Floral repressors in Arabidopsis thaliana
Final Report Abstract
The transition from vegetative growth to flowering is one of the major developmental phase changes during the life cycle of a plant. Especially for annual plants such as Arabidopsis, the decision when to induce flowering is of extreme importance, as it is normally irrevocable. Analysis over the last two decades has revealed several genetically defined pathways that integrate several endogenous and environmental cues to control the transition to flowering. Arabidopsis is a facultative long-day plant and flowering is therefore normally promoted by increasing day length. This induction of flowering is balanced by the action of floral repressors. We had previously shown that two genes with characteristic AP2 domains, SMZ and SNZ, are able to repress flowering when expressed at high levels. Here we show that SMZ represses flowering in a photoperiod specific manner. Late flowering of activation-tagged smz-D plants occurred only under long day conditions, whereas flowering was normal when plants were grown under noninductive short days. Genetic analysis further supports the notion that SMZ is acting specifically in the photoperiod pathway. Loss of SMZ function due to a T-DNA insertion mutant in the relatively early flowering accession Col-0 had no effect on flowering time, i.e. plants were not early flowering. However, constitutive expression of an artificial microRNA targeting the SMZ mRNA in the late flowering Arabidopsis accession Sf-2 led to early flowering, indicating that the contribution of SMZ to the repression of flowering might vary between different accessions. At the molecular level, SMZ blocks the induction of FT even under inductive long day conditions, which easily explains the late flowering observed in smz-D plants. As a consequence, smz-D plants fail to induce at the meristem any of the typical markers associated with flowering, such as FRUITFUL (FUL), CAULIFLOWER (CAL), SUPRESSOR OF CONSTANS 1 (SOC1) or LEAFY (LFY) as revealed by microarray analysis. Furthermore, SMZ belongs to a clade of six AP2-like transcription factors that are targets of a microRNA, miR172. Using various techniques we could show that regulation of SMZ by miR172 occurs in planta and most likely plays an important role in the control of flowering.
