M.Sc Thesis

M.Sc StudentSimaan Hiba
SubjectThe AP-1 Like Transcription Factor ChAP1 Balances
Tolerance and Cell Death in the Response of the
Maize Pathogen Cochliobolus
Heterostrophus to a Plant...
DepartmentDepartment of Biology
Supervisor PROF. Benjamin Horwitz
Full Thesis textFull thesis text - English Version


The oxidant-sensitive transcription factor ChAP1 of Cochliobolus heterostrophus, the Dothideomycete fungus causing Southern corn leaf blight, is an ortholog of yeast YAP1. ChAP1 is retained in the nucleus in response to plant-derived phenolic acids, in addition to the well-studied activation by oxidants. Some phenolics, like ferulic acid (FA) induce rapid regulated cell death (RCD) in the pathogen. The mechanisms of signaling and toxicity are unknown. Tolerance to stress created by the host, and the ability to recover from host-induced cell death, are central to infection by pathogens. Following the intriguing earlier observation that FA promotes nuclear retention of ChAP1 without up-regulating genes for oxidant tolerance, we compared the transcriptomes of the wild type (WT) pathogen and a mutant deleted for ChAP1, to study the function of ChAP1 in response to a phenolic abundant in maize, FA.

The transcriptional signature in response to FA in the wild type compared to the mutant, sheds light on the signaling mechanisms and response pathways by which ChAP1 can mediate tolerance to ferulic acid, distinct from its previously known role in the antioxidant response. 1592 genes were significantly differentially expressed, according to a threshold of 0.05 on the adjusted p-value, between the transcriptome of ∆chap1 and WT under the same conditions. These genes cluster mainly into two groups. Strikingly, the gene expression patterns of Cluster 1 and Cluster 2 are mirror-images. At low concentration of FA (0.5 -1 mM), the gene expression in the mutant in both clusters is up/down regulated in correlation with the wild type. At FA high concentration (2 mM) the regulation of both clusters in ∆chap1 is opposite to that in the WT: genes whose expression continued to increase with FA concentration in the WT are down-regulated in the mutant, and vice-versa. The enrichment of transport and metabolism-related genes in cluster 1 indicates that C. heterostrophus degrades FA and removes it from the cell. When this fails at increasing stress levels, FA provides a signal for cell death, indicated by the enrichment of cell-death related genes in cluster 2, which correspond to some extent with that known from animal cells. By quantitation of survival and by TUNEL assays, we show that ChAP1 promotes survival and mitigates regulated cell death. Despite earlier regulated cell death in the mutant, the whole colony continues to expand. Furthermore, growth rate data show a time window in which the mutant colony expands faster than the wild type. Apparently, RCD at the center of the colony provides nutrients to the remaining viable cells, leading to faster peripheral growth after a specific time.

The results delineate a transcriptional regulatory pattern in which ChAP1 is an important factor in the balance between a detoxification pathway and survival response leading to tolerance to ferulic acid, against a pathway promoting cell death. The findings can be understood in the context of a general model for the transition from a phase where the return to homeostasis dominates, to the onset of cell death in the response of eukaryotic cells to stress.