The timely transition of vegetative to reproductive development is coordinated through quantitative regulation of floral pathway genes in response to physiological and environmental cues. Here, we show that the circadian-controlled expression of the Arabidopsis thaliana floral transition regulators FLOWERING LOCUS T (FT) and CONSTANS (CO) is antiphasic to that of BBX19, a transcription factor with two B-Box motifs. Diminished expression of BBX19 by RNA interference accelerates flowering, and constitutive expression of BBX19 delays flowering under inductive photoperiods. This delay is not accompanied by the alteration of CO expression levels but rather by a reduction of transcript levels of FT and the FT-regulated genes SUPPRESSOR OF OVEREXPRESSION OF CONSTANS1, LEAFY, and FRUITFUL. Similar to CO, BBX19 is expressed in vasculature. BBX19 and CO colocalize in the nucleus and interact physically in vivo. In transient assays, coinfiltration of 10-fold more CO overcomes the BBX19-mediated repression of FT activation. Substitution of the conserved Cys-25 to Ser in the BBX19 Box1 motif abolishes the BBX19-CO interaction and eliminates the negative regulation of flowering time, while the analogous C76S substitution in the Box2 motif is ineffective. Together, these results implicate BBX19 as a circadian clock output that depletes the active CO pool to accurately monitor daylength and precisely time FT expression.

Plants cope with environmental challenges by rapidly triggering and synchronizing mechanisms governing stress-specific and general stress response (GSR) networks. The GSR acts rapidly and transiently in response to various stresses, but the underpinning mechanisms have remained elusive. To define GSR regulatory components we have exploited the Rapid Stress Response Element (RSRE), a previously established functional GSR motif, using Arabidopsis plants expressing a 4xRSRE::Luciferase (RSRE::LUC) reporter. Initially, we searched public microarray datasets and found an enrichment of RSRE in promoter sequences of stress genes. Next, we treated RSRE::LUC plants with wounding and a range of rapidly stress-inducible hormones and detected a robust LUC activity solely in response to wounding. Application of two Ca2+ burst inducers, flagellin22 (flg22) and oligogalacturonic acid, activated RSRE strongly and systemically, while the Ca2+ chelator EGTA significantly reduced wound induction of RSRE::LUC. In line with the signaling function of Ca2+ in transduction events leading to activation of RSRE, we examined role of CALMODULIN-BINDING TRANSCRIPTIONAL ACTIVATORs (CAMTAs) in RSRE induction. Transient expression assays displayed CAMTA3 induction of RSRE and not that of the mutated element mRSRE. Treatment of selected camta mutant lines integrated into RSRE::LUC parent plant, with wounding, flg22, and freezing, established a differential function of these CAMTAs in potentiating the activity of RSRE. Wound response studies using camta double mutants revealed cooperative function of CAMTAs2 and 4 with CAMTA 3 in the RSRE regulation. These studies provide insight into governing components of transduction events and reveal transcriptional modules that tune expression of a key GSR motif.