Ayako Yamaguchi

The MicroRNA-Regulated SBP-Box Transcription Factor SPL3 Is a Direct Upstream Activator of LEAFY, FRUITFULL, and APETALA1

When to form flowers is a developmental decision that profoundly impacts the fitness of flowering plants. In Arabidopsis this decision is ultimately controlled by the induction and subsequent activity of the transcription factors LEAFY (LFY), FRUITFULL (FUL), and APETALA1 (AP1). Despite their central importance, our current understanding of the regulation of LFY, FUL, and AP1 expression is still incomplete. We show here that all three genes are directly activated by the microRNA-targeted transcription factor SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 3 (SPL3). Our findings suggest that SPL3 acts together with other microRNA-regulated SPL transcription factors to control the timing of flower formation. Moreover, the identified SPL activity defines a distinct pathway in control of this vital developmental decision.

Long-distance, graft-transmissible action of Arabidopsis FLOWERING LOCUS T protein to promote flowering

Day length perceived by a leaf is a major environmental factor that controls the timing of flowering. It has been believed that a mobile, long-distance signal called florigen is produced in the leaf under inductive day length conditions, and is transported to the shoot apex where it triggers floral morphogenesis. Grafting experiments have shown that florigen is transmissible from a donor plant that has been subjected to inductive day length to an uninduced recipient plant. However, the nature of florigen has long remained elusive. Arabidopsis FLOWERING LOCUS T (FT) is expressed in cotyledons and leaves in response to inductive long days (LDs). FT protein, with a basic region/leucine zipper (bZIP) transcription factor FD, acts in the shoot apex to induce target meristem identity genes such as APETALA1 (AP1) and initiates floral morphogenesis. Recent studies have provided evidence that the FT protein in Arabidopsis and corresponding proteins in other species are an important part of florigen. Our work shows that the FT activity, either from overexpressing or inducible transgenes or from the endogenous gene, to promote flowering is transmissible through a graft junction, and that an FT protein with a T7 tag is transported from a donor scion to the apical region of recipient stock plants and becomes detectable within a day or two. The sequence and structure of mRNA are not of critical importance for the long-distance action of the FT gene. These observations led to the conclusion that the FT protein, but not mRNA, is the essential component of florigen.

LEAFY Target Genes Reveal Floral Regulatory Logic, cis Motifs, and a Link to Biotic Stimulus Response

The transition from vegetative growth to flower formation is critical for the survival of flowering plants. The plant-specific transcription factor LEAFY (LFY) has central, evolutionarily conserved roles in this process, both in the formation of the first flower and later in floral patterning. We performed genome-wide binding and expression studies to elucidate the molecular mechanisms by which LFY executes these roles. Our study reveals that LFY directs an elaborate regulatory network in control of floral homeotic gene expression. LFY also controls the expression of genes that regulate the response to external stimuli in Arabidopsis. Thus, our findings support a key role for LFY in the coordination of reproductive stage development and disease response programs in plants that may ensure optimal allocation of plant resources for reproductive fitness. Finally, motif analyses reveal a possible mechanism for stage-specific LFY recruitment and suggest a role for LFY in overcoming polycomb repression.

Gibberellin Acts Positively Then Negatively to Control Onset of Flower Formation in Arabidopsis

One Hormone, Two Phases The switch from vegetative growth to flowering in the plant Arabidopsis involves two phases—inflorescence branching and flowering. Yamaguchi et al. (p. 638) examined how the phytohormone gibberellin regulates each phase differently. First, gibberellin levels increase and stimulate production of key flowering factors, one of which degrades gibberellin. As gibberellin levels then fall, the next phase of flowering factors is released from gibberellin repression. By regulating inflorescence branching separately from flowering, this system determines overall seed yield. Inflorescence architecture is shaped by a biphasic signaling network involving the plant hormone gibberellin. The switch to reproductive development is biphasic in many plants, a feature important for optimal pollination and yield. We show that dual opposite roles of the phytohormone gibberellin underpin this phenomenon in Arabidopsis. Although gibberellin promotes termination of vegetative development, it inhibits flower formation. To overcome this effect, the transcription factor LEAFY induces expression of a gibberellin catabolism gene; consequently, increased LEAFY activity causes reduced gibberellin levels. This allows accumulation of gibberellin-sensitive DELLA proteins. The DELLA proteins are recruited by SQUAMOSA PROMOTER BINDING PROTEIN–LIKE transcription factors to regulatory regions of the floral commitment gene APETALA1 and promote APETALA1 up-regulation and floral fate synergistically with LEAFY. The two opposing functions of gibberellin may facilitate evolutionary and environmental modulation of plant inflorescence architecture.

