Isolation and characterization of rice genes regulating disease resistance and cell death

Rice provides nutrition for over half of the world's population. Infection by pathogens poses one of the greatest limitations to rice crop productivity. The use of genetically resistant rice cultivars is among the best strategies for mitigating crop losses due to disease. One disease resistance response in dicotyledonous plants that confers protection against a broad-spectrum of pathogens is termed the Systemic Acquired Resistance (SAR) response. In this study several rice genes that share sequence similarity to components mediating the SAR response in Arabidopsis have been isolated and characterized to determine their contributions to rice disease resistance and cell death. A rice homolog (NH1) of the Arabidopsis SAR regulatory protein (At)NPR1 has been identified. When (At)NPR1 or NH1 are over-expressed in transgenic rice and grown under particular environmental conditions a dramatic cell death response is triggered. This cell death response triggers the expression of defense-related genes and accumulation of reactive oxygen species. Furthermore the levels of the plant metabolite, salicylic acid, are reduced in rice overexpressing either (At)NPR1 or NH1. (At)NPR1 interacts in vitro with a rice bZIP transcription factor, rTGA2.1. Transgenic rice over-expressing a dominant-negative rTGA2.1 mutant, or that have the endogenous gene silenced by dsRNA-mediated silencing, show heightened tolerance to Xanthomonas oryzae pv. oryzae. In addition, these transgenic rice are dwarfed and have increased expression of defense-related genes. This study has identified a suite of genes that will be useful in dissecting out the basic molecular mechanisms of broad-spectrum disease resistance in rice. The roles of some of these genes in resistance to bacterial pathogens, cell death and plant development suggest that they may also be valuable at an applied level for generating rice plants with heightened disease resistance.