The receptor-like cytoplasmic kinase RIPK regulates broad-spectrum ROS signaling in multiple layers of plant immune system

Overexpression of an Antioxidant Enzyme APX1 in cpr5 Mutant Restores its Pleiotropic Growth Phenotype

Breeding crops with enhanced immunity is an effective strategy to reduce yield loss caused by pathogens. The constitutive expresser of pathogenesis-related genes (cpr5) mutant shows enhanced pathogen resistance but retarded growth; thus, it restricts the application of cpr5 in breeding crops with disease resistance. Reactive oxygen species (ROS) play important roles in plant growth and defense. In this study, we determined that the cpr5 mutant exhibited excessive ROS accumulation. However, the mutation of respiratory burst oxidase homolog D (RBOHD), a reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase responsible for the production of ROS signaling in plant immunity, did not suppress excessive ROS levels in cpr5. Furthermore, the cpr5 mutant showed low levels of ascorbate peroxidase 1 (APX1), an important cytosolic ROS-scavenging enzyme. APX1 overexpression in the cpr5 background removed excessive ROS and restored the pleiotropic growth phenotype. Notably, APX1 overexpression did not reduce the resistance of cpr5 mutant to virulent strain Pseudomonas syringae pv. tomato (Pst) DC3000 and avirulent strain Pst DC3000 (avrRpt2). These results suggest that the removal of excessive ROS by APX1 overexpression restored the cpr5 growth phenotype while conserving pathogen resistance. Hence, our study provides a theoretical and empirical basis for utilizing CPR5 in the breeding of crops with disease resistance by effective oxidative stress management via APX1 expression.

Tomato receptor-like cytoplasmic kinase Fir1 interacts with a negative regulator of jasmonic acid signaling

Plant cells detect potential pathogens through plasma membrane-localized pattern recognition receptors (PRRs) that recognize microbe-associated molecular patterns (MAMPs) and activate pattern-triggered immunity (PTI). PRR-mediated MAMP perception is linked to PTI signaling by receptor-like cytoplasmic kinases (RLCKs). In tomato, Flagellin-sensing 2 (Fls2)/Fls3 interacting RLCK 1 (Fir1) is involved in PTI triggered by flagellin perception. Fir1 is necessary for regulation of jasmonic acid (JA) signaling and is involved in pre-invasion immunity. We show that Fir1 physically interacts with JASMONATE-ZIM-DOMAIN PROTEIN 3 (JAZ3), a negative regulator of JA signaling. This finding suggests that Fir1 modulates JA signaling by regulating JAZ3.

Development specifies, diversifies and empowers root immunity

Roots are a highly organised plant tissue consisting of different cell types with distinct developmental functions defined by cell identity networks. Roots are the target of some of the most devastating diseases and possess a highly effective immune system. The recognition of microbe- or plant-derived molecules released in response to microbial attack is highly important in the activation of complex immunity gene networks. Development and immunity are intertwined, and immunity activation can result in growth inhibition. In turn, by connecting immunity and cell identity regulators, cell types are able to launch a cell type-specific immunity based on the developmental function of each cell type. By this strategy, fundamental developmental processes of each cell type contribute their most basic functions to drive cost-effective but highly diverse and, thus, efficient immune responses. This review highlights the interdependence of root development and immunity and how the developmental age of root cells contributes to positive and negative outcomes of development-immunity cross-talk.

ZmGLP1, a Germin-like Protein from Maize, Plays an Important Role in the Regulation of Pathogen Resistance

