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

Jacqueline Meyer,Dave K Berger,Shane L Murray,Shawn A Christensen

doi:10.1186/s12870-017-1137-9

Abstract

BackgroundCercospora 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.ResultsC. 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-regualted, 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 gene, ZmAn2 and a candidate biosynthetic gene, ZmKSL2. We report the expression of a putative copalyl diphosphate synthase gene that is induced by C. zeina in the resistant line exclusively.DiscussionThis 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.

Highlights

Cercospora zeina is a foliar pathogen responsible for maize grey leaf spot in southern Africa that negatively impacts maize production
RIL387 and RIL165 had different alleles at four of the eight quantitative trait loci (QTL) previously identified for severity of Grey leaf spot (GLS) disease (Additional file 1) [21], with only RIL387 carrying the resistance allele for three QTL (QTL6, QTL9b and QTL10) and RIL165 only exhibiting the resistance allele for QTL9a
We found that the C. zeina-susceptible RIL165 accumulates transcripts encoding proteins partaking in both primary and secondary metabolism as the plant defended itself against the fungal infection

Summary

Introduction

Cercospora zeina is a foliar pathogen responsible for maize grey leaf spot in southern Africa that negatively impacts maize production. A variety of biotic and abiotic stressors induce the accumulation of the terpenoid phytoalexins, zealexins and kauralexins. Cereal crops such as maize (Zea mays), wheat (Triticum aestivum), and rice (Oryza sativa) are essential components of consumer diets throughout the world. Grey leaf spot (GLS) is a foliar disease prevalent in South Africa, especially in Kwazulu Natal, where hot and humid climate conditions are favourable for the development of the disease [1]. Fungicide control is expensive and not necessarily effective and C. zeina is rampant in southern Africa [1], severely reducing grain yields, with losses of up to 60% reported [5]. C. zeina spores are allowed to remain on dead leaf material from previous harvests, readily infecting subsequent crops [1]

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