The RCN1‐encoded A subunit of protein phosphatase 2A increases phosphatase activity in vivo

Abstract

SummaryProtein phosphatase 2A (PP2A), a heterotrimeric serine/ threonine-speci®c protein phosphatase, comprises a catalytic C subunit and two distinct regulatory subunits, A and B. The RCN1 gene encodes one of three A regulatory subunits in Arabidopsis thaliana. A T-DNA insertion mutation at this locus impairs root curling, seedling organ elongation and apical hypocotyl hook formation. 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 reduction in catalytic subunit expression. Roots of mutant seedlings exhibit increased sensitivity to the phosphatase inhibitors okadaic acid and cantharidin in organ elongation assays. Shoots of dark-grown, but not light-grown seedlings also show increased inhibitor sensitivity. Furthermore, cantharidin treatment of wildtype seedlings mimics the rcn1 defect in root curling, root waving and hypocotyl hook formation assays. In roots of wild-type seedlings, RCN1 mRNA is expressed at high levels in root tips, and accumulates to lower levels in the pericycle and lateral root primordia. In shoots, RCN1 is expressed in the apical hook and the basal, rapidly elongating cells in etiolated hypocotyls, and in the shoot meristem and leaf primordia of lightgrown seedlings. Our results show that the wild-type RCN1-encoded A subunit functions as a positive regulator of the PP2A holoenzyme, increasing activity towards substrates involved in organ elongation and differential cell elongation responses such as root curling.

The Arabidopsis rcn1-1 Mutation Impairs Dephosphorylation of Phot2, Resulting in Enhanced Blue Light Responses

“…As mentioned above, the null mutation rcn1-1 reduces the PP2A activity to 40 to 60% of that of the wild type (Deruè re et al, 1999). In Arabidopsis, sensing of unilateral blue light mediated by the phototropins activates phototropin autophosphorylation, leading to signaling and eventually hypocotyl curvature (Christie, 2007).…”

“…Furthermore, the mutant seedlings were more sensitive than the wild type to the phosphatase inhibitors okadaic acid and cantharadin. Finally, treatment of wild-type seedlings with the appropriate concentration of cantharadin phenocopied the rcn1-1 mutation (Deruè re et al, 1999;Rashotte et al, 2001).…”

“…PP2A lacking this subunit has 40 to 60% of the activity of the complete wild-type enzyme complex (Deruè re et al, 1999). Since a mutant in this subunit, rcn1-1, was available (a T-DNA insertion line; Garbers et al, 1996), we constructed phot1-5 rcn1-1 Arabidopsis double mutant lines to test whether this protein phosphatase might play a role in two physiological responses: phototropism and blue light-activated stomatal opening.…”

The Clickable Guard Cell, Version II: Interactive Model of Guard Cell Signal Transduction Mechanisms and Pathways

“…The RCN1 gene encoding the PP2A regulatory subunit A is a positive regulator of the ABA signaling cascade in guard cells and seed germination . RCN1 was previously shown to increase PP2A enzyme activity (Deruère et al, 1999). T-DNA insertional mutation in RCN1 causes impairment in both ABA-induced stomatal closure and ABA activation of anion channels, a reduced sensitivity of ABA-triggered cytosolic Ca 2+ increases, ABA-insensitive gene expression, and ABA-insensitive seed germination (Figure 3).…”

Auxin Polar Transport and Automorphosis in Plants

“…Another factor is also demonstrated by the analyses of Arabidopsis mutants that have phenotypes typical for altered auxin transport, namely roots curl in NPA1 (rcn1) and pinoid (pid). These mutants are corresponding to regulatory subunit of protein phosphatase 2A (PP2A) (Deruère et al, 1999) and the serine/threonine protein kinase PINOID (PID) (Christensen et al, 2000), respectively, as factors important for PIN targeting (Friml et al, 2004). The reversible phosphorylation of PIN proteins by PID and PP2A seems to be important for decision about apical or basal delivery of PIN proteins (Michniewicz et al, 2007).…”