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Title: RNA-Seq analysis of resistant and susceptible sub-tropical maize lines reveals a role for kauralexins in resistance to grey leaf spot disease, caused by Cercospora zeina

Author
item MEYER, JACQUELINE - Centre For Proteomic And Genomic Research
item BERGER, DAVE - University Of Pretoria
item Christensen, Shawn
item MURRAY, SHANE - University Of Cape Town

Submitted to: BMC Plant Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/18/2017
Publication Date: 11/13/2017
Citation: Meyer, J., Berger, D.K., Christensen, S.A., Murray, S.L. 2017. RNA-Seq analysis of resistant and susceptible sub-tropical maize lines reveals a role for kauralexins in resistance to grey leaf spot disease, caused by Cercospora zeina. Biomed Central (BMC) Plant Biology. 17(1):197.

Interpretive Summary: A recurring global problem is the infection of maize with pathogenic fungi, leading to billions of dollars in annual losses. Cercospora zeina is a common foliar leaf pathogen, documented to cause gray leaf spot (GLS) in many sub-tropical and tropical regions of the world. Some maize lines have inherent resistance to GLS, but the genetic and defense chemical causes of resistance are largely unknown. Scientists at the Center for Medical, Agricultural and Veterinary Entomology in Gainesville, FL, in collaboration with researchers from South Africa, looked at the global genetic responses to GLS infection in both resistant and susceptible lines and found that the genes responsible for the production of the maize antibiotic compounds, kauralexins, to be prominent in the resistant lines. Analysis of defense chemicals revealed that kauralexins were found in significantly higher concentrations in GLS resistant plants compared to susceptible lines. Collectively, this study elucidates important genetic and defense chemical factors that enhance resistance to GLS. Incorporation of this information into molecular breeding practices will contribute to strategies that improve disease resistance in maize.

Technical Abstract: Cercospora zeina is a foliar pathogen responsible for maize grey leaf spot in southern Africa that negatively impacts maize production. Plants use a variety of chemical and structural mechanisms to defend themselves against invading pathogens such as C. zeina, including the production of secondary metabolites with antimicrobial properties. In maize, a variety of biotic and abiotic stressors induce the accumulation of the terpenoid phytoalexins, zealexins and kauralexins. In this study, we have used global transcriptome analysis to assess the molecular response of a resistant and susceptible maize line to C. zeina. This was followed by targeted metabolomic profiling to characterize the production of foliar phytoalexins in response to C. zeina infection. In addition, corresponding gene expression changes in specific phytoalexin biosynthetic genes were analyzed. C. zeina-susceptible line displayed pervasive rectangular grey leaf spot lesions, running parallel with the leaf veins in contrast to C. zeina-resistant line that had restricted disease symptoms. Analysis of the transcriptome of both lines indicated that genes involved in primary and secondary metabolism were up-regulated, and although different pathways were prioritized in each line, production of terpenoid compounds were common to both. Targeted phytoalexin analysis revealed that C. zeina inoculated leaves accumulated zealexins and kauralexins. The resistant line shows a propensity toward accumulation of the kauralexin B series metabolites in response to infection, which contrasts with the susceptible line that preferentially accumulates the kauralexin A series. Kauralexin accumulation was correlated to expression of the kauralexin biosynthetic genes, ZmAn2 and ZmKSL2. We report the expression of a putative copalyl diphosphate synthase gene that is induced by C. zeina in the resistant line exclusively. This study shows that zealexins and kauralexins, and expression of their biosynthetic genes, are induced by C. zeina in both resistant and susceptible germplasm adapted to the southern African climate. The data presented here indicates that different forms of kauralexins accumulate in the resistant and susceptible maize lines in response to C. zeina, with the accumulation of kauralexin B compounds in a resistant maize line and kauralexin A compounds accumulating in the susceptible line.