Nitric oxide and hydrogen sulfide: an indispensable combination for plant functioning.

During crop cultivation, water-deficit conditions retard growth, thus reducing crop productivity. Therefore, uncovering the mechanisms behind drought tolerance is a critical task for crop improvement. Here, we show that the rice (Oryza sativa) WRKY transcription factor OsWRKY5 negatively regulates drought tolerance. We determined that OsWRKY5 was mainly expressed in developing leaves at the seedling and heading stages, and that its expression was reduced by drought stress and by treatment with NaCl, mannitol, and abscisic acid (ABA). Notably, the genome-edited loss-of-function alleles oswrky5-2 and oswrky5-3 conferred enhanced drought tolerance, measured as plant growth under water-deficit conditions. Conversely, the overexpression of OsWRKY5 in the activation-tagged line oswrky5-D resulted in higher susceptibility under the same conditions. Loss of OsWRKY5 activity increased sensitivity to ABA, thus promoting ABA-dependent stomatal closure. Transcriptome deep sequencing and RT-qPCR analyses demonstrated that the expression of abiotic stress-related genes including rice MYB2 (OsMYB2) was upregulated in oswrky5 knockout mutants and downregulated in oswrky5-D mutants. Moreover, dual-luciferase, yeast one-hybrid, and chromatin immunoprecipitation assays showed that OsWRKY5 directly binds to the W-box sequences in the promoter region of OsMYB2 and represses OsMYB2 expression, thus downregulating genes downstream of OsMYB2 in the ABA signaling pathways. Our results demonstrate that OsWRKY5 functions as a negative regulator of ABA-induced drought stress tolerance, strongly suggesting that inactivation of OsWRKY5 or manipulation of key OsWRKY5 targets could be useful to improve drought tolerance in rice cultivars.

Inactivating transcription factor OsWRKY5 enhances drought tolerance through abscisic acid signaling pathways.

Global climate change and associated adverse abiotic and biotic stress conditions affect plant growth and development, and agricultural sustainability in general. Abiotic and biotic stresses reduce respiration and associated energy generation in mitochondria, resulting in the elevated production of reactive oxygen species (ROS), which are employed to transmit cellular signaling information in response to the changing conditions. Excessive ROS accumulation can contribute to cell damage and death. Production of the non-protein amino acid γ-aminobutyrate (GABA) is also stimulated, resulting in partial restoration of respiratory processes and energy production. Accumulated GABA can bind directly to the aluminum-activated malate transporter and the guard cell outward rectifying K+ channel, thereby improving drought and hypoxia tolerance, respectively. Genetic manipulation of GABA metabolism and receptors, respectively, reveal positive relationships between GABA levels and abiotic/biotic stress tolerance, and between malate efflux from the root and heavy metal tolerance. The application of exogenous GABA is associated with lower ROS levels, enhanced membrane stability, changes in the levels of non-enzymatic and enzymatic antioxidants, and crosstalk among phytohormones. Exogenous GABA may be an effective and sustainable tolerance strategy against multiple stresses under field conditions.

γ-Aminobutyrate (GABA) Regulated Plant Defense: Mechanisms and Opportunities

Photosynthesis is a universal process for plant survival, and immune defense is also a key process in adapting to the growth environment. Various studies have indicated that these two processes are interconnected in a complex network. Photosynthesis can influence signaling pathways and provide both materials and energy for immune defense, while the immune defense process can also have feedback effects on photosynthesis. Pathogen infection inevitably leads to changes in photosynthesis parameters, including Pn, Gs, and Ci; biochemical materials such as SOD and CAT; signaling molecules such as H2O2 and hormones; and the expression of genes involved in photosynthesis. Some researchers have found that changes in photosynthesis activity are related to the resistance level of the host, the duration after infection, and the infection position (photosynthetic source or sink). Interactions between wheat and the main fungal pathogens, such as Puccinia striiformis, Blumeria graminis, and Fusarium graminearum, constitute an ideal study system to elucidate the relationship between changes in host photosynthesis and resistance levels, based on the accessibility of methods for artificially controlling infection and detecting changes in photosynthesis, the presence of multiple pathogens infecting different positions, and the abundance of host materials with various resistance levels. This review is written only from the perspective of plant pathologists, and after providing an overview of the available data, we generally found that changes in photosynthesis in the early stage of pathogen infection could be a causal factor influencing acquired resistance, while those in the late stage could be the result of resistance formation.