The Florigen Genes FT and TSF Modulate Lateral Shoot Outgrowth in Arabidopsis thaliana

Successful sexual reproduction of a plant with prolific seed production requires appropriate timing of flowering and concomitant change of architecture (e.g. internode elongation and branching) to facilitate production of the optimal number of flowers while enabling continued resource production through photosynthesis. Florigen is the prime candidate for a signal linking the two processes. Growth analysis of lateral shoots in mutants of FLOWERING LOCUS T (FT) and TWIN SISTER OF FT (TSF) revealed a delay in the onset of outgrowth and a reduction of the growth rate in ft plants in long-day (LD) conditions and in tsf plants in short-day (SD) conditions. Thus, as in the case of floral transition, FT and TSF play dominant roles in LD and SD conditions, respectively, in the promotion of lateral shoot development. Differential expression patterns of the two genes were in good agreement with their differential roles both in the floral transition and in lateral shoot development under contrasting photoperiod conditions. By manipulating florigen production after bolting of the primary shoot, it was shown that florigen promotes lateral shoot growth independently of its effect on the floral transition of the primary shoot. Analysis of growth and gene expression in lateral shoots of the mutants suggests that the loss of florigen leads to a reduced rate of flower formation on lateral shoots. Together, we propose that the two florigen genes are an important key to linking the floral transition and lateral shoot development to maximize the reproductive success of a plant.

LATE MERISTEM IDENTITY2 acts together with LEAFY to activate APETALA1

The switch from producing vegetative structures (branches and leaves) to producing reproductive structures (flowers) is a crucial developmental transition that significantly affects the reproductive success of flowering plants. In Arabidopsis, this transition is in large part controlled by the meristem identity regulator LEAFY (LFY). The molecular mechanisms by which LFY orchestrates a precise and robust switch to flower formation is not well understood. Here, we show that the direct LFY target LATE MERISTEM IDENTITY2 (LMI2) has a role in the meristem identity transition. Like LFY, LMI2 activates AP1 directly; moreover, LMI2 and LFY interact physically. LFY, LMI2 and AP1 are connected in a feed-forward and positive feedback loop network. We propose that these intricate regulatory interactions not only direct the precision of this crucial developmental transition in rapidly changing environmental conditions, but also contribute to its robustness and irreversibility.

LEAFY controls Arabidopsis pedicel length and orientation by affecting adaxial–abaxial cell fate

Pedicel length and orientation (angle) contribute to the diversity of inflorescence architecture, and are important for optimal positioning of the flowers. However, relatively little is known about pedicel development. We previously described the Arabidopsis CORYMBOSA1 (CRM1)/BIG gene, which affects inflorescence architecture by controlling pedicel elongation and orientation. Here, we performed a suppressor screen using the partial loss-of-function allele crm1-13 to identify genes and pathways that affect pedicel development. We identified a hypomorph allele of the meristem identity regulator LEAFY (LFY) as the suppressor. Consistent with this, crm1 pedicels had elevated LFY levels and conditional gain of LFY function produced downward-bending pedicels. Steroid activation of 35S:LFY-GR plants caused a reduction in the cortical cell length in the abaxial domain and additional defects associated with adaxialization. Further analyses of loss of LFY function revealed that LFY is required for reduced cortical cell elongation at the adaxial side of the pedicel base. Defects in conditional LFY gain-of-function pedicels were correlated with decreased BREVIPEDICELLUS (BP) expression, while ASYMMETRIC LEAVES2 (AS2), a transcriptional repressor of BP, and REVOLUTA, a promoter of adaxial cell fate, were highly and ectopically expressed in LFY gain-of-function pedicels. LFY bound to cis-regulatory regions upstream of AS2, and as2 mutations partially suppressed the pedicel length and orientation defects caused by increased LFY activity. These data suggest that LFY activity promotes adaxial cell fate and hence the proper orientation and length of the pedicel partly by directly activating AS2 expression, which suppresses BP expression.

FE, a phloem-specific Myb-related protein, promotes flowering through transcriptional activation of FLOWERING LOCUS T and FLOWERING LOCUS T INTERACTING PROTEIN 1

In many flowering plants, the transition to flowering is primarily affected by seasonal changes in day length (photoperiod). An inductive photoperiod promotes flowering via synthesis of a floral stimulus, called florigen. In Arabidopsis thaliana, the FLOWERING LOCUS T (FT) protein is an essential component of florigen, which is synthesized in leaf phloem companion cells and is transported through phloem tissue to the shoot apical meristem where floral morphogenesis is initiated. However, the molecular mechanism involved in the long-distance transport of FT protein remains elusive. In this study, we characterized the classic Arabidopsis mutant fe, which is involved in the photoperiodic induction of flowering, and showed that FE encodes a phloem-specific Myb-related protein that was previously reported as ALTERED PHLOEM DEVELOPMENT. Phenotypic analyses of the fe mutant showed that FT expression is reduced in leaf phloem companion cells. In addition, the transport of FT protein from leaves to the shoot apex is impaired in the fe mutant. Expression analyses further demonstrated that FE is also required for transcriptional activation of FLOWERING LOCUS T INTERACTING PROTEIN 1 (FTIP1), an essential regulator for selective trafficking of the FT protein from companion cells to sieve elements. These findings indicate that FE plays a dual role in the photoperiodic induction of flowering: as a transcriptional activator of FT on the one hand, and its transport machinery component, FTIP1, on the other hand. Thus, FE is likely to play a role in regulating FT by coordinating FT synthesis and FT transport in phloem companion cells.