Analysis of the Cercospora zeina transciptome in the early, mid and late stages of gray leaf spot disease on maize

Cercospora zeina is a fungal pathogen that causes gray leaf spot (GLS) disease on maize (Zea mays L.) in South Africa. Upon landing on a maize leaf, the pathogen rapidly enters through the stomata and continues to develop asymptomatically for up to 28 days before symptoms are seen. As previous work has yet to adequately determine how the pathogen behaves during its infective period, we used transcriptomics to gain insights about the in-planta development of the pathogen and explore how it uses its effectors to facilitate this process. Samples from B73 maize inbreds infected with C. zeina were harvested in a time course experiment and used for RNA sequencing. We used reads mapped to the C. zeina genome as a proxy for biomass accumulation. At the end of the latent period, C. zeina was found to rapidly accumulate biomass and showed a nearly 50-fold increase in biomass as symptoms appeared. There were two distinct transcriptional waves occurring across the infection period. The first wave showed expression of genes for cellular growth, maintenance and immune avoidance, whereas the second wave was enriched with genes involved in detoxification and carbohydrate catabolism. A total of 140 putative effector genes were differentially expressed over the time-courses, with most up-regulated during the mid stage when the switch to necrotrophy occurs. Transient expression of three of these C. zeina effectors (CzEcp2, CzNis1a, CzNis1b) induced plant immunity in Nicotiana spp. resulting in the development of a hypersensitive response. This suggests that a cohort of C. zeina effectors expressed at this time have functions for which receptors have evolved in non-host species like tobacco. Altogether, this work suggests C. zeina behaves as a latent necrotroph during infection and provides a foundation for future research into the infection biology of C. zeina on maize. Overall design: Infection trials were set up by inoculating B73 maize plants with the C. zeina CMW 25467 strain. Samples were harvested at three timepoints: 23, 30 and 44-days post infection (dpi) with four replicates harvested per timepoint. Mock-inoculated plants inoculated with Tween 20 (0.02%) were used as a control for the experiment.