The four-member SPA protein family of Arabidopsis acts in concert with the E3 ubiquitin ligase COP1 to suppress photomorphogenesis in dark-grown seedlings. Here, we demonstrate that SPA proteins are, moreover, essential for photoperiodic flowering. Mutations in SPA1 cause phyA-independent early flowering under short day (SD) but not long day (LD) conditions, and this phenotype is enhanced by additional loss of SPA3 and SPA4 function. These spa1 spa3 spa4 triple mutants flower at the same time in LD and SD, indicating that the SPA gene family is essential for the inhibition of flowering under non-inductive SD. Among the four SPA genes, SPA1 is necessary and sufficient for normal photoperiodic flowering. Early flowering of SD-grown spa mutant correlates with strongly increased FT transcript levels, whereas COtranscript levels are not altered. Epistasis analysis demonstrates that both early flowering and FT induction in spa1 mutants is fully dependent on CO. Consistent with this finding, SPA proteins interact physically with CO in vitro and in vivo, suggesting that SPA proteins regulate CO protein function. Domain mapping shows that the SPA1-CO interaction requires the CCT-domain of CO, but is independent of the B-box type Zn fingers of CO. We further show that spa1 spa3 spa4 mutants exhibit strongly increased CO protein levels, which are not caused by a change in COgene expression. Taken together, our results suggest, that SPA proteins regulate photoperiodic flowering by controlling the stability of the floral inducer CO.
Shoot branching is a pivotal process during plant growth and development, antagonistically orchestrated by auxin and sugars. By contrast to extensive investigations on hormonal regulatory networks, our current knowledge on the role of sugar signalling pathways in bud outgrowth is still scarce. Based on a stepwise and comprehensive strategy, we investigated the role of glycolysis/the tricarboxylic acid (TCA) cycle and the oxidative pentose phosphate pathway (OPPP) in the control of bud outgrowth. We demonstrated that these two pathways are necessary for bud outgrowth promotion upon plant decapitation and in response to sugar availability. They are also targets of the antagonistic crosstalk between auxin and sugar availability. These two pathways act synergistically to downregulate the expression of BRC1, a conserved inhibitor of shoot branching. Using Rosa calluses stably transformed with GFP-fused promoter sequences of RhBRC1 (pRhBRC1), glycolysis/TCA-cycle and the OPPP were found to repress the transcriptional activity of pRhBRC1 cooperatively. Glycolysis/TCA-cycle- and OPPP-dependent regulations involve the -1973bp/-1611bp and -1206bp/-709bp regions of pRhBRC1, respectively. Taken together, our findings indicate that glycolysis/the tricarboxylic acid cycle and the OPPP are integrative parts of shoot branching control and can link endogenous factors to the developmental program of bud outgrowth, more likely through two distinct mechanisms.
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