Changes in Photosynthesis Could Provide Important Insight into the Interaction between Wheat and Fungal Pathogens.

In environmental conditions, plants are affected by abiotic and biotic stressors which can be heterogenous. This means that the systemic plant adaptive responses on their actions require long-distance stress signals including electrical signals (ESs). ESs are based on transient changes in the activities of ion channels and H+-ATP-ase in the plasma membrane. They influence numerous physiological processes, including gene expression, phytohormone synthesis, photosynthesis, respiration, phloem mass flow, ATP content, and many others. It is considered that these changes increase plant tolerance to the action of stressors; the effect can be related to stimulation of damages of specific molecular structures. In this review, we hypothesize that programmed cell death (PCD) in plant cells can be interconnected with ESs. There are the following points supporting this hypothesis. (i) Propagation of ESs can be related to ROS waves; these waves are a probable mechanism of PCD initiation. (ii) ESs induce the inactivation of photosynthetic dark reactions and activation of respiration. Both responses can also produce ROS and, probably, induce PCD. (iii) ESs stimulate the synthesis of stress phytohormones (e.g., jasmonic acid, salicylic acid, and ethylene) which are known to contribute to the induction of PCD. (iv) Generation of ESs accompanies K+ efflux from the cytoplasm that is also a mechanism of induction of PCD. Our review argues for the possibility of PCD induction by electrical signals and shows some directions of future investigations in the field.

Electrical Signals, Plant Tolerance to Actions of Stressors, and Programmed Cell Death: Is Interaction Possible?

Abscisic acid (ABA) is a stress hormone that accumulates under different abiotic and biotic stresses. A typical effect of ABA on leaves is to reduce transpirational water loss by closing stomata and parallelly defend against microbes by restricting their entry through stomatal pores. ABA can also promote the accumulation of polyamines, sphingolipids, and even proline. Stomatal closure by compounds other than ABA also helps plant defense against both abiotic and biotic stress factors. Further, ABA can interact with other hormones, such as methyl jasmonate (MJ) and salicylic acid (SA). Such cross-talk can be an additional factor in plant adaptations against environmental stresses and microbial pathogens. The present review highlights the recent progress in understanding ABA's multifaceted role under stress conditions, particularly stomatal closure. We point out the importance of reactive oxygen species (ROS), reactive carbonyl species (RCS), nitric oxide (NO), and Ca2+ in guard cells as key signaling components during the ABA-mediated short-term plant defense reactions. The rise in ROS, RCS, NO, and intracellular Ca2+ triggered by ABA can promote additional events involved in long-term adaptive measures, including gene expression, accumulation of compatible solutes to protect the cell, hypersensitive response (HR), and programmed cell death (PCD). Several pathogens can counteract and try to reopen stomata. Similarly, pathogens attempt to trigger PCD of host tissue to their benefit. Yet, ABA-induced effects independent of stomatal closure can delay the pathogen spread and infection within leaves. Stomatal closure and other ABA influences can be among the early steps of defense and a crucial component of plants' innate immunity response. Stomatal guard cells are quite sensitive to environmental stress and are considered good model systems for signal transduction studies. Further research on the ABA-induced stomatal closure mechanism can help us design strategies for plant/crop adaptations to stress.

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Abscisic Acid-Induced Stomatal Closure: An Important Component of Plant Defense Against Abiotic and Biotic Stress.

