RESEARCH ARTICLE The ERECTA, CLAVATA and class III HD-ZIP Pathways Display Synergistic Interactions in Regulating Floral Meristem Activities

In angiosperms, the production of flowers marks the beginning of the reproductive phase. At the emergence of flower primordia on the flanks of the inflorescence meristem, the WUSCHEL (WUS) gene, which encodes a homeodomain transcription factor starts to be ex-pressed and establishes de novo stem cell population, founder of the floral meristem (FM). Similarly to the shoot apical meristem a precise spatial and temporal expression pattern of WUS is required and maintained through strict regulation by multiple regulatory inputs to maintain stem cell homeostasis. However, following the formation of a genetically deter-mined fixed number of floral organs, this homeostasis is shifted towards organogenesis and the FM is terminated. In here we performed a genetic study to test how a reduction in ERECTA, CLAVATA and class III HD-ZIP pathways affects floral meristem activity and flow-er development. We revealed strong synergistic phenotypes of extra flower number, super-numerary whorls, total loss of determinacy and extreme enlargement of the meristem as compared to any double mutant combination indicating that the three pathways, CLV3, ER and HD-ZIPIII distinctively regulate meristem activity and that they act in parallel. Our find-ings yield several new insights into stem cell-driven development. We demonstrate the cru-cial requirement for coupling floral meristem termination with carpel formation to ensure successful reproduction in plants. We also show how regulation of meristem size and alter-nation in spatial structure of the meristem serve as a mechanism to determine flower organ-ogenesis. We propose that the loss of FM determinacy due to the reduction in CLV3, ER and HD-ZIPIII activity is genetically separable from the AGAMOUS core mechanism of meristem termination