Perception of a divergent family of phytocytokines by the Arabidopsis receptor kinase MIK2

The Phloem Intercalated With Xylem-Correlated 3 Receptor-Like Kinase Constitutively Interacts With Brassinosteroid Insensitive 1-Associated Receptor Kinase 1 and Is Involved in Vascular Development in Arabidopsis

Leucine-rich repeat receptor-like kinases (LRR-RLKs) play fundamental roles in cell-to-cell and plant-environment communication. LRR-RLKs can function as receptors perceiving endogenous or external ligands, or as coreceptors, which stabilize the complex, and enhance transduction of the intracellular signal. The LRR-RLK BAK1 is a coreceptor for different developmental and immunity pathways. In this article, we identified PXY-CORRELATED 3 (PXC3) as a BAK1-interacting LRR-RLK, which was previously reported to be transcribed in vascular tissues co-expressed with PHLOEM INTERCALATED WITH XYLEM (PXY), the receptor of the TDIF/CLE41 peptide. Characterization of pxc3 loss-of-function mutants revealed reduced hypocotyl stele width and vascular cells compared to wild type, indicating that PXC3 plays a role in the vascular development in Arabidopsis. Furthermore, our data suggest that PXC3 might function as a positive regulator of the CLE41/TDIF-TDR/PXY signaling pathway.

Mechanisms of plant cell wall surveillance in response to pathogens, cell wall-derived ligands and the effect of expansins to infection resistance or susceptibility

Cell wall integrity is tightly regulated and maintained given that non-physiological modification of cell walls could render plants vulnerable to biotic and/or abiotic stresses. Expansins are plant cell wall-modifying proteins active during many developmental and physiological processes, but they can also be produced by bacteria and fungi during interaction with plant hosts. Cell wall alteration brought about by ectopic expression, overexpression, or exogenous addition of expansins from either eukaryote or prokaryote origin can in some instances provide resistance to pathogens, while in other cases plants become more susceptible to infection. In these circumstances altered cell wall mechanical properties might be directly responsible for pathogen resistance or susceptibility outcomes. Simultaneously, through membrane receptors for enzymatically released cell wall fragments or by sensing modified cell wall barrier properties, plants trigger intracellular signaling cascades inducing defense responses and reinforcement of the cell wall, contributing to various infection phenotypes, in which expansins might also be involved. Here, we review the plant immune response activated by cell wall surveillance mechanisms, cell wall fragments identified as responsible for immune responses, and expansin's roles in resistance and susceptibility of plants to pathogen attack.

The MIK2/SCOOP Signaling System Contributes to Arabidopsis Resistance Against Herbivory by Modulating Jasmonate and Indole Glucosinolate Biosynthesis

Initiation of plant immune signaling requires recognition of conserved molecular patterns from microbes and herbivores by plasma membrane-localized pattern recognition receptors. Additionally, plants produce and secrete numerous small peptide hormones, termed phytocytokines, which act as secondary danger signals to modulate immunity. In Arabidopsis, the Brassicae-specific SERINE RICH ENDOGENOUS PEPTIDE (SCOOP) family consists of 14 members that are perceived by the leucine-rich repeat receptor kinase MALE DISCOVERER 1-INTERACTING RECEPTOR LIKE KINASE 2 (MIK2). Recognition of SCOOP peptides elicits generic early signaling responses but knowledge on how and if SCOOPs modulate specific downstream immune defenses is limited. We report here that depletion of MIK2 or the single PROSCOOP12 precursor results in decreased Arabidopsis resistance against the generalist herbivore Spodoptera littoralis but not the specialist Pieris brassicae. Increased performance of S. littoralis on mik2-1 and proscoop12 is accompanied by a diminished accumulation of jasmonic acid, jasmonate-isoleucine and indolic glucosinolates. Additionally, we show transcriptional activation of the PROSCOOP gene family in response to insect herbivory. Our data therefore indicate that perception of endogenous SCOOP peptides by MIK2 modulates the jasmonate pathway and thereby contributes to enhanced defense against a generalist herbivore.

