Comparison of gene activation by two TAL effectors from Xanthomonas axonopodis pv. manihotis reveals candidate host susceptibility genes in cassava

Xanthomonas axonopodis pv. manihotis (Xam) employs transcription activator-like (TAL) effectors to promote bacterial growth and symptom formation during infection of cassava. TAL effectors are secreted via the bacterial type III secretion system into plant cells, where they are directed to the nucleus, bind DNA in plant promoters and activate the expression of downstream genes. The DNA-binding activity of TAL effectors is carried out by a central domain which contains a series of repeat variable diresidues (RVDs) that dictate the sequence of bound nucleotides. TAL14(Xam668) promotes virulence in Xam strain Xam668 and has been shown to activate multiple cassava genes. In this study, we used RNA sequencing to identify the full target repertoire of TAL14(Xam668) in cassava, which includes over 50 genes. A subset of highly up-regulated genes was tested for activation by TAL14(CIO151) from Xam strain CIO151. Although TAL14(CIO151) and TAL14(Xam668) differ by only a single RVD, they display differential activation of gene targets. TAL14(CIO151) complements the TAL14(Xam668) mutant defect, implying that shared target genes are important for TAL14(Xam668)-mediated disease susceptibility. Complementation with closely related TAL effectors is a novel approach to the narrowing down of biologically relevant susceptibility genes of TAL effectors with multiple targets. This study provides an example of how TAL effector target activation by two strains within a single species of Xanthomonas can be dramatically affected by a small change in RVD-nucleotide affinity at a single site, and reflects the parameters of RVD-nucleotide interaction determined using designer TAL effectors in transient systems.