Figure 7.

Possible models for PAN in regulating floral organ number. We postulate that there is an inhibitory mechanism controlling the relative positions of first and second whorl floral organ primordia in ancestral plants (i.e., Capparaceae) of the mustard family (i.e., Arabidopsis) and PAN participates in the pre-existing inhibitory pathway to establish tetramerous flowers in the mustard family. (A) Introduction of PAN into the morphogen pathway leads to more distant spacing of organ primordia and thus fewer organs. When PAN activity is reduced or eliminated (pan), plants produce flowers with a pattern resembling the ancestral flower form. (B–D) The inhibitor concentration [I] is plotted as function of distance. The diffusing inhibitor decays with time and thus distance. P1, the first primordium. (█) pan; (□) PAN. (B) PAN might facilitate inhibitor diffusion or persistence to allow the inhibitor to spread more widely, resulting in the formation of the second primordium (P2) at a more distant position than in pan (P2′). (C) PAN might enhance inhibitor reception, leading to formation of the next primordium at a lower inhibitor concentration than in pan. (D) PAN might increase inhibitor production, indirectly leading to its wider diffusion. (se) Sepal; (pe) petal; (st) stamen; (ca) carpel; (solid ovals) inflorescence meristem; (□) and (█) the lowest concentration in the inhibitory field allowing the new primordium to arise in PAN and pan floral meristems, respectively.