A transcription activator-like effector from Xanthomonas oryzae pv. oryzicola elicits dose-dependent resistance in rice

HUMMEL, AARON W; WILKINS, KATHERINE E; WANG, LI; CERNADAS, RAUL ANDRES; BOGDANOVE, ADAM JOSEPH

MOLECULAR PLANT PATHOLOGY

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2016

1464-6722

Xanthomonas spp. reduce crop yields and quality worldwide. During infection of their plant hosts, many strains secrete transcription activator-like (TAL) effectors, which enter the host cell nucleus and activate specific corresponding host genes at effector binding elements (EBEs) in the promoter. TAL effectors may contribute to disease by activating expression of susceptibility genes or trigger resistance associated with the hypersensitive reaction (HR) by activating an executor resistance (R) gene. The rice bacterial leaf streak pathogen X. oryzae pv. oryzicola (Xoc) is known to suppress host resistance, and no host R gene has been identified against it, despite considerable effort. To further investigate Xoc suppression of host resistance, we conducted a screen of effectors from BLS256 and identified Tal2a as an HR elicitor in rice when delivered heterologously by a strain of the closely related rice bacterial blight pathogen X. oryzae pv. oryzae (Xoo) or by the soybean pathogen X. axonopodis pv. glycines. The HR required the Tal2a activation domain, suggesting an executor R gene. Tal2a activity was differentially distributed among geographically diverse Xoc isolates, being largely conserved among Asian isolates. We identified four genes induced by Tal2a in next generation RNA sequencing experiments and confirmed them with quantitative real time RT-PCR. However, neither individual nor collective activation of these genes by designer TAL effectors resulted in the HR. A tal2a knockout mutant of BLS256 showed virulence comparable to wild type, but plasmid-based overexpression of tal2a at different levels in the wild type reduced virulence in a directly corresponding way. The results overall reveal that host resistance suppression by Xoc plays a critical role in pathogenesis. Further, the dose-dependent avirulence activity of Tal2a and the apparent lack of a single, canonical target that accounts for the HR point to a novel, activation-domain-dependent mode of action, which might involve, for example, a non-coding gene or a specific pattern of activation across multiple targets.