In the rosette plant Arabidopsis thaliana, different environmental cues trigger upward (hyponastic) leaf movement which is evolutionary beneficial when it comes to avoid shading by neighboring plants. Hyponasty results from longitudinal cell expansion at the lower side of the leaf petiole. Here, we report that clade I TGA transcription factors TGA1 and TGA4 are required for hyponastic growth irrespective of whether it is induced by low light intensities, ethylene or heat. Expression of genes encoding cell wall-loosening or auxin-induced proteins was diminished in petioles of the tga1 tga4 double mutant subjected to low light for six hours. Cysteines C260 and C266 of TGA1, which have been reported to be redox-modulated by the defense hormone salicylic acid (SA) in vivo, do not play a role for the function of TGA1 in hyponastic growth. Likewise, they are not important for the inhibition of hyponastic growth by SA. We identified several TGA1/4-interacting glutaredoxins (ROXY8, ROXY9, ROXY20) as potential negative regulators of TGA1/4 activity. Ectopic expression of ROXY8 and ROXY9 led to plants that show the same hyponasty-deficient phenotype as the tga1 tga4 mutant. Expression of ROXY9 was particularly high when hyponastic plants were transferred back to control light conditions. We therefore favour the hypothesis that the above mentioned ROXYs interfere with TGA1/4 activity in order to mediate the downward orientation of hyponastic leaves after transfer to control conditions. The repressive activity of ROXY9 depends on the integrity of the active site motif CCLC, which might be involved in oxidoreductase activity, binding of iron sulfur complexes or other yet unknown functions.
