Role of Polycomb-group genes in commitment to flowering in Arabidopsis

Unlike animals, the plant body plan is formed continuously during adult life and can change in response to environment. One of the most significant changes that occurs during the life of a plant is the switch to flowering. Although flowers and vegetative shoots both develop from groups of stem cells at the tips of shoots (meristems), they differ from plant shoots in that they are usually determinate and stop growing once the female reproductive organs are made. In most plants, the switch from shoot to flower formation is controlled environmentally by daylength and temperature. Once the switch happens, it is very stable even if the environment is changed. This is important in ensuring that flower/shoot intermediates are not formed (these would not make seed effectively). Understanding how this stable commitment to flowering is controlled is important, for example for helping us to control when and where flowering occurs. We are studying a group of proteins that are important in both animals and plants for giving cells a 'memory' of their identity, so that / for example / skin cells don't change into gut cells during our lifetime (inappropriate changes in cell identity are thought to underlie many cancers). Unexpectedly, our recent BBSRC funded research showed that when these proteins (called Polycomb) are missing in flowers, flower/shoot intermediates are formed. We think therefore that the stable switch to flowering may in part occur because Polycomb proteins make sure that genes promoting shoot formation are not activated in flowers. In addition, we found that we could identify which genes are switched off by Polycomb proteins as there are characteristic changes which occur to the histone proteins which package genes in the nucleus. Our aim in this project is to use these techniques to identify which targets have to be switched off by Pc-G during flowering in order to make flower development stable.

Although plant development is generally labile, the switch to flowering is unusual in that, once it occurs, it is very stable. This is important in ensuring that flower/shoot intermediates don't form, as these would be maladaptive. The molecular basis for this commitment to flowering is unclear, but appears to involve both stable changes in production of a promotive signal from leaves, and also in the response of shoot apical meristems. Using conditional mutants, we find that plant Polycomb-group (Pc-G) genes are required for maintaining floral meristem identity during flowering. This promotive effect is unexpected, as earlier in development the Pc-G repress premature transition to flowering. Consistent with Pc-G genes maintaining commitment to flowering, several genes that are down-regulated early in flower development and that are known to promote shoot development or repress flowering are likely targets of the Pc-G as they have characteristic histone methylation properties. The aim of this project is to test the hypothesis that Pc-G mediate floral commitment and to identify the key target genes responsible. To address this we will profile the changes in histone methylation and Pc-G binding that occur early during floral transitions. In parallel we will profile transcriptional changes that occur when Pc-G activity is depleted during flowering. This will identify candidate targets, whose importance we will establish by loss and gain of function genetic analysis.