The bipartite function of Clade I TGAs: involvement in basal immunity and systemic acquired resistance

Abstract

When pathogens attack, each plant cell must mount its defense response against the pathogen. Understanding the process of pathogenic infiltration, detection, signalling, and defense is required to produce more resistant crop species and understand the complexities involved. The transcription factor family TGAs contains two members, TGA1/4, which were found to be basal immunity and systemic acquired resistance in plants by large-scale transcriptomic analysis. TGA1/4 were found to negatively regulate WRKYs 15, 40, and 48 genes, which are negative regulators of basal immunity involved in the growth and immunity tradeoff controlled by the phytohormone Brassinoteroid. TGA1/4 was found to directly associate with each WRKY gene promoter and negatively regulate their expression. As a proxy to basal immunity, reactive oxygen species were quantified in COL-0 and tga1tga4 double mutant lines, showing a reduction in ROS expression in the mutant lines when treated with the brassinosteroid-pathway inhibitor, brassinazole. Additionally, TGA1/4 was found to form a repressosome with GRX480 and NPR1 to negatively regulate early-SAR inducible genes, like the SAR marker gene PR-1, by TGA1/4 and GRX480 directly interacting with PR-1 gene promoter. How TGA1/4 mechanistically regulates basal immunity, and SAR is not entirely known, in part due to the lack of structural information about TGA family members as a whole. In silico derived TGA1 bZip homodimer-DNA 3D model, developed through molecular dynamics simulations and alanine scanning, identified essential amino acids required for TGA1-DNA association and stabilization. Some residues outside the range of DNA interaction are critical in DNA stabilization, namely Y106, V107 and E111. Through EMSA experiments TGA1 WT and mutant-dsLS7 dissociation constant (KD) values were determined, showing TGA1-DNA K¬D to be about 22 nM, with a melting temperature of 66.5°C. Continued development of the TGA1-DNA model is required. However, the current model was successfully validated. Taken together, clade I TGAs possess bipartite functions within plant immunity, now being rooted in both the growth and immunity tradeoff and early-SAR induction.