32004:

Novel systems for plant disease resistance engineering, basing on cell death and defense-related gene activation.

Authors:	He, Zuhua(A)Zhu, Qun(B)Li, Debao(C)Chory, Joanne(B)Lamb, Chris(B)Ronald, Pamela(A)
Affiliations:	(A): Department of Plant Pathology, UC Davis (B): Plant Biology Laboratory, The Salk Institute for Biological Studies (C): Biotechnology Institute, Zhejiang University
Presenter:	He, Zuhua , zhe@ucdavis.edu

Incompatible interactions between plants and pathogens are characterized by defense responses such as the oxidative burst, hypersensitive cell death and defense gene activation. These interactions are often highly specific. We are interested in developing a non-specific disease resistance strategy. To this end, the chimeric receptors were constructed which combined the catalytic domain of the rice bacterial blight resistance gene Xa21 encoding a receptor-like kinase with ligand binding domains from other genes. We first tested whether an incompatible interaction between rice and Xanthomonas oryzae pv. oryzae (Xoo) could cause cell death. Race-specific cell death was observed in cell lines expressing the Xa21 gene when inoculated with an avirulent strain of Xoo but not with a virulent strain nor in the wildtype cell line. One of the chimeric receptors, which consisted of the Brassinosteroid (BR) binding/interaction domains of the Arabidopsis BR receptor (BRI.1) and the XA21 kinase domain was constitutively expressed in rice cell lines. These cell lines displayed specific cell death and defense gene activation when treated with the presumed ligand, BR. Another chimeric receptor which combined the chitin binding (Hevein) domain of rice chitinase with the XA21 transmembrane and kinase domains also showed significant cell death in constitutive expression cell lines after treatment with chitin. Hydrogen peroxide (H2O2) is produced during the oxidative burst in incompatible interactions and has been recognized as a key component of plant defense responses. As another strategy for engineering non-specific disease resistance, we generated transgenic rice carrying the rice phenylalanine ammonia-lyase (PAL) gene promoter driving glucose oxidase gene (GOX). H2O2 levels were elevated when GOX gene expression was induced by wounding in PAL::GOX plants. Significant cell death was observed in wounded and pathogen infected leaves. Rice defense-related genes such as chitinase and PIR2 (thaumatin-like) were activated earlier and stronger in GOX-expressing plants. In contrast PIR3 (peroxidase) gene expression was completely inhibited in GOX-expressing plants, suggesting that GOX protein or GOX generated H2O2 inhibits peroxidase gene expression. Bacterial growth and rice blast disease development were significantly inhibited in the PAL::GOX plants. This work suggests new approaches for engineering durable and broad spectrum disease resistance in plants.