A gene encoding a protein similar to germin-like proteins (GLPs) was obtained from maize (Zea mays) and designated as ZmGLP1. Based on the ZmGLP1 conserved domain and phylogenetic status, ZmGLP1 was grouped into GLP subfamily b and has high similarity to OsGLP8-14 from Oryza sativa. ZmGLP1 is expressed in different maize tissues during different growth stages and is mainly expressed in the stems and leaves. The induced expression patterns confirmed that ZmGLP1 is differentially expressed under abiotic and hormone stress; it had an early response to jasmonic acid (JA) and ethephon (ET) but a late response to salicylic acid (SA) and was significantly upregulated under Bipolaris maydis infection. The overexpression of ZmGLP1 in Arabidopsis improved the resistance to biotrophic Pseudomonas syringae pv. tomato DC3000 (PstDC3000) and necrotrophic Sclerotinia sclerotiorum by inducing the expression of JA signaling-related genes. Moreover, the hydrogen peroxide (H₂O₂) content increased due to the overexpression of ZmGLP1 in Arabidopsis after pathogen infection. Compared to the wild-type control, the H₂O₂ content of ZmGLP1-overexpressing Arabidopsis infected by PstDC3000 increased significantly but was lower in transgenic plants infected with S. sclerotiorum. Furthermore, high-performance liquid chromatography-tandem mass (HPLC-MS/MS) spectrometry showed that the JA contents of ZmGLP1-overexpressing Arabidopsis markedly increased after pathogen infection. However, the improved resistance of ZmGLP1-overexpressing Arabidopsis pretreated with the JA biosynthetic inhibitor, sodium diethyldithiocarbamate trihydrate (DIECA), was suppressed. Based on these findings, we speculate that ZmGLP1 plays an important role in the regulation of Arabidopsis resistance to biotrophic PstDC3000 and necrotrophic S. sclerotiorum; the regulatory effects are achieved by inducing plant oxidative burst activity and activation of the JA signaling pathway.

Regulation of PaRBOH1-mediated ROS production in Norway spruce by Ca2+ binding and phosphorylation

Plant respiratory burst oxidase homologs (RBOHs) are plasma membrane-localized NADPH oxidases that generate superoxide anion radicals, which then dismutate to H₂O₂, into the apoplast using cytoplasmic NADPH as an electron donor. PaRBOH1 is the most highly expressed RBOH gene in developing xylem as well as in a lignin-forming cell culture of Norway spruce (Picea abies L. Karst.). Since no previous information about regulation of gymnosperm RBOHs exist, our aim was to resolve how PaRBOH1 is regulated with a focus on phosphorylation. The N-terminal part of PaRBOH1 was found to contain several putative phosphorylation sites and a four-times repeated motif with similarities to the Botrytis-induced kinase 1 target site in Arabidopsis AtRBOHD. Phosphorylation was indicated for six of the sites in in vitro kinase assays using 15 amino-acid-long peptides for each of the predicted phosphotarget site in the presence of protein extracts of developing xylem. Serine and threonine residues showing positive response in the peptide assays were individually mutated to alanine (kinase-inactive) or to aspartate (phosphomimic), and the wild type PaRBOH1 and the mutated constructs transfected to human kidney embryogenic (HEK293T) cells with a low endogenous level of extracellular ROS production. ROS-producing assays with HEK cells showed that Ca²⁺ and phosphorylation synergistically activate the enzyme and identified several serine and threonine residues that are likely to be phosphorylated including a novel phosphorylation site not characterized in other plant species. These were further investigated with a phosphoproteomic study. Results of Norway spruce, the first gymnosperm species studied in relation to RBOH regulation, show that regulation of RBOH activity is conserved among seed plants.

Interactions of melatonin, reactive oxygen species, and nitric oxide during fruit ripening: an update and prospective view

Fruit ripening is a physiological process that involves a complex network of signaling molecules that act as switches to activate or deactivate certain metabolic pathways at different levels, not only by regulating gene and protein expression but also through post-translational modifications of the involved proteins. Ethylene is the distinctive molecule that regulates the ripening of fruits, which can be classified as climacteric or non-climacteric according to whether or not, respectively, they are dependent on this phytohormone. However, in recent years it has been found that other molecules with signaling potential also exert regulatory roles, not only individually but also as a result of interactions among them. These observations imply the existence of mutual and hierarchical regulations that sometimes make it difficult to identify the initial triggering event. Among these 'new' molecules, hydrogen peroxide, nitric oxide, and melatonin have been highlighted as prominent. This review provides a comprehensive outline of the relevance of these molecules in the fruit ripening process and the complex network of the known interactions among them.

