STUNTED-PLANT-1, A GENE REQUIRED FOR EXPANSION IN RAPIDLY ELONGATING BUT NOT IN DIVIDING CELLS AND MEDIATING ROOT-GROWTH RESPONSES TO APPLIED CYTOKININ

To understand the control of spatial patterns of expansion, we have st udied root growth in wild type and in the stunted plant 1 mutant, stp1 , of Arabidopsis thaliana. We measured profiles of cell length and cal culated the distribution of elongation rate. Slow growth of stp1 resul ts both from a failure of dividing cell number to increase and from lo w elongation rates in the zone of rapid expansion. However, elongation of dividing cells was not greatly affected, and stp1 and wild-type ca llus grew at identical rates. Thus, rapid cellular expansion differs i n mechanism from expansion in dividing cells and is facilitated by the STP1 gene. Additionally, there was no difference between stp1 and wil d-type roots for elongation in response to abscisic acid, auxin, ethyl ene, or gibberellic acid or for radial expansion in response to ethyle ne; however, stp1 responded to cytokinin much less than wild type. In contrast, both genotypes responded comparably to hormones when explant s were cultured; in particular, there was no difference between genoty pes in shoot regeneration in response to cytokinin. Thus, effects on r oot expansion mediated by cytokinin, but not effects mediated by other hormones or effects on other cytokinin-mediated responses, require th e STP1 locus.