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Actin CAPped for cell enlargement
The size and shape of plant organs results from a combination of many interacting factors. These include cell numbers, polarity of cell divisions, and the direction and extent of cell elongation. Environmental cues and cell-cell signalling are involved, and within cells the activity of the cytoskeleton is important. This latter aspect has been the subject of a study by Barrero and colleagues (Tokyo and Iwate, pp. 599-603). Cyclase-associated protein (CAP) is an actin-binding protein that occurs widely in eukaryotes, including plants. The protein is specific for actin monomers and promotes depolymerization of F-actin filaments. CAP-induced depolymerization has been shown to be involved in regulating cell size and polarity in drosophila, and the question is whether the same is true in plants. The authors have cloned a CAP-encoding gene from Arabidopsis thaliana and have inserted it into tobacco plants under the control of an inducible promoter. The latter feature enabled them to activate the gene at particular stages of seedling growth. The results of over-expression of the CAP gene were very clear. Leaves formed after induction of the gene were misshapen and very much reduced in size compared with leaves of control plants. This was the result of significantly reduced cell elongation of epidermal and mesophyll cells; there was no decrease in cell number. The authors compare these results with those previously obtained with A. thaliana itself where over-expression of the CAP gene led to a reduction in cell number and in cell expansion. The difference between the effects in the two plants may be genuine, i.e. cell numbers are under different controls in the two species, or may simply arise because of the differences between homologous and heterologous over-expression. In either case, the results imply a role for the actin cytoskeleton in regulation of organ size and shape, and thus pave the way for further investigation.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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