Shoot-to-root mobile CEPD proteins regulate TGA transcription factors to allow nitrate root-to-shoot transport in Arabidopsis thaliana

In Arabidopsis thaliana, nitrogen (N) starvation leads to increased synthesis of CEPD (C-TERMINALLY ENCODED PEPTIDE DOWNSTREAM) proteins in the shoot. CEPDs travel to the roots, where they activate expression of genes required for high affinity nitrate transport. CEPDs belong to a plant-specific class of glutaredoxin-like proteins that interact with TGACG-binding transcription factors (TGAs). Here we identified the redundant clade-I TGAs TGA1 and TGA4 as the link between CEPDs and target promoters. In the absence of CEPDs, TGA1/4 have a strong negative effect on N starvation-induced gene expression leading to reduced translocation of N from the root to the shoot and to reduced shoot fresh weight. Basal levels of CEPDs were sufficient to completely release TGA1/4-mediated repression of nitrate acquisition. The antagonism between CEPDs and TGA1/4 was also detected in shoots, where CEPDs dampened the activating function of TGA1/4 on hyponasty and defense. CEPDs encode the conserved putative active site motif CxxC/S that was suggested to mediate redox regulation of target proteins. Complementation of the tga1 tga4 mutant with a TGA1 variant containing amino acid substitutions of all four potentially redox-active cysteines showed that CEPDs do not regulate TGA1/4 by modulating their redox state.