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Exception to the norm: Inactive TAL effectors trigger an atypical resistance mechanism in rice

Alejandra Huerta: Colorado State University

<div>Plant pathogenic <em>Xanthomonas</em> spp. produce numerous transcription activator-like effectors (TALEs) induce expression of host susceptibility genes. TALE-DNA binding is conferred by the central repeat region (CRR) which pairs one repeat to one DNA base in the promoter of host genes. Plants have evolved TALE-associated resistance mechanisms to circumvent pathogen attack, which include dominant TALE-dependent resistance, loss of susceptibility, and dominant non-transcriptional resistance. Comprised in the latter are resistance loci Bs4 and the recently described Xo1 from tomato and rice, respectively. The resistance mechanism of both Bs4 and Xo1 is hypothesized to be dependent on cytoplasmic recognition of the highly conserved CRR but its mechanistic details remain unknown. To better understand how TALE trigger or escape Xo1 detection we aligned the CRR protein sequences of eight Xo1 triggering TALE with two non-triggering effectors. We found consistent variation in the amino acid (AA) residues at position 4, 8, 33 and 35. In addition, these same AA residues were predicted to be solvent exposed. We tested the role of these key AAs in triggering Xo1 by swapping AA residues at these key positions in resistance- and susceptibility-inducing TALEs. A mutated resistance-inducing PthXo1 TALE lost its resistance-triggering ability, indicating that Xo1 recognition is sensitive to solvent exposed AA residues. The strong similarity between Xo1 and Bs4 TAL effectors triggered resistance phenotype suggest that monocots and dicots may share an ancient or convergently evolved mechanism to recognize the conserved structure of TAL effectors.</div>

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