Environmental stress and flowering time

After the f loral transition the shoot apical meristem (SAM) changes its identity switching from vegetative to reproductive. In annual Arabidopsis ecotypes, the transition to f lowering is strongly promoted by variations in day length (photoperiod). The photoperiodic pathway promotes f lowering when Arabidopsis plants are exposed to long days (LDs) conditions (typical of spring and summer). Photoperiodic f lowering is the result of complex interactions between the circadian clock (an endogenous timekeeping mechanism) and external cues, which ultimately results in the activation of a set of f loral genes. Central to photoperiod-dependent f lowering is the pattern of accumulation of the f lowering protein CONSTANS (CO). CO expression is regulated transcriptionally by the circadian clock through the GIGANTEA (GI)-FLAVIN-BINDING, KELCH REPEAT, F-BOX (FKF1) complex. LDs also promote the stabilization of CO protein at the end of a LD via activation of the photoreceptors PHYTOCROME A, CRYPTOCHROME 1 and 2 (CRY1 and 2). CO protein promotes the transcriptional activation of the f lorigen genes FLOWERING LOCUS T (FT ) and TWIN SISTER OF FT (TSF ) in the phloem companion cells. FT and FT-likes proteins encode small proteins with similarity to the Raf Kinase Inhibitor Proteins (RKIP). They usually act as systemic signals, since these proteins are able to move between cells. FT protein moves from the leaves to the SAM where it interacts with the SAM-specific bZIP transcription factors FLOWERING LOCUS D (FD) and FD PARALOG (FDP) to initiate the f loral transition. Here, the FT/FD heterodimer activates several MADS box-type transcription factors, namely SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1), APETALA1, and FRUITFUL, responsible for triggering the f loral transition. Florigen gene expression has been demonstrated to play a pivotal role in photoperiodic f lowering in different plants including Arabidopsis, a facultative LD plant and Rice (Oryza sativa), a facultative short day (SD) plant. However, f lorigen expression is not always dependent upon photoperiod variations as in the case of the day neutral plant Tomato (Solanum lycopersicum). This implies that f lorigen upregulation can also occur in response to internal or external stimuli other than variations in day length. The data reviewed here reinforces the idea that the photoperiodic pathway and the f lorigen genes are central nodes of a wider network receiving a multitude of external inputs. Furthermore, mechanisms that couple photoperiodic f lowering with stress acclimation are emerging.