Dichotomous branching: the plant form and integrity upon the apical meristem bifurcation

Self perpetuation of the shoot meristem is essential for the repetitive initiation of shoot structures during plant development. In Arabidopsis shoot meristem maintenance is disrupted by recessive mutations in the WUSCHEL (WUS) gene. The defect is evident at all developmental stages and is restricted to shoot and floral meristems, whereas the root meristem is not affected. wus mutants fail to properly organize a shoot meristem in the embryo. Postembryonically, defective shoot meristems are initiated repetitively but terminate prematurely in aberrant flat structures. In contrast to wild-type shoot meristems, primordia initiation occurs ectopically across mutant apices, including the center, and often new shoot meristems instead of organs are initiated. The cells of wus shoot apices are larger and more vacuolated than wild-type shoot meristem cells. wus floral meristems terminate prematurely in a central stamen. Double mutant studies indicate that the number of organ primordia in the center of wus flowers is limited, irrespective of organ identity and we propose that meristem cells are allocated into floral whorl domains in a sequential manner. WUS activity also appears to be required for the formation of supernumerary organs in the center of agamous, superman or clavata1 flowers, suggesting that the WUS gene acts upstream of the corresponding genes. Our results suggest that the WUS gene is specifically required for central meristem identity of shoot and floral meristems to maintain their structural and functional integrity.