The host response of maize towards Exserohilum turcicum and its toxin, monocerin

Abstract

Northern leaf blight (NLB) is a devastating foliar disease of maize (Zea mays L.) throughout the maize growing regions of the world. The causal agent of NLB is the hemibiotrophic fungal pathogen, Exserohilum turcicum. Exserohilum turcicum produces a secondary metabolite, monocerin, which is phytotoxic and could aid the fungus in causing NLB in maize. This study was conceptualised to assess the infection strategy of E. turcicum in maize, as well as the host response of maize towards E. turcicum and its toxin monocerin. The infection strategy of E. turcicum was evaluated through the use of high resolution light (LM), scanning (SEM) and transmission electron microscopy (TEM) to obtain a better understanding of the hemibiotrophic lifestyle of the fungus. During the biotrophic phase of fungal infection, the disease was characterised by chlorotic spots whereas cigar shaped lesions formed during the necrotrophic phase. Infection structures as well as conidiophores were observed for the first time through SEM. At 9 days post inoculation (9 dpi) the fungus was observed in the epidermal cells, visible in the xylem at 11 dpi, at 14 dpi the xylem was almost completely blocked, and at 18 dpi conidiophores formed through the stomata, and the fungus completed its life cycle. The results of this study provide updated insight into the infection strategy of the fungus in maize as well as supporting previous findings that E. turcicum is a hemibiotrophic pathogen. Pathogenesis-related (PR) proteins are one of the many defence mechanisms plants use to protect themselves against fungal infection. Reverse transcription-quantitative PCR was applied to evaluate whether PR protein genes were upregulated in maize in response to E. turcicum infection and the presence of the E. turcicum toxin, monocerin. Expression of selected PR protein genes (PR-1, PR-2, PR-3, PR10) associated with fungal infection was induced in response to the fungus but only during the necrotrophic phase of the fungal growth. Monocerin did induce the gene expression of PR proteins but at a low level when compared to the fungus. PR-10 (ribonuclease-like) was the only PR protein gene which was induced at a higher level by monocerin as compared to the fungus. The phytotoxic effects of monocerin on the maize leaf cell ultrastructure were studied using LM and TEM. The cytoplasm as well as the vacuole and chloroplast were most affected by the phytotoxic nature of monocerin. The chloroplast was the most sensitive to the toxin due to disruption of the double-membrane, stroma and thylakoid membranes. As monocerin treatment caused an over accumulation of starch granules in the chloroplast, the gene expression of enzymes (gwd, pwd, amy3) involved in degradation of starch granules in the chloroplast was assessed following fungal infection and monocerin treatment. Expression of the all the starch degradation enzymes genes was inhibited during fungal infection but only amy3 was inhibited by monocerin treatment. Response of the maize host to E. turcicum infection and monocerin infiltration provided new understanding in the host-pathogen interaction which could be exploited in developing new control strategies against NLB in maize.