The frequency of plasmodesmata increases early in the whole shoot apical meristem of Sinapis alba L. during floral transition

The frequency of plasmodesmata increases in the shoot apical meristem of pl ants of Sinapis alba L. induced to fewer by exposure to a single long day. This increase is observed within all cell layers (L1, L2, L3) as well as at the interfaces between these layers, and it occurs in both the central and peripheral zones of the shoot apical meristem. The extra plasmodesmata are formed only transiently, from 28 to 48 h after the start of the long day, and acropetally since they are detectable in L3 4 h before they are seen in L1 and L2. These observations indicate that (i) in the Sinapis shoot apica l meristem at floral transition, there is an unfolding of a single field wi th increased plasmodesmatal connectivity, and (ii) this event is an early e ffect of the arrival at this meristem of the floral stimulus of leaf origin . Since (i) the wave of increased frequency of plasmodesmata is 12 h later than the wave of increased mitotic frequency (A. Jacqmard et al. 1998, Plan t cell proliferation and its regulation in growth and development, pp. 67-7 8, Wiley), and (ii) the increase in frequency of plasmodesmata is observed in all cell walls, including in walls not deriving from recent divisions (p ericlinal walls separating the eel layers), it is concluded that the extra plasmodesmata seen at floral transition do not arise in the forming cell pl ate during mitosis and are thus of secondary origin.