The symplasmic coupling of L2-cells diminishes in early floral development of Iris
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The transition from vegetative to reproductive development involves extensive revisions of cellular collaboration at the apical meristem and results in the production of novel appendages. In Dutch Iris (Iris xiphium) the transition from vegetative apical meristem to inflorescence meristem was morphologically signalled by the appearance of a `spathe leaf '. After enlargement of the inflorescence meristem, a second spathe leaf and a double floral meristem were formed. The then undulated surface corresponded to general topological changes and a beginning of altered cell division patterns. Throughout, all cells produced in the meristem remained in contact via plasmodesmata (Pd), thus maintaining the symplasmic unity of the meristem. Since the symplasm harbours part of the signal network that coordinates the activities of the meristem cells, we investigated if alterations in Pd numbers could underlie meristem transitions. Prior to Pd counting, potentially important borders inside the meristem, representing primary and secondary cell contacts, were identified by the construction of a symplasmetric map. During the transition, significant alterations did take place at some of the borders defined by the map. Within the second tunica layer (L2), and between the L2 and adjacent cells, Pd numbers were strongly reduced. Within the first tunica layer (L1) and within the corpus they remained the same. The reduction was 25% between L2-cells, and between L2- and L1-cells; it was 40% between the L2 and the outer corpus layer. The reductions appeared to be due to a lowered production of primary and secondary Pd by L2-cells towards each other and towards adjacent cells. As a result of this the integration of all L2-cells and, as a consequence, of the L1 as a whole in the symplasmic network of the meristem was reduced. The implied gain in autonomy of the individual L2-cells and of the L1-layer may reflect their new functions in the floral meristem.
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