and Drought Stress Tolerance in Transgenic Nicotiana benthamiana

As a large family of regulatory proteins, WRKY transcription factors play essential roles in the processes of adaptation to diverse environmental stresses and plant growth and development. Although several studies have investigated the role of WRKY transcription factors during these processes, the mechanisms underlying the function of WRKY members need to be further explored, and research focusing on the WRKY family in cotton crops is extremely limited. In the present study, a gene encoding a putative WRKY family member, GhWRKY15, was isolated from cotton. GhWRKY15 is present as a single copy gene, and a transient expression analysis indicated that GhWRKY15 was localised to the nucleus. Additionally, a group of cis-acting elements associated with the response to environmental stress and plant growth and development were detected in the promoter. Consistently, northern blot analysis showed that GhWRKY15 expression was significantly induced in cotton seedlings following fungal infection or treatment with salicylic acid, methyl jasmonate or methyl viologen. Furthermore, GhWRKY15-overexpressing tobacco exhibited more resistance to viral and fungal infections compared with wild-type tobacco. The GhWRKY15-overexpressing tobacco also exhibited increased RNA expression of several pathogen-related genes, NONEXPRESSOR OF PR1, and two genes that encode enzymes involved in ET biosynthesis. Importantly, increased activity of the antioxidant enzymes POD and APX during infection and enhanced expression of NtAPX1 and NtGPX in transgenic tobacco following methyl viologen treatment were observed. Moreover, GhWRKY15 transcription was greater in the roots and stems compared with the expression in the cotyledon of cotton, and the stems of transgenic plants displayed faster elongation at the earlier shooting stages compared with wide type tobacco. Additionally, exposure to abiotic stresses, including cold, wounding and drought, resulted in the accumulation of GhWRKY15 transcripts. Overall, our data suggest that overexpression of GhWRKY15 may contribute to the alteration of defence resistance to both viral and fungal infections, probably through regulating the ROS system via multiple signalling pathways in tobacco. It is intriguing that GhWRKY15 overexpression in tobacco affects plant growth and development, especially stem elongation. This finding suggests that the role of the WRKY proteins in disease resistance may be closely related to their function in regulating plant growth and development.

https://link.springer.com/content/pdf/10.1186%2F1471-2229-12-144.pdf

GhWRKY15, a member of the WRKY transcription factor family identified from cotton (Gossypium hirsutum L.), is involved in disease resistance and plant development

Mitogen-activated protein kinase (MAPK) cascades play important roles in mediating biotic and abiotic stress responses. In plants, MAPKs are classified into four major groups (A–D) according to their sequence homology and conserved phosphorylation motifs. Compared with well-studied MAPKs in groups A and B, little is known about group C. In this study, we functionally characterised a stress-responsive group C MAPK gene (GhMPK2) from cotton (Gossypiumhirsutum). Northern blot analysis indicated that GhMPK2 was induced by abscisic acid (ABA) and abiotic stresses, such as NaCl, PEG, and dehydration. Subcellular localization analysis suggested that GhMPK2 may activate its specific targets in the nucleus. Constitutive overexpression of GhMPK2 in tobacco (Nicotianatabacum) conferred reduced sensitivity to ABA during both seed germination and vegetative growth. Interestingly, transgenic plants had a decreased rate of water loss and exhibited enhanced drought and salt tolerance. Additionally, transgenic plants showed improved osmotic adjustment capacity, elevated proline accumulation and up-regulated expression of several stress-related genes, including DIN1, Osmotin and NtLEA5. β-glucuronidase (GUS) expression driven by the GhMPK2 promoter was clearly enhanced by treatment with NaCl, PEG, and ABA. These results strongly suggest that GhMPK2 positively regulates salt and drought tolerance in transgenic plants.

A cotton group C MAP kinase gene, GhMPK2, positively regulates salt and drought tolerance in tobacco

MicroRNAs (miRNAs) processed from nuclear-encoded transcripts control expression of target transcripts by directing cleavage or translational inhibition. Artificial miRNAs (amiRNAs) that exploit this endogenous gene silencing mechanism can be designed to target any gene of interest and provide a highly specific approach for effective post-transcriptional gene silencing (PTGS) in plants. Here, using Arabidopsis thaliana miR159a, miR167b and miR171a precursors as backbones, we designed two types of amiRNA targeting sequence that encode the silencing suppressor HC-Pro of Potato virus Y (PVY) and the TGBp1/p25 (p25) of Potato virus X (PVX). The detected amiRNAs efficiently inhibited HC-Pro and p25 gene expression and conferred highly specific resistance against PVY or PVX infection in transgenic Nicotiana tabacum; this resistance was also maintained under conditions of increased viral pressure. Moreover, resistance was strongly influenced by the complementarity between the target sequence and amiRNA, and was well correlated to amiRNA expression level; the expression level of amiRNAs was also well related to the precursor backbones. We further showed that transgenic N. tabacum developed highly effective resistance to both PVY and PVX through expression of a dimeric amiRNA precursor. Together, our findings indicate that transgenic plants with multiple virus-specific resistance can be obtained through co-expression of several amiRNAs targeting multiple viruses.

Xingqi Guo

Papers

The Cotton WRKY Transcription Factor GhWRKY17 Functions in Drought and Salt Stress in Transgenic Nicotiana benthamiana Through ABA Signaling and the Modulation of Reactive Oxygen Species Production

The Cotton WRKY Gene GhWRKY41 Positively Regulates Salt and Drought Stress Tolerance in Transgenic Nicotiana benthamiana

GhWRKY15, a member of the WRKY transcription factor family identified from cotton (Gossypium hirsutum L.), is involved in disease resistance and plant development

A cotton group C MAP kinase gene, GhMPK2, positively regulates salt and drought tolerance in tobacco

Highly efficient virus resistance mediated by artificial microRNAs that target the suppressor of PVX and PVY in plants