Melatonin as a regulator of plant ionic homeostasis: implications for abiotic stress tolerance

Melatonin is a highly conserved and ubiquitous molecule that operates upstream of a broad array of receptors in animal systems. Since melatonin was discovered in plants in 1995, hundreds of papers have been published revealing its role in plant growth, development, and adaptive responses to the environment. This paper summarizes the current state of knowledge of melatonin's involvement in regulating plant ion homeostasis and abiotic stress tolerance. The major topics covered here are: (i) melatonin's control of H+-ATPase activity and its implication for plant adaptive responses to various abiotic stresses; (ii) regulation of the reactive oxygen species (ROS)-Ca2+ hub by melatonin and its role in stress signaling; and (iii) melatonin's regulation of ionic homeostasis via hormonal cross-talk. We also show that the properties of the melatonin molecule allow its direct scavenging of ROS, thus preventing negative effects of ROS-induced activation of ion channels. The above 'desensitization' may play a critical role in preventing stress-induced K+ loss from the cytosol as well as maintaining basic levels of cytosolic Ca2+ required for optimal cell operation. Future studies should focus on revealing the molecular identity of transporters that could be directly regulated by melatonin and providing a bioinformatic analysis of evolutionary aspects of melatonin sensing and signaling.

Concerted actions of PRR- and NLR-mediated immunity

Plants utilise cell-surface immune receptors (functioning as pattern recognition receptors, PRRs) and intracellular nucleotide-binding leucine-rich repeat receptors (NLRs) to detect pathogens. Perception of pathogens by these receptors activates immune signalling and resistance to infections. PRR- and NLR-mediated immunity have primarily been considered parallel processes contributing to disease resistance. Recent studies suggest that these two pathways are interdependent and converge at multiple nodes. This review summarises and provides a perspective on these convergent points.

Tomato receptor-like cytosolic kinase RIPK confers broad-spectrum disease resistance without yield penalties

Production of reactive oxygen species (ROS) is an important immune response in plant multilayer defense mechanisms; however, direct modification of ROS homeostasis to breed plants with broad-spectrum resistance to disease has not yet been successful. In Arabidopsis, the receptor-like cytosolic kinase AtRIPK regulates broad-spectrum ROS signaling in multiple layers of the plant immune system. Upon treatment with immune elicitors, AtRIPK is activated and phosphorylates nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, which leads to ROS production. In this study, we identified an AtRIPK ortholog in tomatoes and generated knockdown mutants using CRISPR/Cas9 technology. Slripk mutants displayed reduced ROS production in response to representative immune elicitors and were susceptible to pathogenic bacteria and fungi from different genera, including Ralstonia solanacearum, Pectobacterium carotovorum, Botrytis cinerea, and Fusarium oxysporum, which are leaf and root pathogens with hemibiotrophic and necrotrophic infection strategies. In contrast, transgenic tomato plants overexpressing SlRIPK are more resistant to these pathogens. Remarkably, the slripk mutants and SlRIPK-overexpressing transgenic plants did not exhibit significant growth retardation or yield loss. These results suggest that overexpression of SlRIPK confers broad-spectrum disease resistance without a yield penalty in tomato plants. Our findings suggest that modifying ROS homeostasis by altering the regulatory components of ROS production in plant immunity could contribute to engineering or breeding broad-spectrum disease-resistant crops without yield penalty.

MxMPK4-1 phosphorylates NADPH oxidase to trigger the MxMPK6-2-MxbHLH104 pathway mediated Fe deficiency responses in apple

Iron (Fe) deficiency is a nutritional stress in plants that commonly occurs in alkaline and calcareous soils. Mitogen-activated protein kinases (MPKs), the terminal player of MAPK cascade, are involved in distinct physiological processes. Once plants suffer from Fe deficiency stress, the mechanism of MPK function remains unclear owing to limited study on the MPK networks including substrate proteins and downstream pathways. Here, the MAP kinase MPK4-1 was induced in roots of Fe efficient apple rootstock Malus xiaojinensis but not in Fe inefficient rootstock Malus baccata under Fe deficiency conditions. Overexpression of MxMPK4-1 in apple calli and apple roots enhanced the responses to Fe deficiency. We found that MxMPK4-1 interacted with NADPH oxidases (NOX)-respiratory burst oxidase homologs MxRBOHD1 and MxRBOHD2, which positively regulated responses to Fe deficiency. Moreover, MxMPK4-1 phosphorylated the C terminus of MxRBOHD2 at Ser797 and Ser906 and positively and negatively regulated NOX activity through these phospho-sites, respectively. When compared with apple calli that overexpressed MxRBOHD2, the coexpression of MxMPK4-1 and MxRBOHD2 prominently enhanced the Fe deficiency responses. We also demonstrated that hydrogen peroxide derived from MxMPK4-1-MxRBOHD2 regulated the MxMPK6-2-MxbHLH104 pathway, illuminating a systematic network that involves different MPK proteins in M. xiaojinensis under Fe deficiency stress.

