The RCN1-encoded A subunit of protein phosphatase 2A increases phosphataseactivity in vivo

Protein phosphatase 2A (PP2A), a heterotrimeric serine/threonine-specific p rotein phosphatase, comprises a catalytic C subunit and two distinct regula tory subunits, A and B. The RCN1 gene encodes one of three A regulatory sub units in Arabidopsis thaliana. A T-DNA insertion mutation at this locus imp airs root curling, seedling organ elongation and apical hypocotyl hook form ation. We have used in vivo and in vitro assays to gauge the impact of the rcn1 mutation on PP2A activity in seedlings. PP2A activity is decreased in extracts from rcn1 mutant seedlings, and this decrease is not due to a redu ction in catalytic subunit expression. Roots of mutant seedlings exhibit in creased sensitivity to the phosphatase inhibitors okadaic acid and canthari din in organ elongation assays. Shoots of dark-grown, but not light-grown s eedlings also show increased inhibitor sensitivity. Furthermore, cantharidi n treatment of wild-type seedlings mimics the rcn1 defect in root curling, root waving and hypocotyl hook formation assays. In roots of wild-type seed lings, RCN1 mRNA is expressed at high levels in root tips, and accumulates to lower levels in the pericycle and lateral root primordia. In shoots, RCN 1 is expressed in the apical hook and the basal, rapidly elongating cells i n etiolated hypocotyls, and in the shoot meristem and leaf primordia of lig ht-grown seedlings. Our results show that the wild-type RCN1-encoded A subu nit functions as a positive regulator of the PP2A holoenzyme, increasing ac tivity towards substrates involved in organ elongation and differential cel l elongation responses such as root curling.