Abstract
rrd4 is one of temperature-sensitive mutants of Arabidopsis thaliana that were isolated by screening with callus-mediated root redifferentiation as an index phenotype. The rrd4 mutant exhibits a severe defect of dedifferentiation and a leaky defect of cell proliferation in a cytokinin-dependent manner at the restrictive temperature. This feature implies that the RRD4 gene product functions in sequestering negative actions of cytokinin on dedifferentiation and cell proliferation, which are normally observed only at high concentrations of cytokinin. The RRD4 gene encodes a G-patch-containing protein very similar to mouse TIP39 and Drosophila SIP1. To understand how RRD4 modulates various actions of cytokinin, we examined effects of the rrd4 mutation on the expression patterns of cytokinin-responsive genes such as ARR5 in hypocotyl explants. The results obtained so far suggest that the rrd4 mutation does not alter all aspects of cytokinin responses, and that RRD4 is involved in a limited part of cytokinin actions.
