Cell division and morphological changes in the shoot apex of Arabidopsis thaliana during floral transition.

Eight-week-old vegetative plants of Arabidopsis thaliana, ecotype Columbia, were induced to flower by a single long day (LD). In this experimental system, it is known that the last component of the floral stimulus moves from the leaves to the apex 24-36 h after the start of the LD, and the first floral meristem is initiated by the shoot apical meristem (SAM) at 44-56 h (Corbesier et al., 1996, The Plant Journal 9: 947-952). Here we show that the rate of cell division is increased at floral transition in all SAM parts but not in the sub-apical pith cells. Mitotic activity starts to increase 24 h after the start of the LD and is two- to three-fold higher at peak times than that in non-induced plants. This activation is followed by the start of SAM enlargement at 44 h, SAM doming at 48 h, and the elongation of apical internodes (bolting) at 52 h.