Fighting salt or enemies: shared perception and signaling strategies

Plants react to a myriad of biotic and abiotic environmental signals through specific cellular mechanisms required for survival under stress. Although pathogen perception has been widely studied and characterized, salt stress perception and signaling remain largely elusive. Recent observations, obtained in the model plant Arabidopsis thaliana, show that perception of specific features of pathogens also allows plants to mount salt stress resilience pathways, highlighting the possibility that salt sensing and pathogen perception mechanisms partially overlap. We discuss these overlapping strategies and examine the emerging role of A. thaliana cell wall and plasma membrane components in activating both salt- and pathogen-induced responses, as part of exquisite mechanisms underlying perception of damage and danger. This knowledge helps understanding the complexity of plant responses to pathogens and salinity, leading to new hypotheses that could explain why plants evolved similar strategies to respond to these, at first sight, very different types of stimuli.

Emerging roles of pathogen-secreted host mimics in plant disease development

Plant pathogens and parasites use multiple virulence factors to successfully infect plants. While most plant-pathogen interaction studies focus on pathogen effectors and their functions in suppressing plant immunity or interfering with normal cellular processes, other virulence factors likely also contribute. Here we highlight another important strategy used by pathogens to promote virulence: secretion of mimics of host molecules, including peptides, phytohormones, and small RNAs, which play diverse roles in plant development and stress responses. Pathogen-secreted mimics hijack the host endogenous signaling pathways, thereby modulating host cellular functions to the benefit of the pathogen and promoting infection. Understanding the mechanisms of pathogen-secreted host mimics will expand our knowledge of host-pathogen coevolution and interactions, while providing new targets for plant disease control.

Pathogen- and plant-derived peptides trigger plant immunity

Plants are constantly exposed to pathogens in their immediate environment. Plants sense the invasion of pathogens by recognizing the components including peptide fragments derived from pathogens, known as pathogen-associated molecular patterns (PAMPs). Plants also produce immunogenic peptides called phytocytokines that regulate immune responses. These molecules are recognized by pattern recognition receptors (PRRs) at plasma membrane. Activated PRRs induce a variety of immune responses including production of reactive oxygen species (ROS), induction of Ca²⁺ influx and activation of mitogen activated protein kinases (MAPKs). Pattern-triggered immunity (PTI) wards off microbes and pests. In this review, we summarize recent our advances in understanding how the peptide fragments are generated and perceived by plant PRRs at cell surface, and the activated PRRs transduce the downstream immune signaling.

The Arabidopsis MIK2 receptor elicits immunity by sensing a conserved signature from phytocytokines and microbes

Sessile plants encode a large number of small peptides and cell surface-resident receptor kinases, most of which have unknown functions. Here, we report that the Arabidopsis receptor kinase MALE DISCOVERER 1-INTERACTING RECEPTOR-LIKE KINASE 2 (MIK2) recognizes the conserved signature motif of SERINE-RICH ENDOGENOUS PEPTIDEs (SCOOPs) from Brassicaceae plants as well as proteins present in fungal Fusarium spp. and bacterial Comamonadaceae, and elicits various immune responses. SCOOP signature peptides trigger immune responses and altered root development in a MIK2-dependent manner with a sub-nanomolar sensitivity. SCOOP12 directly binds to the extracellular leucine-rich repeat domain of MIK2 in vivo and in vitro, indicating that MIK2 is the receptor of SCOOP peptides. Perception of SCOOP peptides induces the association of MIK2 and the coreceptors SOMATIC EMBRYOGENESIS RECEPTOR KINASE 3 (SERK3) and SERK4 and relays the signaling through the cytosolic receptor-like kinases BOTRYTIS-INDUCED KINASE 1 (BIK1) and AVRPPHB SUSCEPTIBLE1 (PBS1)-LIKE 1 (PBL1). Our study identifies a plant receptor that bears a dual role in sensing the conserved peptide motif from phytocytokines and microbial proteins via a convergent signaling relay to ensure a robust immune response.

Peptide signals generated during plant microbe interactions can trigger immune responses in plants. Here the authors show that SCOOP12, a member of a family of peptides present in Brassicaceae plants, and SCOOP12-like motifs in Fusarium fungi, can trigger immune responses following perception by the MIK2 receptor kinase.

