LysM receptors recognize friend and foe

Abstract

Chitin, a β(1→ 4)-linked polymer of N-acetylglucosamine, is the second-most abundant polysaccharide in nature after cellulose and serves as a major structural component in the exoskeleton of arthropods and in the cell walls of fungi. The latter include some of the most important pathogens of plants. Upon microbial attack, plants normally mount a multicomponent defense response that efficiently stops the invasion. This important and durable type of plant disease resistance is called species or nonhost resistance. To become pathogenic, a microbe needs to “learn” to suppress the defense response of a plant species, thereby turning it into a host species. Nonhost resistance is based on a non-self recognition system that perceives at the site of attempted penetration typical microbe-associated molecular patterns (MAMPs), which normally do not occur in plants (1). Prominent MAMPs recognized by plant cells are chitin fragments (chitooligosaccharides) released from fungal cell walls during pathogen attack, which in many plants elicit the plant defense response (oxidative burst, protein phosphorylation, transcriptional activation of defense-related genes, phytoalexin biosynthesis, etc.). High-affinity binding sites were found in suspension-cultured rice and tomato cells (2, 3), and a 75-kDa chitooligosaccharide-binding protein was identified in rice plasma membranes by affinity labeling and cross-linking (4). Results from inhibitor studies using different oligosaccharides were in good agreement with the activities of these oligosaccharides in the induction of phytoalexin biosynthesis and of other cellular responses. Although this suggested the 75-kDa protein to be the functional receptor for the chitooligosaccharide elicitor, the receptor protein at first remained elusive. In this issue of PNAS, Kaku et al. (5) report on the purification of the chitin oligomer-binding protein (CEBiP) from rice and the cloning of its gene. It encodes a 356-aa protein with a 28-aa secretory signal sequence. The mature protein of 328 aa has a calculated molecular weight of 34,640, the higher molecular mass of 75 kDa obtained in previous experiments being due to glycosylation. In CEBiP-RNAi lines, where expression of the gene was knocked down, specific binding of chitooligosaccharides, chitooligosaccharide-elicited oxidative burst, and defense-related reprogramming of gene expression were strongly reduced. In contrast, these cell lines were fully responsive to another typical MAMP, bacterial lipopolysaccharide, indicating that specific, receptor-mediated perception of chitooligosaccharides was affected but not that of other structurally unrelated MAMPs.