Institute of Molecular Agrobiology, Singapore, Malaysia

Background and objectives
The mitogen-activated protein (MAP) kinase is a specific class of serine/threonie protein kinases and has been implicated in a wide variety of physiological processes, such as cell growth, differentiation, oncogenesis and response to environmental stresses. Recently, several stress-induced MAP kinases have also been identified in plants which are responsive to cold, heat, wounding, drought and mechanical stresses. For example, the 48-kDa MAP kinase ERMK is rapidly activated upon high-affinity binding of a fungal elicitor to a plasma membrane receptor in parsley cells [1]. Another MAP kinase, p48 SIP, is identified to be activated in tobacco cells by salicylic acid (SA) treatment, which is an endogenous signal for the activation of several plant defence responses [2]. These results suggest that MAP kinases are important components in the signal transduction pathway of plant defence to blast infection.

Rice blast is the most economically devastating disease of cultivated rice, caused by the filamentous fungus Magnaporthe grisea. The use of resistant cultivars is the most economical and effective method of controlling the disease. Over the past decade, much has been learned about the genetics of resistance to the blast fungus. However, the molecular mechanism of host defences to this pathogen is mostly unknown. The objective of this project is to isolate genes involved in the signal-transduction pathway in rice against pathogen infection.

Results and discussion
When primers CF9-RT and CF9-Rev were used in RT-PCR, four bands were amplified in both rice cultivars C101A51 (compatible) and CO39 (incompatible). These bands were cloned and sequenced. It was found that one clone with a 350-bp insert is highly homologous to mammalian and yeast MAP kinases. To clone the full-length genomic fragment of this gene, a rice BAC library was screened using the 350-bp cDNA fragment as the probe. A 4.5-kb subclone containing the 5' region and a part of the coding region (about 400 bp) was isolated from a positive BAC clone. To isolate a full-length cDNA from rice, RT-PCR was performed using a primer-containing sequence spanning the start codon ATG and an oligo-dT primer, and a 2.0-kbp PCR product was obtained. This clone contains a 1557-bp open reading frame corresponding to 519 amino acids, and is significantly homologous to MAP kinases isolated from a variety of organisms. It contains all 11 highly conserved subdomains which are present in all known MAP kinases in mammals and plants. This gene was designated BIMK1, for blast-induced MAP kinase. Northern blot analysis showed that the expression of BIMK1 was induced as early as 8 h after inoculation, indicating that BIMK1 may be involved in the defence response to the blast fungus.

1. Ligterink WKT, zur Nieden U, Hirt H, Scheel D, 1997. Science 276, 2054-2057.
2. Zhang S, Klessig D, 1997. Plant Cell 9, 809-824.