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SAM’s speedy response to stimulus
Annie Jacqmard has very recently retired from the University of Liege after a long and distinguished career. She has made a significant contribution to our understanding of the cellular processes involved in the induction of flowering, and it is thus fitting that we focus on her paper (Jacqmard et al., pp. 571-576). It is now well known that in many dicot species, acceleration of cell division in the shoot apical meristem (SAM) is an early event in the floral transition. It has been suggested by Dennis Francis at Cardiff and by Georges Bernier at Liege that this is a prerequisite for subsequent reorganization of the SAM. In the paper discussed here, the authors have examined, in more detail, the timing and localization of the changes in cell division pattern in Arabidopsis thaliana. Plants were kept in short days and were thus maintained as rosettes. Flowering was then induced by exposure to a single long day of 22 h. Mitotic activity in the SAM begins to increase within 24 h of the start of the long day, i.e. before the movement of the floral stimulus from leaves to apex is complete. The stimulation of cell division is particularly marked in the central and peripheral zones, and the distinction between these two zones then becomes less marked. The rib meristem (below the central zone) exhibits a smaller increase in mitotic activity and there is no renewal of cell division in the pith derived from the central zone. These changes lead to an enlargement of the SAM that is evident by 44 h, slightly preceding the start of floral meristem initiation. This careful work has thus shown that cell division activity in the SAM is exquisitely sensitive to the floral stimulus and raises interesting questions about the control of the process at the molecular level.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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