M. Isabel Ortiz-Marchena

Polycomb Group (PcG) proteins play important roles in regulating developmental phase transitions in plants; however, little is known about the role the PcG machinery in regulating the transition from juvenile to adult phase. Here, we show that Arabidopsis BMI1 (AtBMI1) PRC1 components participate in the repression of miR156. Loss of AtBMI1 function leads to upregulation of pri-MIR156A/C at the time the levels of miR156 should decline, resulting in an extended juvenile phase and delayed flowering. Conversely, the PRC1 component EMBRYONIC FLOWER (EMF1) participates in the regulation of SPL and MIR172 genes. Accordingly, plants impaired in EMF1 function displayed misexpression of these genes early in development, which contributes to a CONSTANS (CO)-independent upregulation of FLOWERING LOCUS T (FT) leading to the earliest flowering phenotype described in Arabidopsis. Our findings show how the different regulatory roles of two functional PRC1 variants coordinate the acquisition of flowering competence and help to reach the threshold of FT necessary to flower. Furthermore, we show how two central regulatory mechanisms, such as PcG and miRNA, assemble to achieve a developmental outcome.

Flowering is a crucial process that demands substantial resources. Carbon metabolism must be coordinated with development through a control mechanism that optimizes fitness for any physiological need and growth stage of the plant. However, how sugar allocation is controlled during the floral transition is unknown. Recently, the role of a CONSTANS (CO) ortholog (Cr-CO) in the control of the photoperiod response in the green alga Chlamydomonas reinhardtii and its influence on starch metabolism was demonstrated. In this work, we show that transitory starch accumulation and glycan composition during the floral transition in Arabidopsis thaliana are regulated by photoperiod. Employing a multidisciplinary approach, we demonstrate a role for CO in regulating the level and timing of expression of the GRANULE BOUND STARCH SYNTHASE (GBSS) gene. Furthermore, we provide a detailed characterization of a GBSS mutant involved in transitory starch synthesis and analyze its flowering time phenotype in relation to its altered capacity to synthesize amylose and to modify the plant free sugar content. Photoperiod modification of starch homeostasis by CO may be crucial for increasing the sugar mobilization demanded by the floral transition. This finding contributes to our understanding of the flowering process.