STUNTED PLANT 1, a gene required for expansion in rapidly elongating but not in dividing cells, and mediating root growth responses to applied cytokinin.

Tobias I. Baskin*, Ann Cork, Richard E. Williamson, and Janet R. Gorst

*corresponding author
University of Missouri, Biological Sciences,109 Tucker Hall,Columbia, MO 65211 USA
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Plant Physiology, in press.
To understand the control of spatial patterns of expansion, we studied root growth in wildtype and in the stunted plant 1 mutant, stp1, ofArabidopsis thaliana. We measured profiles of cell length and calculatedthe distribution of elongation rate. Slow growth of stp1 results both froma failure of dividing cell number to increase, and from low elongation rates in the zone of rapid expansion. However, elongation of dividing cells was not greatly affected, and stp1 and wildtype callus grew at identical rates. Thus, rapid cellular expansion differs in mechanism from expansion in dividing cells, and is facilitated by the STP1 gene. Additionally, there was no difference between stp1 and wildtype roots for elongation in response to abscisic acid, auxin, ethylene, and gibberellin, or for radial expansion in response to ethylene; however, stp1 responded to cytokinin much less than wildtype. By contrast, both genotypes responded comparably to hormones when explants were cultured; in particular, there was no difference between genotypes in shoot regeneration in response to cytokinin. Thus, effects on root expansion mediated by cytokinin, but not effects mediated by other hormones, nor effects on other cytokinin-mediated responses, require the STP1 locus.