RESEARCH SUMMARY

Light is a crucial environmental signal to plants since provides energy for photosynthesis and in addition serves as an informational cue of the ambient in which the plant grows. Important light-regulated responses are the de-etiolation of seedlings after seed germination and the response to shade in adult plants, aimed to cope with the competition imposed by other plants. These two developmental responses which are triggered by light/darkness or by changes in the R/FR light ratio are to a large extent regulated by the transcription factor PIF4 (PHYTOCHROME INTERACTING FACTOR 4), which is destabilized by the PHYB photoreceptor in the light.

Work in our group aims to identify which signaling cascades govern etiolated seedling development in the dark and how these cascades are affected by light, with a particular focus in the mechanisms involved in integration of the light signal with the own running developmental programs of the plant. The plant hormones gibberellins (GA) and brassinosteroids (BRs) play a central role in transducing the light signal as judged from the dark de-etiolated phenotype of mutants with a block in the synthesis or response to these hormones. As for GA mutants, this phenotype is caused by the stabilization of the DELLAs, a family of nuclear proteins that repress GA-regulated gene expression and are rapidly destabilized by GAs. Evidence provided by our group showed that DELLAs bind the bHLH DNA recognition domain of the PIF factors and block DNA binding ability of these transcriptional regulators. PIF4 and its close homologue PIF5 regulate the expression of several genes driving cell elongation, by recognizing a G-box element in their promoters, the growth restraint imposed by DELLAs hence being mediated through a block in PIF4/PIF5-mediated activation of these genes.

Regarding BR-deficient/-response mutants an important finding was that these seedlings are insensitive to GAs. This impaired response, however, does not correlate with an increased stability of DELLAs, suggesting that a regulatory step downstream of these repressors mediates GA-/BR- cross-talk. BRs were actually found to stabilize the PIF4/PIF5 factors in the light, these results pointing to a master regulatory role of these transcriptional regulators in the control of genes with a role in cell elongation, these TFs integrating not only light- (by PHYB-mediated destabilization) and GA- signals (by inactive complex formation with DELLAs) but playing a role also in BR signaling, thus serving as a molecular link between the signaling cascades controlling plant growth and elongation and the exterior.

A second important line of research in our group is day length control of storage organ formation in potato. It is well established that day length duration is perceived in the leaves, a tuberization signal known as tuberigen being synthesized in these organs and transported to the underground organs of the plant. Our work has demonstrated that this mobile signal is encoded by an FT-like gene (StSP6A), an additional member of the FT-like gene family (StSP3D/SFT) controlling floral transition independent of day length.