Phospholipid signaling is an important component in eukaryotic signal transduction pathways; in plants, it plays a key role in growth and development as well as in responses to environmental stress. We cloned and characterized a gene from rice encoding phosphoinositide-specific phospholipase C (OsPI-PLC1, Oryza sativa L. phosphoinositide-specific phospholipase C1). OsPI-PLC1 encodes a 599 amino acid protein containing the catalytic X and Y domains, as well as a C2-1ike domain, characteristics of this class of enzymes. Expression of OsPI-PLC1 was induced by various chemical and biological inducers of plant defence pathways, including benzothiadiazole (BTH), salicylic acid (SA), dichloroisonicotinic acid, probenazole (PB), jasmonic acid (JA) and its methyl ester, Pseudomonas syringae pv. syringae, and wounding. All of these treatments were shown to induce resistance in rice against blast disease caused by Magrzaporthe grisea. OsPI-PLC1 was activated within 6 h after inoculation with the blast fungus in BTH-treated rice seedlings and in the incompatible interaction between a resistant genotype of rice and M. grisea, whereas the expression in the BTH-untreated seedlings and in the compatible interaction was only induced to a relatively low level at later time points (30h) after inoculation, indicating that expression of OsPI-PLC1 is associated with the resistance response and/or the incompatible interaction. In addition, BTH treatment also induced systemic expression of OsPI-PLC1. These results suggest that OsPI-PLC1 may be involved in the signaling pathways leading to disease resistance in rice.

In the present study, we cloned and identified a fulllength cDNA of a rice gene, OsBIHD1, encoding a homeodomain type transcriptional factor. OsBIHD1 is predicted to encode a 642 amino acid protein and the deduced protein sequence of OsBIHD1 contains all conserved domains, a homeodomain, a BELL domain, a SKY box, and a VSLTLGL box, which are characteristics of the BELL type homedomain proteins. The recombinant OsBIHD1 protein expressed in Escherichia coil bound to the TGTCA motif that is the characteristic c/s-element D NA sequence of the homeodomain transcriptional factors. Subcellular localization analysis revealed that the OsBIHD1 protein localized in the nucleus of the plant cells. The OsBIHD1 gene was mapped to chromosome 3 of the rice genome and is a single-copy gene with four exons and three introns. Northern blot analysis showed that expression of OsBIHD1 was activated upon treatment with benzothiadiazole (BTH), which is capable of inducing disease resistance. Expression of OsBIHD1 was also up-regulated rapidly during the first 6 h after inoculation with Magnaporthe grisea in BTH-treated rice seedlings and during the incompatible interaction between M. grisea and a resistant genotype. These results suggest that OsBIHD1 is a BELL type of homeodomain transcription factor present in the nucleus, whose induction is associated with resistance response in rice.

The rice OsBIHD1 gene encodes a transcriptional factor belonging to the homeodomain class. It had previously been shown to be activated by treatment with benzothiadiazole, a chemical inducer of disease resistance, and in an incompatible interaction between rice and the blast fungus. To allow a better understanding of the function of OsBIHD1 in plant disease resistance response, the OsBIHD1 gene in tobacco was overexpressed by Agrobacterium-mediated leaf disc transformation with a construct containing the OsBIHD1 ORF under control of the 35S promoter. Overexpression of the rice OsBIHD1 gene in some of the transgenic tobacco lines led to some morphological abnormalities in the top buds and roots. The transgenic tobacco plants showed an elevated level of defence-related PR-1 gene expression and enhanced disease resistance against infection by tomato mosaic virus, tobacco mosaic virus, and Phytophthora parasitica var. nicotianae. However, the transgenic tobacco plants overexpressing OsBIHD1 showed enhanced sensitivity to salt and oxidative stress as compared with the wild-type plants. The results suggested that the OsBIHD1 protein may be positively involved in activating expression of the defence-related genes in disease resistance responses, and is also important in rice development and abiotic stress tolerance.

The activation of mitogen-activated protein kinases (MAPKs) has been previously implicated in signal transduction during plant responses to pathogen attack as well as to various environmental stresses. We have isolated from rice a new MAPK cDNA, OsBIMK1 (Oryza sativa L. BTH-induced MAPK 1), which encodes a 369-amino-acid protein with moderate to high nucleotide sequence similarity to previously reported plant MAPK genes. OsBIMK1 contains all 11 of the MAPK conserved subdomains and the phosphorylation-activation motif, TEY. We analyzed in detail the expression of OsBIMK1 upon treatment with various chemicalan d biologicalinducers of resistance responses in rice and in both incompatible and compatible interactions between rice and Magnaporthe grisea. Expression of OsBIMK1 was activated rapidly upon treatment with benzothiadiazole (BTH) as well as with dichloroisonicotinic acid, probenazole, jasmonic acid and its methyl ester, Pseudomonas syringae pv. syringae, or wounding. Expression of Os-BIMK1 was induced rapidly during the first 36h after inoculation with M. grisea in BTH-treated rice seedlings and in an incompatible interaction between M. grisea and a blast-resistant rice genotype. BTH treatment induced a systemic activation ofOsBIMK1 expression. These results suggest that OsBIMK1 plays an important role in rice disease resistance.