Dean Rouse

In Arabidopsis , the MADS-box protein encoded by FLOWERING LOCUS C ( FLC ) is a repressor of flowering. Vernalization, which promotes flowering in the late-flowering ecotypes and many late-flowering mutants, decreases the level of FLC transcript and protein in the plant. This vernalization-induced reduction in FLC transcript levels is mitotically stable and occurs in all tissues. FLC activity is restored in each generation, as is the requirement of a low-temperature exposure for the promotion of flowering. The level of FLC determines the extent of the vernalization response in the promotion of flowering, and there is a quantitative relationship between the duration of cold treatment and the extent of down-regulation of FLC activity. We conclude that FLC is the central regulator of the induction of flowering by vernalization. Other vernalization-responsive late-flowering mutants, which are disrupted in genes that encode regulators of FLC , are late-flowering as a consequence of their elevated levels of FLC .

Transcription of the AUX/IAA family of genes is rapidly induced by the plant hormone auxin, but evidence that AUX/IAA genes mediate further responses to auxin has been elusive. Changes in diverse auxin responses result from mutations in the Arabidopsis AXR3 gene. AXR3 was shown to be a member of the AUX/IAA family, providing direct evidence that AUX/IAA genes are central in auxin signaling. Molecular characterization of axr3 gain-of-function and loss-of-function mutations established the functional importance of domains conserved among AUX/IAA proteins.

The growth substance auxin mediates many cellular processes, including division, elongation and differentiation. PSIAA6 is a member of the Aux/IAA family of short-lived putative transcriptional regulators that share four conserved domains and whose mRNAs are rapidly induced in the presence of auxin. Here PSIAA6 was shown to serve as a dominant transferable degradation signal when present as a translational fusion with firefly luciferase (LUC), with an in vivo half-life of 13.5 min in transgenic Arabidopsis seedlings. In a transient assay system in tobacco protoplasts using steady-state differences as an indirect measure of protein half-life, LUC fusions with full-length PSIAA6 and IAA1, an Aux/IAA protein from Arabidopsis, resulted in protein accumulations that were 3.5 and 1. 0%, respectively, of that with LUC alone. An N-terminal region spanning conserved domain II of PSIAA6 containing amino acids 18-73 was shown to contain the necessary cis-acting element to confer low protein accumulation onto LUC, while a fusion protein with PSIAA6 amino acids 71-179 had only a slight effect. Single amino acid substitutions of PSIAA6 in conserved domain II, equivalent to those found in two alleles of axr3, a gene that encodes Aux/IAA protein IAA17, resulted in a greater than 50-fold increase in protein accumulation. Thus, the same mutations resulting in an altered auxin response phenotype increase Aux/IAA protein accumulation, providing a direct link between these two processes. In support of this model, transgenic plants engineered to over-express IAA17 have an axr3-like phenotype. Together, these data suggest that rapid degradation of Aux/IAA proteins is necessary for a normal auxin response.

The MADS-box protein encoded by FLOWERING LOCUS C (FLC) is a repressor of flowering. Loci in the autonomous flowering pathway control FLC levels. We show the epistatic groupings of autonomous pathway mutants fca/fy and fve/fpa, based on their effects on flowering time, are consistent with their effects on FLC transcript and protein levels. We demonstrate that synergistic increases in FLC mRNA and protein expression occur in response to interactions between the autonomous pathway mutants fca and fpa and mutants in other pathways (fe, ft, fha) that do not regulate FLC when present as single mutants. These changes in FLC levels provide the molecular basis of the interactions previously shown in genetic analyses. The interactions between genes of multiple pathways emphasize the central position of FLC in the control of floral initiation. FLC protein levels match those of its mRNA for a range of genetic, developmental and environmental variables, indicating that control of FLC is at the level of transcription or transcript stability. The autonomous and photoperiod pathways also interact at the level of SOC1. FLC acts as a repressor of SOC1, and SOC1 levels are low when FLC levels are high. In C24 plants which have moderately high FLC levels, flowering occurs without a decrease in FLC level, but the SOC1 level does increase. Thus SOC1 levels can be upregulated through the activities of other pathways, despite the repression by FLC.