Reactive oxygen species (ROS) play a key role in the acclimation process of plants to abiotic stress. They primarily function as signal transduction molecules that regulate different pathways during plant acclimation to stress, but are also toxic byproducts of stress metabolism. Because each subcellular compartment in plants contains its own set of ROS-producing and ROS-scavenging pathways, the steady-state level of ROS, as well as the redox state of each compartment, is different at any given time giving rise to a distinct signature of ROS levels at the different compartments of the cell. Here we review recent studies on the role of ROS in abiotic stress in plants, and propose that different abiotic stresses, such as drought, heat, salinity and high light, result in different ROS signatures that determine the specificity of the acclimation response and help tailor it to the exact stress the plant encounters. We further address the role of ROS in the acclimation of plants to stress combination as well as the role of ROS in mediating rapid systemic signaling during abiotic stress. We conclude that as long as cells maintain high enough energy reserves to detoxify ROS, ROS is beneficial to plants during abiotic stress enabling them to adjust their metabolism and mount a proper acclimation response.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/tpj.13299

Reactive oxygen species, abiotic stress and stress combination.

As fixed organisms, plants are especially affected by changes in their environment and have consequently evolved extensive mechanisms for acclimation and adaptation. Initially considered by-products from aerobic metabolism, reactive oxygen species (ROS) have emerged as major regulatory molecules in plants and their roles in early signaling events initiated by cellular metabolic perturbation and environmental stimuli are now established. Here, we review recent advances in ROS signaling. Compartment-specific and cross-compartmental signaling pathways initiated by the presence of ROS are discussed. Special attention is dedicated to established and hypothetical ROS-sensing events. The roles of ROS in long-distance signaling, immune responses, and plant development are evaluated. Finally, we outline the most challenging contemporary questions in the field of plant ROS biology and the need to further elucidate mechanisms allowing sensing, signaling specificity, and coordination of multiple signals.

Reactive Oxygen Species in Plant Signaling

Contents Summary 1197 I Introduction 1198 II Measurement and imaging of H2 O2 1198 III H2 O2 and O2 ·- toxicity 1199 IV Production of H2 O2 : enzymes and subcellular locations 1200 V H2 O2 transport 1205 VI Control of H2 O2 concentration: how and where? 1205 VII Metabolic functions of H2 O2 1207 VIII H2 O2 signalling 1207 IX Where next? 1209 Acknowledgements 1209 References 1209 SUMMARY: Hydrogen peroxide (H2 O2 ) is produced, via superoxide and superoxide dismutase, by electron transport in chloroplasts and mitochondria, plasma membrane NADPH oxidases, peroxisomal oxidases, type III peroxidases and other apoplastic oxidases Intracellular transport is facilitated by aquaporins and H2 O2 is removed by catalase, peroxiredoxin, glutathione peroxidase-like enzymes and ascorbate peroxidase, all of which have cell compartment-specific isoforms Apoplastic H2 O2 influences cell expansion, development and defence by its involvement in type III peroxidase-mediated polymer cross-linking, lignification and, possibly, cell expansion via H2 O2 -derived hydroxyl radicals Excess H2 O2 triggers chloroplast and peroxisome autophagy and programmed cell death The role of H2 O2 in signalling, for example during acclimation to stress and pathogen defence, has received much attention, but the signal transduction mechanisms are poorly defined H2 O2 oxidizes specific cysteine residues of target proteins to the sulfenic acid form and, similar to other organisms, this modification could initiate thiol-based redox relays and modify target enzymes, receptor kinases and transcription factors Quantification of the sources and sinks of H2 O2 is being improved by the spatial and temporal resolution of genetically encoded H2 O2 sensors, such as HyPer and roGFP2-Orp1 These H2 O2 sensors, combined with the detection of specific proteins modified by H2 O2 , will allow a deeper understanding of its signalling roles

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/nph.15488

Hydrogen peroxide metabolism and functions in plants.

