, Volume 192, Issue 2, pp 276-286

Bi-directional inflorescence development inArabidopsis thaliana: Acropetal initiation of flowers and basipetal initiation of paraclades

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In this study we investigatedArabidopsis thaliana (L.) Heynh. inflorescence development by characterizing morphological changes at the shoot apex during the transition to flowering. Sixteen-hour photoperiods were used to synchronously induce flowering in vegetative plants grown for 30 d in non-inductive 8-h photoperiods. During the first inductive cycle, the shoot apical meristem ceased producing leaf primordia and began to produce flower primordia. The differentiation of paraclades (axillary flowering shoots), however, did not occur until after the initiation of multiple flower primordia from the shoot apical meristem. Paraclades were produced by the basipetal activation of buds from the axils of leaf primordia which had been initiated prior to photoperiodic induction. Concurrent with the activation of paraclades was the partial suppression of paraclade-associated leaf primordia, which became bract leaves. The suppression of bract-leaf primordia and the abrupt initiation of flower primordia during the first inductive photoperiod is indicative of a single phase change during the transition to flowering in photoperiodically inducedArabidopsis. Morphogenetic changes characteristic of the transition to flowering in plants grown continuously in 16-h photoperiods were qualitatively equivalent to the changes observed in plants which were photoperiodically induced after 30 d. These results suggest thatArabidopsis has only two phases of development, a vegetative phase and a reproductive phase; and that the production of flower primordia, the differentiation of paraclades from the axils of pre-existing leaf primordia and the elongation of internodes all occur during the reproductive phase.

We thank Allen Sessions, Michael Christianson, Lisa Parker, Daxius Gregory and two anonymous reviewers for comments on the manuscript; Ian Sussex, Alemu Belachew, Nancy Kerk, Detlef Weigel, Stacy Steinberg, Sheila McCormick, Eva Huala, Steve Ruzin, Zac Cande and Don Kaplan for insight and discussion during the preparation of the manuscript; Detlef Weigel for access to an unpublished manuscript; Don Pardoe and the UC Berkeley Electron Microscopy Lab for training and technical support; Steve Ruzin and the NSF Center of Plant Developmental Biology for equipment loans and technical assistance. This work was supported by U.S. Department of Agriculture grant # 93-37304-9141 to L.J.F.