elicitor ͉ host-pathogen interaction ͉ N-acetylchitooligosaccharides ͉ pathogen-associated molecular patterns H igher plants have the ability to initiate various defense reactions such as hypersensitive responses, production of phytoalexins and antimicrobial proteins, and reinforcement of cell walls when they are infected by various pathogens (1, 2). They can distinguish self and non-self, or detect specific pathogens, through the perception of signal molecules (elicitors) mostly generated͞secreted from pathogens. Fragments of cell surface macromolecules typical of microorganisms such as cell wall polysaccharides, secreted proteins, as well as a flagella protein, often serve as a potent elicitor to induce defense reactions. They are classified as ''general elicitors'' that are commonly found in various microorganisms and induce defense responses in a wide range of plant species. Perception of general elicitors has been thought to play an important role in the basic resistance, or nonhost resistance, of plants to most potential pathogens. It has also been emerged in recent years that the defense systems mediated by the perception of these ''general elicitors'' have a considerable similarity with mammalian innate immunity, in the recognition of pathogen-associated molecular patterns as well as the molecules involved in the perception and transduction of these signal molecules (3).Chitin oligosaccharides (N-acetylchitooligosaccharides) are a representative general elicitor inducing defense responses in a wide range of plant cells including both monocots and dicots (4). Chitin oligosaccharides were reported to induce defense responses also in mammalian and insect cells (4, 5). Interestingly, specific modifications of chitin oligosaccharides by fatty acids, sulfate, or some sugars, generate ''Nod factors'' that induce nodulation in legume roots in the symbiotic interaction with rhizobial bacteria (6). Thus, the recognition of chitin oligosaccharides and related compounds seems to play a fundamental role in the establishment of basal resistance to potential pathogens in plants and in some cases the symbiotic relationships between leguminous plants and rhizobial bacteria.Concerning to the receptor for chitin oligosaccharide elicitor, we previously identified a high-affinity binding protein for this elicitor in the plasma membrane of rice cells by affinity labeling (7). Similar binding proteins were also detected in various plant cells that could respond to the elicitor (8,9). Correlation between the presence of the binding proteins and the elicitor responsiveness of these cells, correlation between the binding specificity and the preference of the structure of chitin oligosaccharides in defense responses, strongly indicated that the binding proteins function as a receptor, or a part of receptor complex, for chitin oligosaccharide elicitor. Here we report the purification of this chitin oligosaccharide elicitor-binding protein (hereafter designated as CEBiP), cloning of the corresponding cDNA and its functional characte...

The entire chloroplast genome of the monocot rice (Oryza sativa) has been sequenced and comprises 134525 bp. Predicted genes have been identified along with open reading frames (ORFs) conserved between rice and the previously sequenced chloroplast genomes, a dicot, tobacco (Nicotiana tabacum), and a liverwort (Marchantia polymorpha). The same complement of 30 tRNA and 4 rRNA genes has been conserved between rice and tobacco. Most ORFs extensively conserved between N. tabacum and M. polymorpha are also conserved intact in rice. However, several such ORFs are entirely absent in rice, or present only in severely truncated form. Structural changes are also apparent in the genome relative to tobacco. The inverted repeats, characteristic of chloroplast genome structure, have expanded outward to include several genes present only once per genome in tobacco and liverwort and the large single copy region has undergone a series of inversions which predate the divergence of the cereals. A chimeric tRNA pseudogene overlaps an apparent endpoint of the largest inversion, and a model invoking illegitimate recombination between tRNA genes is proposed which accounts simultaneously for the origin of this pseudogene, the large inversion and the creation of repeated sequences near the inversion endpoints.

Chitin is a major molecular pattern for various fungi, and its fragments, chitin oligosaccharides, are known to induce various defense responses in plant cells. A plasma membrane glycoprotein, CEBiP (chitin elicitor binding protein) and a receptor kinase, CERK1 (chitin elicitor receptor kinase) (also known as LysM-RLK1), were identified as critical components for chitin signaling in rice and Arabidopsis, respectively. However, it is not known whether each plant species requires both of these two types of molecules for chitin signaling, nor the relationships between these molecules in membrane signaling. We report here that rice cells require a LysM receptor-like kinase, OsCERK1, in addition to CEBiP, for chitin signaling. Knockdown of OsCERK1 resulted in marked suppression of the defense responses induced by chitin oligosaccharides, indicating that OsCERK1 is essential for chitin signaling in rice. The results of a yeast two-hybrid assay indicated that both CEBiP and OsCERK1 have the potential to form hetero- or homo-oligomers. Immunoprecipitation using a membrane preparation from rice cells treated with chitin oligosaccharides suggested the ligand-induced formation of a receptor complex containing both CEBiP and OsCERK1. Blue native PAGE and chemical cross-linking experiments also suggested that a major portion of CEBiP exists as homo-oligomers even in the absence of chitin oligosaccharides.

Plants use pattern recognition receptors to defend themselves from microbial pathogens. These receptors recognize pathogen-associated molecular patterns (PAMPs) and activate signaling pathways that lead to immunity. In rice (Oryza sativa), the chitin elicitor binding protein (CEBiP) recognizes chitin oligosaccharides released from the cell walls of fungal pathogens. Here, we show that the rice blast fungus Magnaporthe oryzae overcomes this first line of plant defense by secreting an effector protein, Secreted LysM Protein1 (Slp1), during invasion of new rice cells. We demonstrate that Slp1 accumulates at the interface between the fungal cell wall and the rice plasma membrane, can bind to chitin, and is able to suppress chitin-induced plant immune responses, including generation of reactive oxygen species and plant defense gene expression. Furthermore, we show that Slp1 competes with CEBiP for binding of chitin oligosaccharides. Slp1 is required by M. oryzae for full virulence and exerts a significant effect on tissue invasion and disease lesion expansion. By contrast, gene silencing of CEBiP in rice allows M. oryzae to cause rice blast disease in the absence of Slp1. We propose that Slp1 sequesters chitin oligosaccharides to prevent PAMP-triggered immunity in rice, thereby facilitating rapid spread of the fungus within host tissue.

OsWRKY76 encodes a group IIa WRKY transcription factor of rice. The expression of OsWRKY76 was induced within 48h after inoculation with rice blast fungus (Magnaporthe oryzae), and by wounding, low temperature, benzothiadiazole, and abscisic acid. Green fluorescent protein-fused OsWRKY76 localized to the nuclei in rice epidermal cells. OsWRKY76 showed sequence-specific DNA binding to the W-box element in vitro and exhibited W-box-mediated transcriptional repressor activity in cultured rice cells. Overexpression of OsWRKY76 in rice plants resulted in drastically increased susceptibility to M. oryzae, but improved tolerance to cold stress. Microarray analysis revealed that overexpression of OsWRKY76 suppresses the induction of a specific set of PR genes and of genes involved in phytoalexin synthesis after inoculation with blast fungus, consistent with the observation that the levels of phytoalexins in the transgenic rice plants remained significantly lower than those in non-transformed control plants. Furthermore, overexpression of OsWRKY76 led to the increased expression of abiotic stress-associated genes such as peroxidase and lipid metabolism genes. These results strongly suggest that OsWRKY76 plays dual and opposing roles in blast disease resistance and cold tolerance.