Phytocytokines function as immunological modulators of plant immunity

Plant plasma membrane-resident immune receptors regulate plant immunity by recognizing microbe-associated molecular patterns (MAMPs), damage-associated molecular patterns (DAMPs), and phytocytokines. Phytocytokines are plant endogenous peptides, which are usually produced in the cytosol and released into the apoplast when plant encounters pathogen infections. Phytocytokines regulate plant immunity through activating an overlapping signaling pathway with MAMPs/DAMPs with some unique features. Here, we highlight the current understanding of phytocytokine production, perception and functions in plant immunity, and discuss how plants and pathogens manipulate phytocytokine signaling for their own benefits during the plant-pathogen warfare.

Damage-Associated Molecular Patterns (DAMPs) in Plant Innate Immunity: Applying the Danger Model and Evolutionary Perspectives

Danger signals trigger immune responses upon perception by a complex surveillance system. Such signals can originate from the infectious nonself or the damaged self, the latter termed damage-associated molecular patterns (DAMPs). Here, we apply Matzinger's danger model to plant innate immunity to discuss the adaptive advantages of DAMPs and their integration into preexisting signaling pathways. Constitutive DAMPs (cDAMPs), e.g., extracellular ATP, histones, and self-DNA, fulfill primary, conserved functions and adopt a signaling role only when cellular damage causes their fragmentation or localization to aberrant compartments. By contrast, immunomodulatory peptides (also known as phytocytokines) exclusively function as signals and, upon damage, are activated as inducible DAMPs (iDAMPs). Dynamic coevolutionary processes between the signals and their emerging receptors and shared co-receptors have likely linked danger recognition to preexisting, conserved downstream pathways.

Coding of plant immune signals by surface receptors

The detection of molecular signals derived from other organisms is central to the evolutionary success of plants in the colonization of Earth. The sensory coding of these signals is critical for marshaling local and systemic immune responses that keep most invading organisms at bay. Plants detect immune signals inside and outside their cells using receptors. Here, we focus on receptors that function at the cell surface. We present recent work that expands our understanding of the repertoire of immune signals sensed by this family of receptors.

Maintenance of Cell Wall Integrity under High Salinity

Cell wall biosynthesis is a complex biological process in plants. In the rapidly growing cells or in the plants that encounter a variety of environmental stresses, the compositions and the structure of cell wall can be dynamically changed. To constantly monitor cell wall status, plants have evolved cell wall integrity (CWI) maintenance system, which allows rapid cell growth and improved adaptation of plants to adverse environmental conditions without the perturbation of cell wall organization. Salt stress is one of the abiotic stresses that can severely disrupt CWI, and studies have shown that the ability of plants to sense and maintain CWI is important for salt tolerance. In this review, we highlight the roles of CWI in salt tolerance and the mechanisms underlying the maintenance of CWI under salt stress. The unsolved questions regarding the association between the CWI and salt tolerance are discussed.

The Wheat Wall-Associated Receptor-Like Kinase TaWAK-6D Mediates Broad Resistance to Two Fungal Pathogens Fusarium pseudograminearum and Rhizoctonia cerealis

The soil-borne fungi Fusarium pseudograminearum and Rhizoctonia cerealis are the major pathogens for the economically important diseases Fusarium crown rot (FCR) and sharp eyespot of common wheat (Triticum aestivum), respectively. However, there has been no report on the broad resistance of wheat genes against both F. pseudograminearum and R. cerealis. In the current study, we identified TaWAK-6D, a wall-associated kinase (WAK) which is an encoding gene located on chromosome 6D, and demonstrated its broad resistance role in the wheat responses to both F. pseudograminearum and R. cerealis infection. TaWAK-6D transcript induction by F. pseudograminearum and R. cerealis was related to the resistance degree of wheat and the gene expression was significantly induced by exogenous pectin treatment. Silencing of TaWAK-6D compromised wheat resistance to F. pseudograminearum and R. cerealis, and repressed the expression of a serial of wheat defense-related genes. Ectopic expression of TaWAK-6D in Nicotiana benthamiana positively modulated the expression of several defense-related genes. TaWAK-6D protein was determined to localize to the plasma membrane in wheat and N. benthamiana. Collectively, the TaWAK-6D at the plasma membrane mediated the broad resistance responses to both F. pseudograminearum and R. cerealis in wheat at the seedling stage. This study, therefore, concludes that TaWAK-6D is a promising gene for improving wheat broad resistance to FCR and sharp eyespot.