De Novo Purine Nucleotide Biosynthesis Pathway Is Required for Development and Pathogenicity in Magnaporthe oryzae

Purine nucleotides are indispensable compounds for many organisms and participate in basic vital activities such as heredity, development, and growth. Blocking of purine nucleotide biosynthesis may inhibit proliferation and development and is commonly used in cancer therapy. However, the function of the purine nucleotide biosynthesis pathway in the pathogenic fungus Magnaporthe oryzae is not clear. In this study, we focused on the de novo purine biosynthesis (DNPB) pathway and characterized MoAde8, a phosphoribosylglycinamide formyltransferase, catalyzing the third step of the DNPB pathway in M. oryzae. MoAde8 was knocked out, and the mutant (∆Moade8) exhibited purine auxotroph, defects in aerial hyphal growth, conidiation, and pathogenicity, and was more sensitive to hyperosmotic stress and oxidative stress. Moreover, ∆Moade8 caused decreased activity of MoTor kinase due to blocked purine nucleotide synthesis. The autophagy level was also impaired in ∆Moade8. Additionally, MoAde5, 7, 6, and 12, which are involved in de novo purine nucleotide biosynthesis, were also analyzed, and the mutants showed defects similar to the defects of ∆Moade8. In summary, de novo purine nucleotide biosynthesis is essential for conidiation, development, and pathogenicity in M. oryzae.

Essential Acidovorax citrulli Virulence Gene hrpE Activates Host Immune Response against Pathogen

Bacterial fruit blotch (BFB) caused by Acidovorax citrulli (Ac) is a devastating watermelon disease that severely impacts the global watermelon industry. Like other Gram-negative bacteria, the type three secretion system (T3SS) is the main pathogenicity factor of A. citrulli. The T3SS apparatus gene hrpE codes for the Hrp pilus and serves as a conduit to secret effector proteins into host cells. In this study, we found that the deletion of hrpE in A. citrulli results in the loss of pathogenicity on hosts and the hypersensitive response on non-hosts. In addition, the A. citrulli hrpE mutant showed a reduction in in vitro growth, in planta colonization, swimming and twitching motility, and displayed increases in biofilm formation ability compared to the wild type. However, when HrpE was transiently expressed in hosts, the defense responses, including reactive oxygen species bursts, callose deposition, and expression of defense-related genes, were activated. Thus, the A. Citrulli growth in HrpE-pretreated hosts was suppressed. These results indicated that HrpE is essential for A. citrulli virulence but can also be used by hosts to help resist A. citrulli. Our findings provide a better understanding of the T3SS pathogenesis in A. citrulli, thus providing a molecular basis for biopesticide development, and facilitating the effective control of BFB.

A Cytochrome P450 Monooxygenase in Nondefoliating Strain of Verticillium dahliae Manipulates Virulence via Scavenging Reactive Oxygen Species

Verticillium dahliae is a broad host-range phytopathogenic fungus that causes destructive vascular wilt on plants worldwide. Cytochrome P450 monooxygenases, also known as CYPs/P450s, are broadly distributed in organisms and are involved in a diverse array of molecular/metabolic processes. In this study, using reverse transcription quantitative PCR analysis, we observed that the expression of a P450 gene (Chr2g00380) in the E-class P450, group IV from V. dahliae isolate JR2 was highly induced during tomato infection. Targeted deletion of Chr2g00380 in JR2 did not affect hyphal growth and morphology; however, the mutants exhibited increased sensitivity to H₂O₂ and defects in melanized microsclerotia formation compared with the wild type. Loss of Chr2g00380 resulted in reduced virulence on tomato and tobacco plants but did not cause phenotypic changes in infection structure formation or in the penetration of cellophane membranes. These data provide evidence for an involvement of a cytochrome P450 monooxygenase in virulence in V. dahliae.