The plant immune system is fundamental for plant survival in natural ecosystems and for productivity in crop fields. Substantial evidence supports the prevailing notion that plants possess a two-tiered innate immune system, called pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). PTI is triggered by microbial patterns via cell surface-localized pattern-recognition receptors (PRRs), whereas ETI is activated by pathogen effector proteins via predominantly intracellularly localized receptors called nucleotide-binding, leucine-rich repeat receptors (NLRs)1–4. PTI and ETI are initiated by distinct activation mechanisms and involve different early signalling cascades5,6. Here we show that Arabidopsis PRR and PRR co-receptor mutants—fls2 efr cerk1 and bak1 bkk1 cerk1 triple mutants—are markedly impaired in ETI responses when challenged with incompatible Pseudomonas syrinage bacteria. We further show that the production of reactive oxygen species by the NADPH oxidase RBOHD is a critical early signalling event connecting PRR- and NLR-mediated immunity, and that the receptor-like cytoplasmic kinase BIK1 is necessary for full activation of RBOHD, gene expression and bacterial resistance during ETI. Moreover, NLR signalling rapidly augments the transcript and/or protein levels of key PTI components. Our study supports a revised model in which potentiation of PTI is an indispensable component of ETI during bacterial infection. This revised model conceptually unites two major immune signalling cascades in plants and mechanistically explains some of the long-observed similarities in downstream defence outputs between PTI and ETI. Bacteria elicit two distinct immune responses in Arabidopsis thaliana, mediated by diverse signalling receptors but working in a synergistic manner.

Pattern-recognition receptors are required for NLR-mediated plant immunity

Stomata, the pores formed by a pair of guard cells, are the main gateways for water transpiration and photosynthetic CO2 exchange, as well as pathogen invasion in land plants. Guard cell movement is regulated by a combination of environmental factors, including water status, light, CO2 levels and pathogen attack, as well as endogenous signals, such as abscisic acid and apoplastic reactive oxygen species (ROS). Under abiotic and biotic stress conditions, extracellular ROS are mainly produced by plasma membrane-localized NADPH oxidases, whereas intracellular ROS are produced in multiple organelles. These ROS form a sophisticated cellular signaling network, with the accumulation of apoplastic ROS an early hallmark of stomatal movement. Here, we review recent progress in understanding the molecular mechanisms of the ROS signaling network, primarily during drought stress and pathogen attack. We summarize the roles of apoplastic ROS in regulating stomatal movement, ABA and CO2 signaling, and immunity responses. Finally, we discuss ROS accumulation and communication between organelles and cells. This information provides a conceptual framework for understanding how ROS signaling is integrated with various signaling pathways during plant responses to abiotic and biotic stress stimuli.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/jipb.12654

Reactive oxygen species signaling and stomatal movement in plant responses to drought stress and pathogen attack

Hydrogen peroxide (H2O2) is a stable component of reactive oxygen species, and its production in plants represents the successful recognition of pathogen infection and pathogen-associated molecular patterns (PAMPs). This production of H2O2 is typically apoplastic but is subsequently associated with intracellular immunity pathways that regulate disease resistance, such as systemic acquired resistance and PAMP-triggered immunity. Here, we elucidate that an Arabidopsis (Arabidopsis thaliana) aquaporin (i.e. the plasma membrane intrinsic protein AtPIP1;4) acts to close the cytological distance between H2O2 production and functional performance. Expression of the AtPIP1;4 gene in plant leaves is inducible by a bacterial pathogen, and the expression accompanies H2O2 accumulation in the cytoplasm. Under de novo expression conditions, AtPIP1;4 is able to mediate the translocation of externally applied H2O2 into the cytoplasm of yeast (Saccharomyces cerevisiae) cells. In plant cells treated with H2O2, AtPIP1;4 functions as an effective facilitator of H2O2 transport across plasma membranes and mediates the translocation of externally applied H2O2 from the apoplast to the cytoplasm. The H2O2-transport role of AtPIP1;4 is essentially required for the cytoplasmic import of apoplastic H2O2 induced by the bacterial pathogen and two typical PAMPs in the absence of induced production of intracellular H2O2 As a consequence, cytoplasmic H2O2 quantities increase substantially while systemic acquired resistance and PAMP-triggered immunity are activated to repress the bacterial pathogenicity. By contrast, loss-of-function mutation at the AtPIP1;4 gene locus not only nullifies the cytoplasmic import of pathogen- and PAMP-induced apoplastic H2O2 but also cancels the subsequent immune responses, suggesting a pivotal role of AtPIP1;4 in apocytoplastic signal transduction in immunity pathways.

