Independent Control of Gibberellin Biosynthesis and Flowering Time by the Circadian Clock in Arabidopsis

Abstract

Flowering of the facultative long-day plant Arabidopsis is controlled by several endogenous and environmental factors, among them gibberellins (GAs) and day length. The promotion of flowering by long days involves an endogenous clock that interacts with light cues provided by the environment. Light, and specifically photoperiod, is also known to regulate the biosynthesis of GAs, but the effects of GAs and photoperiod on flowering are at least partially separable. Here, we have used a short-period mutant, toc1, to investigate the role of the circadian clock in the control of flowering time by GAs and photoperiod. We show that toc1 affects expression of several floral regulators and a GA biosynthetic gene, but that these effects are independent.Flowering in Arabidopsis is regulated by multiple environmental and endogenous cues, such as light, temperature, nutrient availability, and age (or developmental stage; Simpson et al., 1999). In most Arabidopsis accessions, long days, higher growth temperatures, and transient exposure to winter-like low temperatures (vernalization) promote faster flowering, whereas short days, lower growth temperatures, and absence of vernalization represent noninductive conditions, under which flowering is delayed.In addition to environmental signals, several hormones are known to take part in the control of flowering time. In Arabidopsis, GAs are essential for flowering under noninductive conditions because mutants impaired in GA biosynthesis are unable to flower under short days, whereas mutants with enhanced GA response or plants exogenously treated with GA flower earlier, particularly under short days. The GA antagonist abscisic acid, on the other hand, appears to have a role in delaying flowering because mutants defective in abscisic acid signaling flower slightly earlier than the wild type (Chandler et al., 2000).Several findings have suggested that an increase in GA biosynthesis contributes to the promotion of flowering by long photoperiods. First, the active GA species, GA 1 and GA 4 , accumulate when short daygrown Arabidopsis plants are induced to flower by transferring them to long days (Xu et al., 1997;Gocal et al., 2000); and second, exogenous application of GAs to certain monocots or to Arabidopsis mutants can be as efficient as single long-day treatments in the promotion of flowering (Pharis et al., 1987;Gocal et al., 2000). On the other hand, GA-deficient mutants are able to flower under long days, and the severity of the delay caused by the lack of GAs varies with the experimental conditions employed, indicating that there is not a simple relationship between day length and the role of GAs in floral induction (Wilson et al., 1992;Blázquez et al., 1998).To investigate the relationship between GAs and photoperiod, we have made use of the toc1 mutant, in which photoperiodic promotion of flowering is uncoupled from external inputs by disruption of the circadian clock. TOC1, which encodes a nuclear protein with an atypical response regulator receiver domain, is a critical compo...

Photoperiod pathway regulates expression of MAF5 and FLC that encode MADS-box transcription factors of the FLC family in Arabidopsis

“…The floral activator SOC1 also showed a diurnal expression as previously reported (Blazquez et al 2002). In contrast, the transcript levels of other members of the FLC family were almost constant.…”

“…The MAF1, MAF2, MAF3, MAF4, MAF5, FLC (Ratcliffe et al 2003), SOC1 (Blazquez et al 2002), GI, CCA1, TIMING OF CAB EXPRESSION1 (TOC1; Nakagawa et al 2004), and TUB2 (Kobayashi et al 1999) primers have been previously described.…”

“…, Michaels et al (1999Michaels et al ( , 2001 (2000), Blazquez et al (2002) a They show late flowering when oversxpressed in Ler Sheldon et al 1999;Ratcliffe et al 2001Ratcliffe et al , 2003. b When overexpressed in Col, they show unsettled flowering time phenotype, and significant number of lines show early flowering (Ratcliffe et al 2001(Ratcliffe et al , 2003.…”

Move on up, it’s time for change—mobile signals controlling photoperiod-dependent flowering

“…Finally, apart from being a target of FT at the shoot apex, as first shown by Schmid et al (2003), SOC1 is also expressed in leaves, and it is negatively regulated by FLC at both sites (Borner et al 2000;Lee et al 2000;Samach et al 2000;Hepworth et al 2002;Schmid et al 2003;Michaels et al 2005;Helliwell et al 2006;Searle et al 2006). In leaves, SOC1 is regulated independently of CO by the clock (Blázquez et al 2002;Fujiwara et al 2005;Wigge et al 2005). Whether or how SOC1 in leaves might affect flowering is, however, unknown, since FT expression in leaves appears to be unaffected by loss of SOC1 activity (Lee et al 2000;Moon et al 2005).…”

“…A good candidate for such a pathway is one that relies on gibberellin hormones, which not only regulate, for example, SOC1 (Borner et al 2000;Blázquez et al 2002;Moon et al 2003), but also the flower meristem identity gene LEAFY (LFY) ). In contrast to FT, FD, SOC1, and so on, LFY activity is essential for the development of normal flowers, acting partially redundantly in flower formation with the FT/FD/ AP1 axis (Weigel et al 1992;Ruiz-García et al 1997;Abe et al 2005;Wigge et al 2005).…”