The plant hormones gibberellins (GAs) regulate multiple processes of plant development, such as seed germination, photomorphogenesis, vegetative growth, and flower and fruit development. Most of this regulation occurs at the transcriptional level, through the activity of the DELLAs, which are nuclear-localized proteins subjected to GA-mediated proteolitic degradation. DELLAs do not bind DNA directly, but they have been shown to interact with DNA-binding transcription factors to regulate their targets. Moreover, DELLAs mobilizes distinct set of genes to trigger different GAs responses. In Arabidopsis, DELLAs are encoded by five genes, and genetic studies show that each DELLA displays specific, but also partially overlapping roles with respect to their paralogs. In this Thesis, we have addressed two issues: (1) the contribution of DELLA multiplication to the diversification of functions controlled by GAs; and (2) the identification of direct targets regulated by DELLAs in etiolated seedlings with special attention to those involved in differential growth processes.
Using combinations of mutants and transgenic lines expressing two phylogenetically distant DELLA genes (RGA and RGL2), we have found that these two DELLA proteins can perform each other's role as long as they are expressed under the reciprocal promoters, indicating that DELLA subfunctionalization relies mainly on their differential expression patterns. In agreement with this, none of these DELLA proteins displayed significant differences in their ability to interact with several bHLH transcription factors, again suggesting that the function of each DELLA protein probably depends on the set of transcription factors to which they are exposed, and their mutual interactions.
To identify direct DELLA targets, we have performed transcriptomic analyses of dark-grown seedlings expressing an inducible version of gai-1, a stable, dominant allele of a DELLA gene. This approach rendered a list of over 150 genes differentially expressed between 30 and 240 min after induction of gai-1. Further in silico analysis of this set of targets has confirmed the functional interaction between DELLAs and bHLH transcription factors, but has also allowed the identification of additional transcription factor families putatively involved in transcriptional regulation by DELLAs.
The presence of several auxin-related genes among the primary targets of DELLA proteins has allowed us to establish a new role for GAs in the modulation of hypocotyl gravitropism through the repression of IAA19/MASSUGU2 expression by DELLAs. Our results also suggest that this regulatory module fine tunes the auxin-driven gravitropic response, providing flexibility under competing tropic stimuli. Moreover, the repression of HOOKLESS1 and the auxin efflux carriers PIN3 and PIN7 by DELLAs, is proposed as the molecular mechanism to explain the already known physiological regulation of apical hook development by GAs.