EXPLICIT-Kinase: A gene expression predictor for dissecting the functions of the Arabidopsis kinome

Protein kinases regulate virtually all cellular processes, but it remains challenging to determine the functions of all protein kinases, collectively called the "kinome", in any species. We developed a computational approach called EXPLICIT-Kinase to predict the functions of the Arabidopsis kinome. Because the activities of many kinases can be regulated transcriptionally, their gene expression patterns provide clues to their functions. A universal gene expression predictor for Arabidopsis was constructed to predict the expression of 30,172 non-kinase genes based on the expression of 994 kinases. The model reconstituted highly accurate transcriptomes for diverse Arabidopsis samples. It identified the significant kinases as predictor kinases for predicting the expression of Arabidopsis genes and pathways. Strikingly, these predictor kinases were often regulators of related pathways, as exemplified by those involved in cytokinesis, tissue development, and stress responses. Comparative analyses revealed that portions of these predictor kinases are shared and conserved between Arabidopsis and maize. As an example, we identified a conserved predictor kinase, RAF6, from a stomatal movement module. We verified that RAF6 regulates stomatal closure. It can directly interact with SLAC1, a key anion channel for stomatal closure, and modulate its channel activity. Our approach enables a systematic dissection of the functions of the Arabidopsis kinome.

Regulation of plant responses to biotic and abiotic stress by receptor-like cytoplasmic kinases

As sessile organisms, plants have to cope with environmental change and numerous biotic and abiotic stress. Upon perceiving environmental cues and stress signals using different types of receptors, plant cells initiate immediate and complicated signaling to regulate cellular processes and respond to stress. Receptor-like cytoplasmic kinases (RLCKs) transduce signals from receptors to cellular components and play roles in diverse biological processes. Recent studies have revealed the hubbing roles of RLCKs in plant responses to biotic stress. Emerging evidence indicates the important regulatory roles of RLCKs in plant responses to abiotic stress, growth, and development. As a pivot of cellular signaling, the activity and stability of RLCKs are dynamically and tightly controlled. Here, we summarize the current understanding of how RLCKs regulate plant responses to biotic and abiotic stress.

What's new in protein kinase/phosphatase signalling in the control of plant immunity?

Plant immunity is crucial to plant health but comes at an expense. For optimal plant growth, tight immune regulation is required to prevent unnecessary rechannelling of valuable resources. Pattern- and effector-triggered immunity (PTI/ETI) represent the two tiers of immunity initiated after sensing microbial patterns at the cell surface or pathogen effectors secreted into plant cells, respectively. Recent evidence of PTI-ETI cross-potentiation suggests a close interplay of signalling pathways and defense responses downstream of perception that is still poorly understood. This review will focus on controls on plant immunity through phosphorylation, a universal and key cellular regulatory mechanism. Rather than a complete overview, we highlight “what’s new in protein kinase/phosphatase signalling” in the immunity field. In addition to phosphoregulation of components in the pattern recognition receptor (PRR) complex, we will cover the actions of the major immunity-relevant intracellular protein kinases/phosphatases in the ‘signal relay’, namely calcium-regulated kinases (e.g. calcium-dependent protein kinases, CDPKs), mitogen-activated protein kinases (MAPKs), and various protein phosphatases. We discuss how these factors define a phosphocode that generates cellular decision-making ‘logic gates’, which contribute to signalling fidelity, amplitude, and duration. To underscore the importance of phosphorylation, we summarize strategies employed by pathogens to subvert plant immune phosphopathways. In view of recent game-changing discoveries of ETI-derived resistosomes organizing into calcium-permeable pores, we speculate on a possible calcium-regulated phosphocode as the mechanistic control of the PTI-ETI continuum.

