Regulation of clade I TGA transcription factors of Arabidopsis thaliana during salicylic acid-mediated defense response
von Jelena Budimir
Datum der mündl. Prüfung:2019-12-12
Betreuer:Prof. Dr. Christiane Gatz
Gutachter:Prof. Dr. Christiane Gatz
Gutachter:Prof. Dr. Petr Karlovsky
EnglischTGA1 and TGA4 belong to a family of basic leucine zipper transcription factors. They are important activators of salicylic acid (SA) biosynthesis upon pathogen infection (Sun et al. 2018). Moreover, they interact with a master regulator of SA signaling NPR1 in a redox-dependent manner (Després et al. 2003; Lindermayr et al. 2010). However, the role of TGA1 and TGA4 downstream of SA signaling was not known. Here, we report on a group of TGA1/TGA4-dependent genes which were inducible by SA and pathogen infection. The identified genes belonged to the known signaling pathway downstream of SA consisting of NPR1, TGA2/TGA5/TGA6 and SARD1/CBP60g. Because the marker genes were both TGA1/TGA4- and NPR1-dependent, they were used to investigate the redox regulation of TGA1. The tga1 tga4 mutant was complemented either with a wild-type TGA1 protein or a TGA1 protein mutated in such way to mimic the reduced, active form of the protein. We showed that the two types of complementation lines restore wild-type-like behavior after SA treatment and pathogen infection. Thus, we concluded that the redox state of the four critical cysteine residues is not important for the function of TGA1 downstream of SA signaling. In addition to the direct redox-regulation of the cysteine residues, TGA1 and TGA4 proteins are negatively regulated by CC-type glutaredoxins ROXYs (Li et al. 2019). To investigate regulation of TGA factors by glutaredoxins, we generated mutants of two groups of ROXY proteins. The two groups repress either clade I TGA or clade II TGA factors depending on the presence of the C terminal ALWL motif (Zander et al. 2012; Uhrig et al. 2017). The members of the ALWL group, which repress clade II TGA transcription factors, were highly expressed in tga1 tga4 mutant. We showed here that the mutation in five ALWL ROXY genes (ROXY11-15) did not restore wild-type like behavior in tga1 tga4 mutant. However, it is plausible that the twelve remaining ROXYs repress clade II TGA factors in roxy11-15 tga1 tga4 mutant. Furthermore, we generated a mutant of the four ROXY genes lacking the ALWL motif (ROXY6-9), which repress TGA1 and TGA4 function when overexpressed. Here, we showed that the mutation in ROXY6-9 did not lead to the hyperactivity of clade I TGA factors after SA spraying and pathogen infection. However, it is possible that the repression is lifted under different conditions.
Keywords: NPR1; redox regulation; salicylic acid; TGA transcription factors; glutaredoxins; ROXY; Arabidopsis thaliana; defense responses