/pdf/plant-aquaporin-atpip1-4-links-apoplastic-h2o2-induction-to-265yzn1f9z.pdf

Plant Aquaporin AtPIP1;4 Links Apoplastic H2O2 Induction to Disease Immunity Pathways

Varieties of Gram-negative bacterial pathogens infect their eukaryotic hosts by deploying the type III translocon to deliver effector proteins into the cytosol of eukaryotic cells in which effectors execute their pathological functions. The translocon is hypothetically assembled by bacterial translocators in association with the assumed receptors situated on eukaryotic plasma membranes. This hypothesis is partially verified in the present study with genetic, biochemical, and pathological evidence for the role of a rice aquaporin, plasma membrane intrinsic protein PIP1;3, in the cytosolic import of the transcription activator-like effector PthXo1 from the bacterial blight pathogen. PIP1;3 interacts with the bacterial translocator Hpa1 at rice plasma membranes to control PthXo1 translocation from cells of a well-characterized strain of the bacterial blight pathogen into the cytosol of cells of a susceptible rice variety. An extracellular loop sequence of PIP1;3 and the α-helix motif of Hpa1 determine both the molecular interaction and its consequences with respect to the effector translocation and the bacterial virulence on the susceptible rice variety. Overall, these results provide multiple experimental avenues to support the hypothesis that interactions between bacterial translocators and their interactors at the target membrane are essential for bacterial effector translocation.

/pdf/rice-aquaporin-pip1-3-and-harpin-hpa1-of-bacterial-blight-kvkab0qej8.pdf

Rice aquaporin PIP1;3 and harpin Hpa1 of bacterial blight pathogen cooperate in a type III effector translocation.

Pathogenic Gram-negative bacteria interact with their eukaryotic hosts by deploying the type III translocon to inject effector proteins into the cytosol of eukaryotic cells The translocon compositions, the number and biochemical characteristics of type III translocators in animal-pathogenic bacteria have been well elucidated, but information is lacking for plant-pathogenic bacteria With extensive studies on biological functions of the Hpa1 protein secreted by the type III secretion system in Xanthomonas oryzae pv oryzae (Xoo), we show here that Hpa1 is a type III translocator based on measurements of two proteins categorized as transcription activator-like (TAL) effector Hpa1 was functionally associated with the TAL effector PthXo1 or AvrXa10 by genetic analysis of the wild-type Xoo strain and related mutants or recombinant strains Inoculation experiments suggested that Hpa1 is required not only for the virulent role of PthXo1 in the susceptible rice variety Nipponbare, but also for the avirulent function of AvrXa10 on the resistant rice variety IRBB10 Hpa1 is unrelated to the secretion of PthXo1 and AvrXa10 out of bacterial cells However, Hpa1 is critical for both TAL effectors to be translocated from bacterial cells into the cytosol of rice cells based on replicate experiments performed on the susceptible and resistant varieties, respectively Hpa1-mediated translocation of PthXo1 is coincident with induced expression of rice SWEET11 gene, which is the regulatory target of PthXo1, resulting in the occurrence of the bacterial blight disease in the susceptible rice variety By contrast, the immune hypersensitive response is induced in agreement with induced expression of rice Xa10 gene, which is the target of AvrXa10, only when AvrXa10 is translocated from bacteria into cells of the resistant rice variety All the virulent or avirulent performances of the TAL effectors are nullified by directed mutation that removes the α-helix motif from the Hpa1 sequence The genetic and biochemical data demonstrate that Hap1 is a type III translocator at least for TAL effectors PthXo1 and AvrXa10 The effect of the directed mutation suggests that Hpa1 depends on its α-helical motif to fulfil the translocator function

