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TAL effector targets the abscisic acid biosynthesis pathway for disease susceptibility in bacterial leaf streak of wheat

Zhao Peng: University of Florida

<div>Plant susceptibility (S) genes are targeted by transcription activator-like (TAL) effectors in a variety of disease complexes caused by <em>Xanthomonas</em> species. Here, we show that the TAL effector Tal8 from <em>X. translucens</em> pv. <em>undulosa</em> (Xtu) XT4699 targets a novel S gene, encoding nine-cis-epoxycarotenoid dioxygenase (NCED), to enhance susceptibility in bacterial leaf streak of wheat. The mutant M6 sustained the loss of <em>tal8</em>, resulting in reduced lesion expansion compared to the wild type (WT) strain. Transcriptional profiling of samples inoculated with M6 or WT revealed two candidate upregulated S genes, <em>TaNCED</em> and <em>TaERF</em>. Homeologs of <em>TaNCED</em> encode enzymes that catalyze the rate-limiting step in the biosynthesis of phytohormone abscisic acid (ABA), while homeologs of <em>TaERF</em> encode ethylene-responsive transcription factors. Expression of <em>TaNCED</em> and <em>TaERF </em>was associated with higher virulence of some Xtu strains but not with low virulence strains. The transformation of <em>tal8 </em>into the low virulence strain LW16 resulted in elevated expression of <em>TaNCED</em> and <em>TaERF</em> and enhanced virulence. Synthetic designer TAL effectors (dTALes) targeting <em>TaNCED</em> resulted in elevated ABA levels and enhanced virulence, while dTALes targeting <em>TaERF</em> were ineffective. Exogenous application of ABA onto plant leaves greatly promoted bacterial virulence. The results suggest that Xtu strains gain virulence by targeting ABA biosynthesis genes and elevating the ABA content of the infected tissue.</div>