Cell walls surround all plant cells, and their composition and structure are modified in a tightly controlled, adaptive manner to meet sometimes opposing functional requirements during growth and development. The plant cell wall integrity (CWI) maintenance mechanism controls these functional modifications, as well as responses to cell wall damage (CWD). We investigated how the CWI system mediates responses to CWD in CWD induced by cell wall-degrading enzymes or an inhibitor of cellulose biosynthesis elicited similar, turgor-sensitive stress responses. Phenotypic clustering with 27 genotypes identified a core group of receptor-like kinases (RLKs) and ion channels required for the activation of CWD responses. A genetic analysis showed that the RLK FEI2 and the plasma membrane-localized mechanosensitive Ca channel MCA1 functioned downstream of the RLK THE1 in CWD perception. In contrast, pattern-triggered immunity (PTI) signaling components, including the receptors for plant elicitor peptides (Peps) PEPR1 and PEPR2, repressed responses to CWD. CWD induced the expression of and, which encode the precursors of Pep1 andPep3, and the release of PROPEP3 into the growth medium. Application of Pep1 andPep3 repressed CWD-induced phytohormone accumulation in a concentration-dependent manner. These results suggest that Pep-mediated signaling suppresses CWD-induced defense responses controlled by the CWI mechanism. This suppression was alleviated when PTI signaling downstream of PEPR1 and PEPR2 was impaired. Defense responses controlled by the CWI maintenance mechanism might thus compensate to some extent for the loss of PTI signaling elements.

Plants actively perceive and respond to perturbations in their cell walls which arise during growth, biotic and abiotic stresses. However, few components involved in plant cell wall integrity sensing have been described to date. Using a reverse-genetic approach, we identified the Arabidopsis thaliana leucine-rich repeat receptor kinase MIK2 as an important regulator of cell wall damage responses triggered upon cellulose biosynthesis inhibition. Indeed, loss-of-function mik2 alleles are strongly affected in immune marker gene expression, jasmonic acid production and lignin deposition. MIK2 has both overlapping and distinct functions with THE1, a malectin-like receptor kinase previously proposed as cell wall integrity sensor. In addition, mik2 mutant plants exhibit enhanced leftward root skewing when grown on vertical plates. Notably, natural variation in MIK2 (also named LRR-KISS) has been correlated recently to mild salt stress tolerance, which we could confirm using our insertional alleles. Strikingly, both the increased root skewing and salt stress sensitivity phenotypes observed in the mik2 mutant are dependent on THE1. Finally, we found that MIK2 is required for resistance to the fungal root pathogen Fusarium oxysporum. Together, our data identify MIK2 as a novel component in cell wall integrity sensing and suggest that MIK2 is a nexus linking cell wall integrity sensing to growth and environmental cues.

Summary In higher plants, the two‐component system (TCS) is a signaling mechanism based on a His‐to‐Asp phosphorelay. The Arabidopsis TCS involves three different types of proteins, namely the histidine kinases (AHKs), the histidine phosphotransfer proteins (AHPs) and the response regulators (ARRs). The ARRs comprise three different families, namely A, B and C types, according to their protein structure. While some members of the B‐type family of ARRs have been studied extensively and reported to act as DNA‐binding transcriptional regulators, very limited information is available for other B‐type ARRs such as ARR18. In this study, we characterize in detail the molecular and functional properties of ARR18. ARR18 acts as a transcriptional regulator in plant cells and forms homodimers in planta as shown by FRET–FLIM studies. As demonstrated by mutational analysis, the aspartate at position 70 (D70) in the receiver domain of ARR18 acts as crucial phosphorylation site. The modification of D70 affects the response regulator’s ability to homodimerize and to activate its target genes. Furthermore, physiological investigations of Arabidopsis lines ectopically expressing ARR18 introduce ARR18 as a new member within the cytokinin‐regulated response pathway regulating root elongation.

The multi-step phosphorelay (MSP) system defines a key signal transduction pathway in plants and many eukaryotes. In this system, external stimuli first lead to the activation of a histidine kinase, followed by transfer of a phosphoryl group from the receiver domain of the kinase (HK(RD)) to downstream, cytosolic phosphotransfer proteins (HPs). In order to establish the determinants of specificity for this signaling relay system, we have solved the first crystal structure of a plant HK(RD), AHK5(RD), in complex with one of its cognate HPs, AHP1. AHP1 binds AHK5(RD) via a prominent hydrogen bond docking ridge and a hydrophobic patch. These features are conserved among all AHP proteins, but differ significantly from other structurally characterized prokaryotic and eukaryotic HPs. Surface plasmon resonance experiments show that AHK5(RD) binds to AHP1-3 with similar, micromolar affinity, consistent with the transient nature of this signaling complex. Our correlation of structural and functional data provide the first insight, at the atomic level as well as with quantitative affinity data, into the molecular recognition events governing the MSP in plants.

As the first and rate-limiting enzyme of proline degradation, PROLINE DEHYDROGENASE1 (PDH1) is tightly regulated during plant stress responses, including induction under hypoosmolarity and repression under water deficit. The plant receptor histidine kinases AHKs, elements of the two-component system (TCS) in Arabidopsis thaliana, are proposed to function in water stress responses by regulating different stress-responsive genes. However, little information is available concerning AHK phosphorelay-mediated downstream signaling. Here we show that the Arabidopsis type-B response regulator 18 (ARR18) functions as a positive osmotic stress response regulator in Arabidopsis seeds and affects the activity of the PDH1 promoter, known to be controlled by C-group bZIP transcription factors. Moreover, direct physical interaction of ARR18 with bZIP63 was identified and shown to be dependent on phosphorylation of the conserved aspartate residue in the ARR18 receiver domain. We further show that bZIP63 itself functions as a negative regulator of seed germination upon osmotic stress. Using reporter gene assays in protoplasts, we demonstrated that ARR18 interaction negatively interferes with the transcriptional activity of bZIP63 on the PDH1 promoter. Our findings provide new insight into the function of ARR18 and bZIP63 as antagonistic regulators of gene expression in Arabidopsis.