Xa7, a new executor R gene that confers durable and broad-spectrum resistance to bacterial blight disease in rice

Abstract

Bacterial blight (BB) is a globally devastating rice disease caused by <i>Xanthomonas oryzae</i> pv. <i>oryzae</i> (<i>Xoo</i>). The use of disease resistance (<i>R</i>) genes in rice breeding is an effective and economical strategy for the control of this disease. Nevertheless, a majority of <i>R</i> genes lack durable resistance for long-term use under global warming conditions. Here, we report the isolation of a novel executor <i>R</i> gene, <i>Xa7</i>, that confers extremely durable, broad-spectrum, and heat-tolerant resistance to <i>Xoo</i>. The expression of <i>Xa7</i> was induced by incompatible <i>Xoo</i> strains that secreted the transcription activator-like effector (TALE) AvrXa7 or PthXo3, which recognized effector binding elements (EBEs) in the <i>Xa7</i> promoter. Furthermore, <i>Xa7</i> induction was faster and stronger under high temperatures. Overexpression of <i>Xa7</i> or co-transformation of <i>Xa7</i> with <i>avrXa7</i> triggered a hypersensitive response in plants. Constitutive expression of <i>Xa7</i> activated a defense response in the absence of <i>Xoo</i> but inhibited the growth of transgenic rice plants. In addition, analysis of over 3000 rice varieties showed that the <i>Xa7</i> locus was found primarily in the <i>indica</i> and <i>aus</i> subgroups. A variation consisting of an 11-bp insertion and a base substitution (G to T) was found in <i>EBE</i>_{<i>AvrXa7</i>} in the tested varieties, resulting in a loss of <i>Xa7</i> BB resistance. Through a decade of effort, we have identified an important BB resistance gene and characterized its distinctive interaction with <i>Xoo</i> strains; these findings will greatly facilitate research on the molecular mechanism of <i>Xa7</i>-mediated resistance and promote the use of this valuable gene in breeding.