Phloem transport of flowering signals

Abstract

Seasonal variability in environmental parameters such as day length regulates many aspects of plant development. The transition from vegetative growth to flowering in Arabidopsis is regulated by seasonal changes in day length through a genetically defined molecular cascade known as the photoperiod pathway. Recent advances were made in understanding the tissues in which different components of the photoperiod pathway act to regulate floral induction. These studies highlighted the key role of the FT protein, which is produced in the leaves in response to inductive day lengths and traffics through the phloem to initiate flowering at the shoot apex. Unveiling the cellular and molecular details of this systemic signaling process will be required for a complete understanding of flowering regulation and other photoperiodic processes. IntroductionIn many plants flowering is induced only on exposure to appropriate environments. Day length and temperature are the most important of these environmental signals, providing seasonal cues that enable varieties of a species to become adapted to life at particular latitudes or altitudes. Flower development occurs at the shoot apical meristem or in lateral meristems, but these are often covered in growing leaves so that their exposure to light is limited. Thus, it is, perhaps, not surprising that day length should be perceived in the leaves, as these evolved to maximize light perception for photosynthesis. The spatial separation between the organs that perceive light and those in which floral development occurs raises issues of the mechanisms by which the floral signal is communicated between organs of the plant. Recently, work in Arabidopsis provided evidence that the small FLOWERING LOCUS T (FT) protein is at least a component of this signal, and work in other species strengthened this conclusion. These results were reviewed at length during the last year [1,2]. Here, we briefly summarize the molecular-genetic data and give emphasis to papers that appeared most recently, as well as to the questions of cell biology that arise from current models. Pathways that regulate flowering in the leafArabidopsis flowers early under long days characteristic of summer and late under short winter days. Isolation of lateflowering mutants of Arabidopsis that are insensitive to day length, flowering at similar times under long and short days, provided access to genes that regulate flowering in response to photoperiod [1,2]. These genes comprise a regulatory pathway often referred to as the photoperiodic flowering pathway or long-day pathway. The mRNA of two of the genes in this pathway, FT and its close homolog TWIN SISTER OF FT (TSF), are expressed under long days but not short days [3][4][5]. The FT and TSF proteins belong to the CETS family, named after the three founding members, CENTRORADIALIS, TERMINAL FLOWER, and SELF PRUNING [6], and strongly promote flowering [3,7,5]. The mechanisms that lead to FT activation specifically under long days involve transcriptional and post-transcriptional ...

TheFT-LikeZCN8Gene Functions as a Floral Activator and Is Involved in Photoperiod Sensitivity in Maize

“…Thus, transient FT mRNA accumulation caused by a short inductive photoperiod is sufficient to promote flowering (Corbesier et al, 2007). Using transgenic plants ectopically expressing fluorescent-tagged FT protein, it was shown that the FT protein moved from the vascular tissue to the SAM (Corbesier et al, 2007;Jaeger and Wigge, 2007;Lin et al, 2007;Mathieu et al, 2007;Tamaki et al, 2007;Giakountis and Coupland, 2008). There, the FT protein interacts with the bZIP transcription factor encoded by FLOWERING LOCUS D (FD) to induce expression of a number of floral identity genes Wigge et al, 2005).…”

VEGETATIVE1 is essential for development of the compound inflorescence in pea

“…This indicates that VEG1 acts downstream of VEG2 and GIGAS and suggests that they participate in the activation of VEG1 expression. This seems particularly likely for GIGAS 16 , in view of the fact that its Arabidopsis homologue FLOWERING LOCUS T (FT) is a direct activator of the floral identity genes AP1 and FUL [32][33][34][35] , which are MADS-box genes from the same lineage as VEG1 (Supplementary Fig. S3).…”

Orchestration of the Floral Transition and Floral Development inArabidopsisby the Bifunctional Transcription Factor APETALA2

“…It is therefore no surprise that a finely nuanced gene regulatory network evolved to translate environmental and endogenous information, integrate the diverse inputs, and commit resources to initiate flowering when the time is most favorable. The emerging view of the regulatory network governing the reproductive transition in Arabidopsis thaliana is that of a tuned balance of repressors and activators, both classes operating throughout the plant tissues and at every level in the flowering time network topology (Kobayashi and Weigel, 2007;Giakountis and Coupland, 2008;Yant et al, 2009). Both positive and negative inputs are conveyed along initially distinct, but later crosstalking pathways to the shoot apex, where the transition is finally realized in morphological changes.…”

“…In addition to this balancing act of FT promotion and transcriptional repression, another level of repression occurs when FT reaches the shoot apex. TERMINAL FLOWER1 (TFL1), itself a floral repressor with a high degree of homology to FT is thought to compete directly with FT for heterodimerization with FD at the shoot apex (Abe et al, 2005;Hanzawa et al, 2005;Wigge et al, 2005;Ahn et al, 2006;Giakountis and Coupland, 2008).…”