The Study On The Regulation Mechanism Of Xcc SuxB In Response To Sucrose Signaling

Xanthomonas campestris pv.campestris(Xcc)is an important pathogen of cruciferous plants and an important factor of the decreasing crops yields.Xcc enters plants through water holes or wounds and reproduces in vascular bundles.The nutrients in plant vascular bundles are mainly sucrose,a product of photosynthesis and one of the most important nutritional sources for Xcc growth.Type III secretion system(T3SS)is essential to Xanthomonas virulence.Extracellular signals activate T3 SS to release virulence factors to enable bacteria to rapidly reproduce in host,eventually causing the host leaves to wither and necrosis.The hrp gene clusters encode T3 SS structure proteins and its regulatory components.Hrp G and Hrp X are the key elements regulating T3 SS expressin.However,what signals from plants indcuce T3 SS expression is not clear.In this study,we have screened the signals and investigated the transduction mechanism.Using the T3 SS reporter strain hrc U-lac,we first screened the signals that induce Xcc T3 SS expression and found the photosynthesis product,sucrose,is the most effective induction signal.Then we analyzed the roles of the sucrose transporter and hydrolyase(sux A,sux B,sux C,XC3665,XC3670)in sucrose signaling.Mutation of sux B,not sux A,sux C,XC3665 or XC3670 reduced the abilities of sucrose utilization and T3 SS induction,indicating that sux B is invloved in Xcc metabolism and vital for T3 SS expression.We also detected the virulence of these mutants and found only sux B mutation decreased Xcc infection ability,suggesting sux B is also vital Xcc-plant interaction.As Hrp G and Hrp X are the key regulator for T3 SS induction,we wonder what relationship between sux B and Hrp G as well as Hrp X.We first analyzed whether they can interact with each other by pulldown and bacterial two hybrid assays and found sux B can not directly interact with Hrp G or Hrp X.We further wonder whether sux Bmediated sucrose signaling depends on Hrp G or Hrp X.Double mutation analysis showed that this sucrose signaling is indispensible for Hrp G not Hrp X,suggesting that sux B-mediated sucrose signaling is dependent of Hrp G.Moreover,we found that sux B regulates hrp G expression dependent of sucrose.In summary,our results suggest that sux B is the main sucrose utilization protein of Xcc 8004,and its sucrose utilization ability is positively correlated with pathogenicity.Morover,sux B is also a sucrose signaling componet that regulates T3 SS expression and vital for Xcc-plant interaction.Our data provide new ideas for the control of black rot disease.