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Title: Code-assisted discovery of TAL effector targets in bacterial leaf streak of rice reveals contrast with bacterial blight and a novel susceptibility gene

Author
item CERNADAS, RAUL - Iowa State University
item DOYLE, ERIN - Iowa State University
item NINO-LIU, DAVID - Iowa State University
item WILKINS, KATHERINE - Cornell University
item BANCROFT, TIMOTHY - Iowa State University
item WANG, LI - Iowa State University
item SCHMIDT, CLARICE - Iowa State University
item CALDO, RICO - Iowa State University
item YANG, BING - Iowa State University
item WHITE, FRANK - Kansas State University
item NETTLETON, DAN - Iowa State University
item Wise, Roger
item BOGDANOVE, ADAM - Iowa State University

Submitted to: PLoS Pathogens
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/17/2014
Publication Date: 2/27/2014
Citation: Cernadas, R.A., Doyle, E.L., Nino-Liu, D., Wilkins, K.E., Bancroft, T., Wang, L., Schmidt, C., Caldo, R., Yang, B., White, F.F., Nettleton, D., Wise, R.P., Bogdanove, A. 2014. Code-assisted discovery of TAL effector targets in bacterial leaf streak of rice reveals contrast with bacterial blight and a novel susceptibility gene. PLoS Pathogens. DOI: 10.1371/journal.ppat.1003972.

Interpretive Summary: Plant diseases are among the greatest deterrents to crop production worldwide. Pathogenic microbes are able to infect plant cells by producing effector proteins that supersede the host’s immune response. Transcription-activator like (TAL) effectors of plant pathogenic Xanthomonas species induce targeted gene expression in host plants to facilitate bacterial colonization and growth. After injection via the type III secretion system, TAL effectors localize to the nucleus where they bind specific DNA sequences and induce host gene expression. In this work we focused on two important bacterial rice pathogens that cause bacterial blight and bacterial leaf streak. We conducted highly parallel gene expression analyses, followed by computational predictions and experimental validations to pinpoint a novel set of TAL effector targets encoded by the bacterial leaf streak, but not the bacterial blight, pathogen. Using TAL effector mutagenesis and in plant virulence assays, we identified the major TAL effector for bacterial leaf streak virulence, Tal2g, and its biologically relevant host target. We describe a distinct set of host genes induced by bacterial leaf streak TAL effectors. We further present a candidate susceptibility gene for this important pathogen of rice, which should contribute to novel strategies to improve host resistance. Because common themes govern all plant-pathogen interactions, this finding provides new knowledge of broad significance to plant scientists, and to growers who utilize disease resistance to protect their crops.

Technical Abstract: Transcription activator-like (TAL) effectors found in Xanthomonas spp. promote bacterial growth and plant susceptibility by binding specific DNA sequences or, effector-binding elements (EBEs), and inducing host gene expression. In this study, we have found substantially different transcriptional profiles of rice (Oryza sativa) cv. Nipponbare after infiltration with Xanthomonas oryzae pv. oryzae (Xoo) or Xanthomonas oryzae pv. oryzicola (Xoc), two closely related rice pathogens that cause Bacterial blight and Bacterial leaf streak diseases, respectively. Previous studies have revealed the role of several TAL effectors in Bacterial blight, but Xoc TAL effectors have not been characterized. Using the gene expression data and computational analyses, we predicted potential Xoc- and Xoo-TAL effector targets among the rice genes induced after the bacterial infiltration. All characterized Xoo TAL effectors targets were identified in our analysis. For Xoc, out of the 45 predicted targets, we confirmed experimentally that TAL effectors induce 19 host genes. By virulence tests with TAL effector mutants, we identified the major virulence TAL effector of Xoc, Tal2g, which plays a role in disease symptoms development and bacteria exit onto the leaves surface. We found two Tal2g targets among the top Xoc-induced genes. To distinguish which of the Tal2g-targeted genes is biologically relevant for the pathogen, we generated designer TAL effectors (dTALes) to selectively induce the targets and analyze their role separately. These results pointed to a sulfate transporter as a principal susceptibility gene for bacterial leaf streak of rice caused by Xoc.