FLOWERING NEWSLETTER REVIEW Flowering and determinacy in Arabidopsis

Meristems provide new cells to produce organs throughout the life of a plant, and their continuous activity depends on regulatory genes that balance the proliferation of meristem cells with their recruitment to organogenesis. During flower development, this balance is shifted towards organogenesis, causing the meristem to terminate after producing a genetic-ally determined number of organs. In Arabidopsis, WUSCHEL (WUS) specifies the self-renewing cells at the core of the shoot meristems and is a key target in the control of meristem stability. The development of a determinate floral meristem is initiated by APETALA1/ CAULIFLOWER (AP1/CAL) and LEAFY (LFY). The latter activates AGAMOUS (AG), partly in co-operation with WUS. AG then directs the development of the inner-most floral organs and at the same time antagonizes WUS to terminate the meristem, although the mechan-ism of WUS repression remains unknown. All these genes participate in a series of regulatory feedback loops that maintain stable expression patterns or pro-mote sharp developmental transitions. Although the regulators of meristem maintenance and determinacy in Arabidopsis are widely conserved, their interactions may vary in other species. Key words: AGAMOUS, determinacy, flower development, shoot meristem, WUSCHEL