PopW Increases PAMP-triggered Immunity To Suppress Xanthomonas Euvesicatoria Infection In Tomato

Tomato bacterial spot disease is a wild spread foliar disease occurrence in tomato or pepper caused by Xanthomonas euvesicatoria(X.e.).X.e.infection effects the foliage,fruit,blossoms,and stems of tomato plants caused by disease incidence of 80%and yield loss of more than 60%.The quality of tomato fruit is seriously affected during high diurnal temperature and humidity.Harpin is acidic,heat-stable,glycine-rich,water-soluble protein coding by the hrp gene in many Gram-negative phytopathogens.Harpins can induce the systemic acquired resistance in diverse plant species against various diseases by reducing bacterial growth.Besides,harpins have the ability to decrease the population density of the insects,improve plant growth and drought tolerance.As a new identified harpin protein from Ralstonia solanacearum ZJ3721,PopW performed well in controlling many plant diseases,e.g.tomato leaf mold disease caused by Fulvia fulva,tobacco mosaic virus disease caused by Tobacco mosaic virus(TMV),rice false smut caused by Usrtilaginoidea virens,revealed a broad-spectrum biocontrol capacity.Relevant gene transcription level in plants treated by PopW indicated PopW plays an important role in biocontrol of plant diseases as an SAR(system acquired resistance)elicitor.In this study,PopW as a protein elicitor,improves the plant disease resistance and controls tomato bacterial spot disease caused by X.e.with more than 50%biocontrol efficacy.Moreover,colonizing concentration of X.e.obviously decreased on pretreated tomato leaves by 50 mg/L PopW protein at 4 dpi after X.e.treatment.Apparent reactive oxygen species(ROS)burst and callose deposition were displayed on PopW treatment in cellular level.Along with phenotype changes in cellular level,gene expression level of several PTI pathway relevant genes(PTI5、LRR22、GRAS2)were positively regulated by PopW.We demonstrated that PopW can improve plant resistance against X.e.and confer tomato to early resistance against tomato bacterial spot disease for the first time.Stomata provide an important entrance point for X.e.into the inner leaf tissue of plant.Stomatal immunity is now been well-concerned as one subdiscipline of PTI and can suppress the bacteria into the plant tissue.In this study,we found that X.e.mediates stomatal open,PopW primes stomatal closure,and PopW suppresses X.e.-mediated stomatal opening on tomato leaves.The duration of stomatal closure triggered by PopW was transient(12 h).Challenge inoculation with X.e.reactivated PopW-triggered stomatal closure and extended as far as to 48 hpi.In the progress,the content of ABA and expression of ABA-associated genes CYP707A1 and ABF4 were increased corresponding to reduced stomatal aperture in PopW-pretreated plant leaves,and the ABA-induced stomatal closure signaling pathway inhibitors-butyrate and K252a abolished the stomatal closure and increase of ABA content triggered by PopW.These results revealed that ABA signaling pathway was involved in and could be mainly responsible for PopW-primed stomatal immunity.It is the first report that a harpin protein PopW,as a PAMP factor,restricts the invasion of a foliar pathogen through priming ABA-associated plant stomatal immunity.