Cell elongation during seedling development is antagonistically regulated by light and gibberellins (GAs) 1,2 . Light induces photomorphogenesis, leading to inhibition of hypocotyl growth, whereas GAs promote etiolated growth, characterized by increased hypocotyl elongation. The mechanism underlying this antagonistic interaction remains unclear. Here we report on the central role of the Arabidopsis thaliana nuclear transcription factor PIF4 (encoded by PHYTOCHROME INTERACTING FACTOR 4) 3 in the positive control of genes mediating cell elongation and show that this factor is negatively regulated by the light photoreceptor phyB (ref. 4) and by DELLA proteins that have a key repressor function in GA signalling 5 . Our results demonstrate that PIF4 is destabilized by phyB in the light and that DELLAs block PIF4 transcriptional activity by binding the DNA-recognition domain of this factor. We show that GAs abrogate such repression by promoting DELLA destabilization, and therefore cause a concomitant accumulation of free PIF4 in the nucleus. Consistent with this model, intermediate hypocotyl lengths were observed in transgenic plants over-accumulating both DELLAs and PIF4. Destabilization of this factor by phyB, together with its inactivation by DELLAs, constitutes a protein interaction framework that explains how plants integrate both light and GA signals to optimize growth and development in response to changing environments.Seedlings undergo alternative developmental programmes depending on whether they are germinated in the dark or in the light. Dark-grown seedlings exhibit etiolated growth, characterized by long hypocotyls, small and closed cotyledons with undifferentiated chloroplasts, and the repression of light-regulated genes 1 . During photomorphogenesis, light inhibits hypocotyl growth and promotes cotyledon opening and expansion, chloroplast differentiation and the activation of light-regulated genes. phyB is the main photoreceptor mediating de-etiolation in red light 4,6 . Absorption of red light converts this photoreceptor into a Pfr active form that is translocated into the nucleus 7,8 ; Pfr interacts there with members of the bHLH family of phytochrome-interacting factors (PIFs), involved in modulation of light-regulated genes with a role in photomorphogenesis 1,4 . Gibberellins (GAs) exert an opposite effect to light on photomorphogenesis 2 . GAs promote etiolated growth, whereas GA-deficiency induces a partially de-etiolated phenotype in the dark, which is reverted by a lack of DELLA function 2,9 . DELLAs function as key repressors of GA-responsive growth, by inhibiting GA-regulated gene expression 5 . These repressors accumulate in the nucleus and are rapidly degraded in response to GA 10,11 . In Arabidopsis, RGA (encoded by repressor of ga1-3) and GAI (encoded by GA insensitive) are the main repressors controlling hypocotyl growth and stem elongation 12,13 . Mutations within the DELLA domain render these proteins resistant to degradation, and result in a GA-insensitive dwarf phenotype 12,14 . This ...

Fluctuations in day length determine the time to flower in many plants and in potato are critical to promote differentiation of tubers. Day length is perceived in the leaves and under inductive conditions these synthesize a systemic signal that is transported to the underground stolons to induce tuber development. Flowering tobacco shoots grafted into potato stocks promote tuberization in the stocks, indicating that the floral and tuber-inducing signals might be similar. We describe recent progress in the identification of the molecular mechanisms underlying day-length recognition in potato. Evidence has been obtained for a conserved function of the potato orthologs of the CONSTANS (CO) and FLOWERING LOCUS T (FT) proteins in tuberization control under short days (SDs). These observations indicate that common regulatory pathways are involved in both flowering and tuberization photoperiodic responses in plants.