Abstract
Cortical microtubule arrays in plants are predicted to be regulated by reversible protein phosphorylation and the PHS1 protein phosphatase, which contains an active MAPK phosphatase catalytic domain, was implicated in this process. The phs1-1 mutation partially compromised catalytic activity of PHS1 in vitro, and here we show that transgenic plants expressing the inactivated version behind its own promoter display a severe isotropic cell expansion phenotype and substantially decreased levels of cortical microtubules in the root transition zone. We further evaluated the strong microtubule depolymerization effect of the inactive PHS1 in a set of transient assays. Null alleles of PHS1, however, do not show any microtubule related phenotype suggesting a functional overlap with other phosphatases. We explain the dominant effects of decreased PHS1 activity either as a formation of non-productive signaling complexes or as an effective competition for substrate with other functionally related phosphatases.