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Stomatal Closure Sets in Motion Long-Term Strategies of Plant Defense Against Microbial Pathogens

Protein phosphatases (PPs) are indispensable for growth, development, and various signaling pathways in plants. Opening or closure of stomata, the tiny pores on leaf surface, is essential for optimizing plant performance, under abiotic stress. PPs are important links in the signal transduction events to connect signal perception with subsequent steps of ion efflux/influx leading to the decrease/increase in turgor of the guard cells. Of the diverse spectrum of PPs in plants, only a few are involved in stomatal movements, particularly protein phosphatase 2C (PP2C), protein phosphatase 2A (PP2A), protein phosphatase 1 (PP1), and protein phosphatase 7 (PP7). PP2C plays a pivotal role in abscisic acid (ABA)-induced stomatal closure as it forms a ternary complex with ABA and receptors, switching on the Open STomata 1 (OST1) kinase by phosphorylation, which in turn triggers events leading to ion efflux. On the other hand PP1 participates during stomatal opening by blue light perceived by phototropins (Phots) or cryptochromes (Crys). On sensing the blue light, stomatal phototropin can modulate PP1 by a blue light-activated kinase. As a consequence, H+-ATPase is activated, followed by membrane hyperpolarization leading to K+ influx, increased turgor, and opening. The tripartite interaction of ABA, ABA receptors, and PP2C provides the missing link to complete the chain of perception of ABA and its regulatory effects. The presence of ABA is perceived by receptors PYLs and stabilized by complex with PP2C. This tripartite interaction activates OST1 kinase and in turn phosphorylates NADPH oxidase to produce ROS and initiate events of closure. Arabidopsis mutants were instrumental in demonstrating the roles of PPs in signal transduction of stomatal guard cells. This chapter concludes with remarks on existing gaps in our knowledge and suggestions for further studies.

Protein Phosphatases in Guard Cells: Key Role in Stomatal Closure and Opening

Moderate modulation by S-nitrosoglutathione of photorespiratory enzymes in pea (Pisum sativum) leaves, compared to the strong effects of high light.

The reported effects of nitric oxide (NO), a signaling molecule, on the photochemical components of leaves are ambiguous. We examined the changes by a natural NO donor, S-nitrosoglutathione (GSNO). The effect of GSNO on Pisum sativum leaves was studied after a 3-hour exposure in dark, moderate (ML), or high light (HL). The NO levels in GSNO-treated samples were at their maximum under HL, compared to those under ML or dark. Most of the elevated NO was decreased by 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), a NO scavenger, confirming the NO increase. Treatment with GSNO caused inhibition of photosynthesis/respiration and restricted electron transport mediated by both photosystem (PS)II and PSI. However, the inhibition by NO-donor of PSII components was stronger than those of PSI. A marked increase in the PSI acceptor side limitation [Y(NA)] and a decrease in PSI donor side limitation [Y(ND)] indicated an upregulation of cyclic electron transport, possibly to balance the damage to PSII by GSNO. We suggest that NO aggravated the HL-induced inhibition of photosynthesis and dark respiration.

Modulation by S-nitrosoglutathione (a natural nitric oxide donor) of photosystem in Pisum sativum leaves, as revealed by chlorophyll fluorescence: Light-dependent aggravation of nitric oxide effects

Pulimamidi Bharath

Papers

Abscisic Acid-Induced Stomatal Closure: An Important Component of Plant Defense Against Abiotic and Biotic Stress.

Stomatal Closure Sets in Motion Long-Term Strategies of Plant Defense Against Microbial Pathogens

Protein Phosphatases in Guard Cells: Key Role in Stomatal Closure and Opening

Moderate modulation by S-nitrosoglutathione of photorespiratory enzymes in pea (Pisum sativum) leaves, compared to the strong effects of high light.

Modulation by S-nitrosoglutathione (a natural nitric oxide donor) of photosystem in Pisum sativum leaves, as revealed by chlorophyll fluorescence: Light-dependent aggravation of nitric oxide effects