Insight from expression profiles of FT orthologs in plants: conserved photoperiodic transcriptional regulatory mechanisms

Abstract

Floral transition from the vegetative to the reproductive stages is precisely regulated by both environmental and endogenous signals. Among these signals, photoperiod is one of the most important environmental factors for onset of flowering. A florigen, FLOWERING LOCUS T (FT) in Arabidopsis, has thought to be a major hub in the photoperiod-dependent flowering time regulation. Expression levels of FT likely correlates with potence of flowering. Under long days (LD), FT is mainly synthesized in leaves, and FT protein moves to shoot apical meristem (SAM) where it functions and in turns induces flowering. Recently, it has been reported that Arabidopsis grown under natural LD condition flowers earlier than that grown under laboratory LD condition, in which a red (R)/far-red (FR) ratio of light sources determines FT expression levels. Additionally, FT expression profile changes in response to combinatorial effects of FR light and photoperiod. FT orthologs exist in most of plants and functions are thought to be conserved. Although molecular mechanisms underlying photoperiodic transcriptional regulation of FT orthologs have been studied in several plants, such as rice, however, dynamics in expression profiles of FT orthologs have been less spotlighted. This review aims to revisit previously reported but overlooked expression information of FT orthologs from various plant species and classify these genes depending on the expression profiles. Plants, in general, could be classified into three groups depending on their photoperiodic flowering responses. Thus, we discuss relationship between photoperiodic responsiveness and expression of FT orthologs. Additionally, we also highlight the expression profiles of FT orthologs depending on their activities in flowering. Comparative analyses of diverse plant species will help to gain insight into molecular mechanisms for flowering in nature, and this can be utilized in the future for crop engineering to improve yield by controlling flowering time.

Keywords: FLOWERING LOCUS T; expression profiles; florigen; flowering plants; photoperiodic flowering.

Figure 1

Diurnal expression profiles of FLOWERING LOCUS T (FT) in Arabidopsis seedlings. FT expression peaks once in a day at the end of day under laboratory long day (LD) conditions (A). Under natural LDs or when far-red (FR) light is supplemented in LD conditions (LD + FR), expression of FT peaks twice in a day in the morning and the evening (B). FT expression is low in short day (SD) conditions, both in the absence and presence of supplementary FR light (SD+FR) (C). Yellow and orange boxes indicate day period without and with supplementary FR light, respectively. Night period is indicated as dark or grey boxes.

Figure 2

Gene structures and photoperiod-dependent expression profiles of FT and its ortholog genes. The gene structures of FT and FT orthologs in LDPs (A), SDPs (B), and a DNP (C) are indicated with their respective photoperiod-dependent expression profiles. Floral activities are denoted below the FT orthologs in parenthesis. In the gene structures, black and grey boxes represent exons and UTRs, respectively, and numbers above the exons indicate size of each exon in base pairs. A set of three circles or rectangles indicates expression patterns of FT ortholog in either SD or LD, depending on the photoperiod in which expression levels are high. The photoperiod-dependent expression profiles are denoted by the shape type; circles mean higher expression in SD than in LD, while rectangles mean higher expression in LD than in SD. Time-of-day expression profiles are indicated by the position of filled shapes; filled circles or rectangles denote expression peaks observed in the morning (position 1) or in the evening (position 2). A filled circle or rectangle at position 3 indicates that a gene is expressed irrespective of the time of day. Scale bar = 500 bp.