In plants, developmental timing is coordinately regulated by a complex signaling network that integrates diverse intrinsic and extrinsic signals. miR172 promotes photoperiodic flowering. It also regulates adult development along with miR156, although the molecular mechanisms underlying this regulation are not fully understood. Here, we demonstrate that miR172 modulates the developmental transitions by regulating the expression of a subset of the SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) genes, which are also regulated by miR156. The SPL3/4/5 genes were upregulated in the miR172-overproducing plants (35S:172) and its target gene mutants that exhibit early flowering. In contrast, expression of other SPL genes was not altered to a discernible level. Kinetic measurements of miR172 abundance in the transgenic plants expressing the MIR156a gene driven by a β-estradiol-inducible promoter revealed that expressions of miR172 and miR156 are not directly interrelated. Instead, the 2 miRNA signals are integrated at the SPL3/4/5 genes. Notably, analysis of developmental patterns in the 156 × 172 plants overproducing both miR172 and miR156 showed that whereas vegetative phase change was delayed as observed in the miR156-overproducing plants (35S:156), flowering initiation was accelerated as observed in the 35S:172 transgenic plants. Together, these observations indicate that although miR172 and miR156 play distinct roles in the timing of developmental phase transitions, there is a signaling crosstalk mediated by the SPL3/4/5 genes.

The plasma membrane is an important cellular organ that perceives incoming developmental and environmental signals and integrates these signals into cellular regulatory mechanisms. It also acts as a barrier against unfavorable extracellular factors to maintain cell viability. Despite its importance for cell viability, molecular components determining cell viability and underlying mechanisms are largely unknown. Here, we show that a plasma membrane-localized MtN3 protein SAG29 regulates cell viability under high salinity in Arabidopsis. The SAG29 gene is expressed primarily in senescing plant tissues. It is induced by osmotic stresses via an abscisic acid-dependent pathway. Whereas the SAG29-overexpressing transgenic plants (35S:SAG29) exhibited an accelerated senescence and were hypersensitive to salt stress, the SAG29-deficient mutants were less sensitive to high salinity. Consistent with this, the 35S:SAG29 transgenic plants showed reduced cell viability in the roots under normal growth condition. In contrast, cell viability in the SAG29-deficient mutant roots was indistinguishable from that in the roots of control plants. Notably, the mutant roots exhibited enhanced cell viability under high salinity. Our observations indicate that the senescence-associated SAG29 protein is associated with cell viability under high salinity and other osmotic stress conditions. We propose that the SAG29 protein may serve as a molecular link that integrates environmental stress responses into senescing process.

SUMMARYThere has been a long-standing interest in the role played by sugars in flowering. Of particular interest is how sugar-related signals are integrated into flowering genetic pathways. Here, we demonstrate that the INDETERMINATE DOMAIN transcription factor AtIDD8 regulates photoperiodic flowering by modulating sugar transport and metabolism. We found that whereas AtIDD8-deficient idd8 mutants exhibit delayed flowering under long days, AtIDD8-overexpressing plants (35S:IDD8) show early flowering. In addition, the sucrose synthase genes SUS1 and SUS4 were upregulated in 35S:IDD8 plants but downregulated in idd8 mutants, in which endogenous sugar levels were altered. AtIDD8 activates the SUS4 gene by binding directly to its promoter, resulting in promoted flowering in SUS4-overexpressing plants. SUS4 expression also responds to photoperiodic signals. Notably, the AtIDD8 gene is suppressed by sugar deprivation. Therefore, we conclude that AtIDD8 regulation of sugar transport and metabolism is linked to photoperiodic flowering.