/pdf/hpa1-is-a-type-iii-translocator-in-xanthomonas-oryzae-pv-4xenksu5sx.pdf

Hpa1 is a type III translocator in Xanthomonas oryzae pv. oryzae.

The translocation of type III effectors of plant-pathogenic bacteria is required both for the expression of their role in virulence, and for the triggering of plant defenses. The effector translocation needs molecular associations between bacterial translocators and their recognition compounds situated in plant plasma membranes. We show here that rice aquaporin PIP1;3, a member of the plasma membrane intrinsic protein family, is implicated in translocation of a transcription activator-like (TAL) effector from the bacterial blight pathogen into the cytosol of rice cells. The TAL protein PthXo1 targets rice disease-susceptible gene SWEET11 to determine virulence of the bacterial strain PXO99A on susceptible rice variety Nipponbare. In Nipponbare, post-transcriptional gene silencing of PIP1;3 results in highly alleviated susceptibility and concomitantly decreased expression of SWEET11 as compared to levels of the bacterial virulence and the plant SWEET11 expression in the wild-type plant with normal transcription of PIP1;3. In coincidence, the efficiency in PthXo1 translocation is substantially reduced in PIP1;3-silenced rice lines in contrast to the wild-type plant. In the case of PIP1;3 silencing, moreover, the bacterial type III translocator Hpa1 loses the ability to mediate PthXo1 translocation from the bacterial cells into the cytosol of rice cells. By contrast, PIP1;3 silencing does not affect the performance of isolated Hpa1 as a pattern molecule to induce immune responses in rice. Our data suggest that rice aquaporin PIP1;3 is a candidate of disease-susceptible factor with functional relevance to PthXo1 translocation as a prerequisite of the bacterial virulence on the susceptible rice variety.

Silencing of an aquaporin gene diminishes bacterial blight disease in rice

Many species of plant-pathogenic gram-negative bacteria deploy the type III (T3) secretion system to secrete virulence components, which are mostly characteristic of protein effectors targeting the cytosol of the plant cell following secretion. Xanthomonas oryzae pv. oryzae (Xoo), a rice pathogen causing bacterial blight disease, uses the T3 accessory protein HrpE to assemble the pilus pathway, which in turn secretes transcription activator-like (TAL) effectors. The hrpE gene can execute extensive physiological and pathological functions beyond effector secretion. As evidenced in this study, when the hrpE gene was deleted from the Xoo genome, the bacteria incur seriouimpairments in multiplication, motility, and virulence. The virulence nullification is attributed to reduced secretion and translocation of PthXo1, which is a TAL effector that determines the bacterial virulence in the susceptible rice varieties. When the HrpE protein produced by prokaryotic expression is applied to plants, the recombinant protein is highly effective at inducing the defense response. Moreover, leaf photosynthesis efficiency is enhanced in HrpE-treated plants. These results provide experimental avenues to modulate the plant defense and growth tradeoff by manipulating a bacterial T3 accessory protein.

Ping Li

Papers

Plant Aquaporin AtPIP1;4 Links Apoplastic H2O2 Induction to Disease Immunity Pathways

Rice aquaporin PIP1;3 and harpin Hpa1 of bacterial blight pathogen cooperate in a type III effector translocation.

Hpa1 is a type III translocator in Xanthomonas oryzae pv. oryzae.

Silencing of an aquaporin gene diminishes bacterial blight disease in rice

The Type III Accessory Protein HrpE of Xanthomonas oryzae pv. oryzae Surpasses the Secretion Role, and Enhances Plant Resistance and Photosynthesis