New molecules in plant defence against pathogens

Plants host a multipart immune signalling network to ward off pathogens. Pathogen attack upon plant tissues can often lead to an amplified state of (induced) defence against subsequent infections in distal tissues; this is known as systemic acquired resistance (SAR). The interaction of plants with beneficial microbes of the rhizosphere microbiome can also lead to an induced resistance in above-ground plant tissues, known as induced systemic resistance. Second messengers such as calcium (Ca2+), reactive oxygen species (ROS), and nitric oxide (NO) are necessary for cell-to-cell signal propagation during SAR and show emergent roles in the mediation of other SAR metabolites. These include the lysine-derived signals pipecolic acid (Pip) and N-hydroxypipecolic acid (NHP), which are key signalling metabolites in SAR. Emerging evidence additionally pinpoints plant volatiles as modulators of defence signalling within and between plants. Plant volatile organic compounds (VOCs) such as monoterpenes can promote SAR by functioning through ROS. Furthermore, plant-derived and additionally also microbial VOCs can target both salicylic acid and jasmonic acid signalling pathways in plants and modulate defence against pathogens. In this review, an overview of recent findings in induced defence signalling, with a particular focus on newer signalling molecules and how they integrate into these networks is discussed.

The receptor-like cytosolic kinase RIPK activates NADP-malic enzyme 2 to generate NADPH for fueling ROS production

Reactive oxygen species (ROS) production is a conserved immune response in Arabidopsis primarily mediated by respiratory burst oxidase homolog D (RBOHD), a nicotinamide adenine dinucleotide phosphate (NADPH) oxidase associated with the plasma membrane. A rapid increase in NADPH is necessary to fuel RBOHD proteins and thus maintain ROS production. However, the molecular mechanism by which NADPH is generated to fuel RBOHD remains unclear. In this study, we isolated a new mutant allele of FLAGELLIN-INSENSITIVE 4 (FIN4), which encodes the first enzyme in de novo NAD biosynthesis. fin4 mutants show reduced NADPH levels and impaired ROS production. However, FIN4 and other genes involved in NAD- and NADPH-generating pathways are not highly upregulated upon elicitor treatment, raising a possibility that a cytosolic NADP-linked dehydrogenase might be post-transcriptionally activated to maintain the NADPH supply close to RBOHD. To verify this possibility, we isolated the proteins associated with RPM1-INDUCED PROTEIN KINASE (RIPK), a receptor-like cytoplasmic kinase that regulates broad-spectrum ROS signaling in plant immunity, and identified NADP-malic enzyme 2 (NADP-ME2), an NADPH-generating enzyme. Compared with wild-type plants, nadp-me2 mutants display decreased NADP-ME activity, lower NADPH levels, and reduced ROS production in response to immune elicitors. Furthermore, we found that RIPK can directly phosphorylate NADP-ME2 and enhance its activity in vitro. The phosphorylation of the NADP-ME2 S371 residue contributes to ROS production upon immune elicitor treatment and susceptibility to the necrotrophic bacterium Pectobacterium carotovorum. Collectively, our study suggests that RIPK phosphorylates and activates NADP-ME2 to rapidly increase cytosolic NADPH, thus fueling RBOHD to sustain ROS production in plant immunity.

RIPK: a crucial ROS signaling component in plants

Reactive oxygen species (ROS) are involved in plant development and stress acclimation. Recently, Li et al. reported that ROS production is controlled by receptor-like cytoplasmic kinase (RLCK)-mediated respiratory burst oxidase homolog D (RBOHD) phosphorylation, which subsequently regulates pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI), damage-associated molecular pattern (DAMP)-triggered immunity (DTI), effector-triggered immunity (ETI), and systemic acquired resistance (SAR) in plants, thus enhancing our understanding of biotic stress tolerance.

Molecular mechanisms of early plant pattern-triggered immune signaling

All eukaryotic organisms have evolved sophisticated immune systems to appropriately respond to biotic stresses. In plants and animals, a key part of this immune system is pattern recognition receptors (PRRs). Plant PRRs are cell-surface-localized receptor kinases (RKs) or receptor proteins (RPs) that sense microbe- or self-derived molecular patterns to regulate pattern-triggered immunity (PTI), a robust form of antimicrobial immunity. Remarkable progress has been made in understanding how PRRs perceive their ligands, form active protein complexes, initiate cell signaling, and ultimately coordinate the cellular reprogramming that leads to PTI. Here, we discuss the critical roles of PRR complex formation and phosphorylation in activating PTI signaling, as well as the emerging paradigm in which receptor-like cytoplasmic kinases (RLCKs) act as executors of signaling downstream of PRR activation.