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Introduction

I I For thousands of years, people have relied on important crops such as rice to survive. More than 2.7 billion people utilize rice as their major source of food worldwide (International Rice Research Institute, website). However, the natural lack of immunity in some rice plants make them susceptible to pathogenic attacks. The usual method of cross breeding and artificial selection is slow and non specific (limited genetic stock, less control of phenotypic expressions), thus making genetic engineering a plausible alternative. Intervarietal, interspecific or intergeneric gene transfer would bypass sterility problems common to interspecific hybridization (Ronald, 1997).
In 1990, Dr. Pamela Ronald began research on the ability of a multicellular organism to defend itself against invasion by pathogens (innate immunity) in rice (Ronald, 1997). Since that time, advances in biology including the sequencing of the rice genome have enabled more rapid characterization of important genes. Nevertheless, little is known about the specific pathway in which an immune response is induced.

helical DNA structure

Immunity in plants is induced by a signal transduction pathway (STP), triggered by the interactions between the plant and pathogenic gene products. If a plant carries the correct resistance gene (R gene), and the pathogen carries a corresponding avirulence gene (Avr gene), recognition occurs leading to the hypersensitive response (HR), accumulation of highly reactive derivative of oxygen that contributes to cell death and damages the pathogens (Pontier et al., 1998). Avr gene products in pathogens are hypothesized to play a role in facilitating the infection process. Proteins coded by Avr genes are detected by a receptor-like kinase (RLK) coded by resistance genes. Some researchers also believe that the Avr inducing pathway is involved with a number of proteins that ultimately activates defense products such as ions, reactive oxygen species, and phospholipids (Ronald et al., 1997). These substances are integral to a cascade reaction and ultimately lead to plant immunological gene expression.

rice view

Xa21 is a vital resistance gene in rice involved in the elicitation of innate immunity (Figure 1.) It is a membrane-bound RLK that may form a receptor dimmer to activate STP components (Romeis, 2001). The gene was isolated by positional cloning and transferred to the susceptible variety TP309. Transgenic rice plants carrying the cloned Xa21 gene displayed high levels of resistance to the pathogen, Xanthomonas oryzae pv. oryzae (Xoo) (Cohn et al., 2001). The sequence of Xa21, which carries both a leucine-rich repeat motif and a serine-threonine kinase-like domain, suggests a role in cell surface recognition of a pathogen ligand and subsequent activation of an intracellular defense response (Shen et al., 2000).
It is not known how many proteins are involved in the resistant reaction. At least one protein - Xb3 - found to be associated with Xa21 (via the yeast two hybrid system (Y2H)) has been linked to the pathway (Fields and Song, 1994). An in vivo study knocked out xb3 in plants and yielded no immune response when stimulated by pathogens. In prior Xa21 kinase domain screens against the C64 and the Moroberekan libraries (Dardick et al., unpublished data), Xb10, along with 4 other proteins, were identified to interact with Xa21. From the DNA sequence, Xb10 was determined to be a WRKY transcription factor involved in the activation of innate immunity downstream. This paper focuses on proteins that interact with Xb10. Utilization of the yeast two hybrid system on Xb10 allowed for the determination of more transcription factors involved in the Xa21 STP and altered the original pathway hypothesized.

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