Site-directed mutagenesis and functional analysis of hrcU gene from rice pathogen Xanthomonas oryzae pv. oryzae.

The hrp gene cluster is composed of 27 members in rice pathogen Xanthomonas oryzae pv.oryzae(Xoo).To analyze the function of hrcU gene,the hrcU gene was knocked out by allelic exchange.The resulting hrcU mutant lost the pathogenicity in rice and the ability to trigger hypersensitive response in nonhost plant tobacco.The sole hrcU gene fully restored the phenotypic alterations in pathogenicity in rice,hypersensitive response in tobacco,and bacterial growth in rice.Western blot displayed that the typeⅢ secretion system was destroyed in hrcU mutant in which the T3S effectors Hpa1 could not be secreted into extracellular.This indicates that the hrcU gene of Xoo was essential for type Ⅲ secretion system to deliver effectors into plants.