A Pathovar of Xanthomonas oryzae Infecting Wild Grasses Provides Insight Into the Evolution of Pathogenicity in Rice Agroecosystems

Abstract

<i>Xanthomonas oryzae</i> (<i>Xo</i>) are globally important rice pathogens. Virulent lineages from Africa and Asia and less virulent strains from the United States have been well characterized. <i>Xanthomonas campestris</i> pv. <i>leersiae</i> (<i>Xcl</i>), first described in 1957, causes bacterial streak on the perennial grass, <i>Leersia hexandra</i>, and is a close relative of <i>Xo</i>. <i>L. hexandra</i>, a member of the Poaceae, is highly similar to rice phylogenetically, is globally ubiquitous around rice paddies, and is a reservoir of pathogenic <i>Xo</i>. We used long read, single molecule real time (SMRT) genome sequences of five strains of <i>Xcl</i> from Burkina Faso, China, Mali, and Uganda to determine the genetic relatedness of this organism with <i>Xo</i>. Novel transcription activator-like effectors (TALEs) were discovered in all five strains of <i>Xcl</i>. Predicted TALE target sequences were identified in the <i>Leersia perrieri</i> genome and compared to rice susceptibility gene homologs. Pathogenicity screening on <i>L. hexandra</i> and diverse rice cultivars confirmed that <i>Xcl</i> are able to colonize rice and produce weak but not progressive symptoms. Overall, based on average nucleotide identity (ANI), type III (T3) effector repertoires, and disease phenotype, we propose to rename <i>Xcl</i> to <i>X. oryzae</i> pv. <i>leersiae</i> (<i>Xol</i>) and use this parallel system to improve understanding of the evolution of bacterial pathogenicity in rice agroecosystems.