A novel Arabidopsis CHITIN ELICITOR RECEPTOR KINASE 1 (CERK1) mutant with enhanced pathogen‐induced cell death and altered receptor processing

Abstract

SummaryPlants detect pathogens by sensing microbe-associated molecular patterns (MAMPs) through pattern recognition receptors. Pattern recognition receptor complexes also have roles in cell death control, but the underlying mechanisms are poorly understood. Here, we report isolation of cerk1-4, a novel mutant allele of the Arabidopsis chitin receptor CERK1 with enhanced defense responses.We identified cerk1-4 in a forward genetic screen with barley powdery mildew and consequently characterized it by pathogen assays, mutant crosses and analysis of defense pathways. CERK1 and CERK1-4 proteins were analyzed biochemically.The cerk1-4 mutation causes an amino acid exchange in the CERK1 ectodomain. Mutant plants maintain chitin signaling capacity but exhibit hyper-inducible salicylic acid concentrations and deregulated cell death upon pathogen challenge. In contrast to chitin signaling, the cerk1-4 phenotype does not require kinase activity and is conferred by the N-terminal part of the receptor. CERK1 undergoes ectodomain shedding, a well-known process in animal cell surface proteins. Wild-type plants contain the full-length CERK1 receptor protein as well as a soluble form of the CERK1 ectodomain, whereas cerk1-4 plants lack the N-terminal shedding product.Our work suggests that CERK1 may have a chitin-independent role in cell death control and is the first report of ectodomain shedding in plants.

An Overdose of the Arabidopsis Coreceptor BRASSINOSTEROID INSENSITIVE1-ASSOCIATED RECEPTOR KINASE1 or Its Ectodomain Causes Autoimmunity in a SUPPRESSOR OF BIR1-1-Dependent Manner

“…The epidermal growth factor receptor is modified at the transmembrane domain to release an active intracellular domain: the latter is translocated to the nucleus, where it may regulate gene expression (Carpenter and Liao, 2013). In plants, cleavage at the ectodomain has been proposed for several RLKs, like the rice PRR Xa21, the Arabidopsis regulator CHITIN ELICITOR RECEPTOR KINASE1, and the symbiotic receptor kinase from Lotus japonicus (Park and Ronald, 2012;Antolín-Llovera et al, 2014;Petutschnig et al, 2014). However, the biological significance of these observations needs to be further shown experimentally.…”

Nucleoporin NUP88/MOS7 is required for manifestation of phenotypes associated with the Arabidopsis CHITIN ELICITOR RECEPTOR KINASE1 mutant cerk1–4

“…14 Interestingly, cerk1-4 plants lack a soluble CERK1 ectodomain fragment that is released into the apoplast of wild-type plants by an N-terminal shedding process, suggesting that breakdown products of the ectodomain shed from the CERK1-4 receptor variant cause the mutant phenotype. 14 To test, whether the nucleoporin MOS7/NUP88 is required for manifestation of cerk1-4 associated phenotypes, we generated the cerk1-4 mos7-1 double mutant and analyzed its growth morphology and cell death response upon inoculation with Bgh. We found that mos7-1 (in ecotype Col-0) fully suppresses the stunted rosette growth of cerk1-4 (in ecotype Col-3 gl1) (Fig.…”

“…14 Both, the Bgh-induced cell death response and growth retardation of cerk1-4 at old age are independent of chitin signaling, yet require accumulation of the defense hormone SA and an intact SA signaling pathway, since cerk1-4 plants deficient in the SA biosynthetic enzyme SID2 or the downstream SA regulators EDS1 and PAD4 resemble Col-3 gl1 wild-type plants. 14 Interestingly, cerk1-4 plants lack a soluble CERK1 ectodomain fragment that is released into the apoplast of wild-type plants by an N-terminal shedding process, suggesting that breakdown products of the ectodomain shed from the CERK1-4 receptor variant cause the mutant phenotype. 14 To test, whether the nucleoporin MOS7/NUP88 is required for manifestation of cerk1-4 associated phenotypes, we generated the cerk1-4 mos7-1 double mutant and analyzed its growth morphology and cell death response upon inoculation with Bgh.…”

“…This indicates that downstream signal transduction into the nucleus and MOS7-dependent nuclear accumulation of downstream regulatory proteins is essential for manifestation of cerk1-4 related phenotypes. As mos7-1 mutants are impaired in nuclear accumulation of EDS1 7 and the investigated phenotypes of cerk1-4 are EDS1-dependent, 14 the nucleocytoplasmic shuttling protein EDS1 is an obvious candidate to fulfill such downstream regulatory function. However, we cannot exclude the possibility that cerk1-4 associated phenotypes depend on additional unknown signal transducers that translocate into the nucleus and are subject to MOS7-regulated nuclear accumulation.…”

“…hordei (Bgh). 14 The dominant L 124 F mutation (named cerk1-4) results in a deregulated celldeath response upon inoculation with Bgh. At later developmental stages cerk1-4 plants show spontaneous cell death on older leaves and newly emerging leaves often appear smaller and crinkly, eventually leading to reduced rosette diameter in comparison to the wild-type.…”