Consequences of adaptation of TAL effectors on host susceptibility to Xanthomonas.

Doron Teper,Nian Wang,Gregory P Copenhaver

doi:10.1371/journal.pgen.1009310

Doron Teper, Nian Wang + Show 1 more

Open Access

https://doi.org/10.1371/journal.pgen.1009310

Copy DOI

Journal: PLoS Genetics	Publication Date: Jan 19, 2021
Citations: 22	License type: CC BY 4.0

Affiliation: Florida Department of Citrus, University of Florida

Abstract

Transcription activator-like effectors (TALEs) are virulence factors of Xanthomonas that induce the expression of host susceptibility (S) genes by specifically binding to effector binding elements (EBEs) in their promoter regions. The DNA binding specificity of TALEs is dictated by their tandem repeat regions, which are highly variable between different TALEs. Mutation of the EBEs of S genes is being utilized as a key strategy to generate resistant crops against TALE-dependent pathogens. However, TALE adaptations through rearrangement of their repeat regions is a potential obstacle for successful implementation of this strategy. We investigated the consequences of TALE adaptations in the citrus pathogen Xanthomonas citri subsp. citri (Xcc), in which PthA4 is the TALE required for pathogenicity, whereas CsLOB1 is the corresponding susceptibility gene, on host resistance. Seven TALEs, containing two-to-nine mismatching-repeats to the EBEPthA4 that were unable to induce CsLOB1 expression, were introduced into Xcc pthA4:Tn5 and adaptation was simulated by repeated inoculations into and isolations from sweet orange for a duration of 30 cycles. While initially all strains failed to promote disease, symptoms started to appear between 9-28 passages in four TALEs, which originally harbored two-to-five mismatches. Sequence analysis of adapted TALEs identified deletions and mutations within the TALE repeat regions which enhanced putative affinity to the CsLOB1 promoter. Sequence analyses suggest that TALEs adaptations result from recombinations between repeats of the TALEs. Reintroduction of these adapted TALEs into Xcc pthA4:Tn5 restored the ability to induce the expression of CsLOB1, promote disease symptoms and colonize host plants. TALEs harboring seven-to-nine mismatches were unable to adapt to overcome the incompatible interaction. Our study experimentally documented TALE adaptations to incompatible EBE and provided strategic guidance for generation of disease resistant crops against TALE-dependent pathogens.

Highlights

Transcription activator-like effectors (TALEs) are bacteria-encoded eukaryotic transcriptional activators delivered into host cells through the type III secretion system (T3SS) [1]
Multiple X. oryzae pv. oryzae (Xoo) TALEs induce the expression of rice SWEET sugar transporter genes to facilitate sucrose and glucose efflux [12,13,14], Tal2g of X. oryzae pv. oryzicola promotes lesion expansion and bacteria exudation by inducing the expression of sulfate transporter gene [15], AvrHah1of X. gardneri indirectly stimulates the expression of a pectate lyase gene to promote the accumulation of apoplectic fluid [16], AvrBs3 of X. euvesicatoria causes cell hypertrophy through increasing expression of pepper UPA20 [17], Tal8 of X. translucens promotes accumulation of ABA through induction of NCED in wheat [18], and PthA4 of X. citri ssp. citri (Xcc) induces hypertrophy and hyperplasia through induction of citrus CsLOB1 [19,20,21,22]
We investigated the variations among TALEs (PthA4 and homologs) that target LOB1 by analyzing all available Xcc and X. citri ssp. aurantifolii (Xca) deposits in the NCBI database

Summary

Introduction

Transcription activator-like effectors (TALEs) are bacteria-encoded eukaryotic transcriptional activators delivered into host cells through the type III secretion system (T3SS) [1]. The 12th and 13th amino acids of each repeat, known as the “repeat-variable diresidue” (RVD), vary between repeats and dictate the affinity of each repeat to an individual nucleotide [3] Through this recognition mechanism, the TALE repeat array determines the binding specificity of each TALE to a DNA sequence located in the promoter of host target genes that serves as an effector-binding element (EBE) [4]. Oryzicola promotes lesion expansion and bacteria exudation by inducing the expression of sulfate transporter gene [15], AvrHah1of X. gardneri indirectly stimulates the expression of a pectate lyase gene to promote the accumulation of apoplectic fluid [16], AvrBs3 of X. euvesicatoria causes cell hypertrophy through increasing expression of pepper UPA20 [17], Tal of X. translucens promotes accumulation of ABA through induction of NCED in wheat [18], and PthA4 of X. citri ssp. Multiple X. oryzae pv. oryzae (Xoo) TALEs induce the expression of rice SWEET sugar transporter genes to facilitate sucrose and glucose efflux [12,13,14], Tal2g of X. oryzae pv. oryzicola promotes lesion expansion and bacteria exudation by inducing the expression of sulfate transporter gene [15], AvrHah1of X. gardneri indirectly stimulates the expression of a pectate lyase gene to promote the accumulation of apoplectic fluid [16], AvrBs3 of X. euvesicatoria causes cell hypertrophy through increasing expression of pepper UPA20 [17], Tal of X. translucens promotes accumulation of ABA through induction of NCED in wheat [18], and PthA4 of X. citri ssp. citri (Xcc) induces hypertrophy and hyperplasia through induction of citrus CsLOB1 [19,20,21,22]

Methods

Results

Discussion

Conclusion