The significance of bolting and floral transitions as indicators of reproductive phase change in Arabidopsis.

Reproductive phase change in Arabidopsis thaliana is characterized by the floral transition (initiation of the first flower) and the bolting transition (elongation of the first internode). Here, the relationship between these transitions is examined by comparing variation in cauline and total leaf numbers in wild-type plants and 49 early-flowering mutants under a wide range of photoperiods. The timing of these transitions was also evaluated by subjecting wild-type plants to photoperiodic perturbations at different developmental stages. Coupling between the bolting and floral transitions was altered in the wild type under non-optimal flowering conditions but could be restored by optimal conditions that activate the progression to flowering, including continuous light treatments and early flowering mutations. Under non-optimal photoperiodic conditions, the floral node was specified a few days before the bolting node. Altered definitions of long days for the cauline and total leaf responses were frequently coupled in early flowering mutants and were associated with similar photomorphogenetic defects. By contrast, altered definitions of short days were often opposite for the two leaf responses and were associated with different photomorphogenetic and circadian phenotypes. It is concluded that the bolting and floral transitions are regulated by different signalling pathways under non-optimal conditions and that phase change is a multidimensional process. This paper also proposes that, in contrast to the floral transition which is contingent on different factors, the bolting transition may be a robust indicator of reproductive phase change, especially when the progression to flowering is not optimal.