Tal6b/AvrXa27A,a hidden TALE targeting the susceptibility gene OsSWEET11a and the resistance gene Xa27 in rice

Xanthomonas oryzae pv.oryzae(Xoo)secretes transcription activator-like effectors(TALEs)to activate rice susceptibility(S)genes,causing bacterial blight(BB),as well as resistance(R)genes,leading to de-fense against BB.This activation follows a gene-for-gene paradigm that results in an arms race between the TALE of the pathogen and effector-binding elements(EBEs)in the promoters of host genes.In this study,we characterized a novel TALE,designated Tal6b/AvrXa27A,that activates the rice S gene OsSWEET11a and the rice R gene Xa27.Tal6b/AvrXa27A is a member of the AvrXa27/TalAO class and contains 16 repeat variable diresidues(RVDs);one RVD is altered and one is deleted in Tal6b/AvrXa27A compared with AvrXa27,a known avirulence(avr)effector of Xa27.Tal6b/AvrXa27A can transcriptionally activate the expression of Xa27 and OsSWEET11a via EBEs in their corresponding promoters,leading to effector-triggered immunity and susceptibility,respectively.The 16 RVDs in Tal6b/AvrXa27A have no obvious similarity to the 24 RVDs in the effector PthXo1,but EBETal6b and EBEPthXo1 are overlapped in the OsSWEET11a promoter.Tal6b/AvrXa27A is prevalent among Asian Xoo isolates,but PthXo1 has only been reported in the Philippine strain PXO99A.Genome editing of EBETal6b in the OsSWEET11a pro-moter further confirmed the requirement for OsSWEET11a expression in Tal6b/AvrXa27A-dependent susceptibility to Xoo.Moreover,Tal6b/AvrXa27A resulted in higher transcription of Xa27 than of OsSWEET11a,which led to a strong,rapid resistance response that blocked disease development.Thesefindings suggest that Tal6b/AvrXa27A has a dual function:triggering resistance by activating Xa27 gene expression as an avirulence factor and inducing transcription of the S gene OsSWEET11a,resulting in virulence.Intriguingly,Tal6b/AvrXa27A,but not AvrXa27,can bind to the promoter of OsSWEET11a.The underlying recognition mechanism for this binding remains unclear but appears to deviate from the currently accepted TALE code.

Designer TAL Effectors Induce Disease Susceptibility and Resistance to Xanthomonas oryzae pv. Oryzae in Rice

TAL （transcription activator-like） effectors from Xanthomonas bacteria activate the cognate host genes, leading to disease susceptibility or resistance dependent on the genetic context of host target genes. The modular nature and DNA recognition code of TAL effectors enable custom-engineering of designer TAL effectors （dTALE） for gene activation. However, the feasibility of dTALEs as transcription activators for gene functional analysis has not been demonstrated. Here, we report the use of dTALEs, as expressed and delivered by the pathogenic Xanthomonas oryzae pv. oryzae （Xoo）, in revealing the new function of two previously identified disease-related genes and the potential of one developmental gene for disease susceptibility in rice/Xoo interactions. The dTALE gene dTALE-xa27, designed to target the susceptible allele of the resistance gene Xa27, elicited a resistant reaction in the otherwise susceptible rice cultivar IR24. Four dTALE genes were made to induce the four annotated Xa27 homologous genes in rice cultivar Nipponbare, but none of the four induced Xa27-1ike genes conferred resistance to the dTALE-containing Xoo strains. A dTALE gene was also generated to activate the recessive resistance gene xa13, an allele of the disease-susceptibility gene Os8N3 （also named Xa13 or OsSWEETll, a member of sucrose efflux transporter SWEET gene family）. The induction of xa13 by the dTALE rendered the resistant rice IRBB13 （xa13/xa13） susceptible to Xoo. Finally, OsSWEET12, an as-yet uncharacterized SWEET gene with no corresponding naturally occurring TAL effector identified, conferred susceptibility to the Xoo strains expressing the corresponding dTALE genes. Our results demonstrate that dTALEs can be delivered through the bacterial secretion system to activate genes of interest for functional analysis in plants.