Specialized functions of the PP2A subfamily II catalytic subunits PP2A‐C3 and PP2A‐C4 in the distribution of auxin fluxes and development in Arabidopsis

Abstract

SUMMARYProtein phosphorylation is a key molecular switch used to transmit information in biological signalling networks. The output of these signalling circuits is governed by the counteracting activities of protein kinases and phosphatases that determine the direction of the switch. Whereas many kinases have been functionally characterized, it has been difficult to ascribe precise cellular roles to plant phosphatases, which are encoded by enlarged gene families that may provide a high degree of genetic redundancy. In this work we have analysed the role in planta of catalytic subunits of protein phosphatase 2A (PP2A), a family encoded by five genes in Arabidopsis. Our results indicate that the two members of subfamily II, PP2A-C3 and PP2A-C4, have redundant functions in controlling embryo patterning and root development, processes that depend on auxin fluxes. Moreover, polarity of the auxin efflux carrier PIN1 and auxin distribution, determined with the DR5 pro :GFP proxy, are affected by mutations in PP2A-C3 and PP2A-C4. Previous characterization of mutants in putative PP2A regulatory subunits had established a link between this class of phosphatases and PIN dephosphorylation and subcellular distribution. Building on those findings, the results presented here suggest that PP2A-C3 and PP2A-C4 catalyse this reaction and contribute critically to the establishment of auxin gradients for proper plant development.

A protein phosphatase 2A complex spatially controls plant cell division

“…We provide evidence that all three A-type, but presumably only two (PP2AC3 and PP2AC4 subunits) of the five Arabidopsis C-type PP2A subunits take part in the complex, the three other PP2AC subunits indeed representing a distinct clade among the five PP2AC subunits 33 . This is further corroborated by the recently published analysis of a complete knock-out double pp2ac3-c4 mutant whose phenotype is strikingly reminiscent of ton1 or fass mutants 34 . TON1 also comes into two isoforms, and recent data suggest that TON1a and TON1b are not strictly functionally redundant 12 able to interact with both TON1 and FASS, thanks to their M2 and M3 motif, respectively.…”

PP2A-3 interacts with ACR4 and regulates formative cell division in the Arabidopsis root

“…S1) (34). However, this doublemutant phenotype appeared to be less severe than the one obtained by Ballesteros and coworkers (36), which is possibly due to the use of different T-DNA lines. Short root phenotypes associated with disrupted cell division and/or cellular patterning in the root tip are often associated with a loss of quiescent center cell identity (37,38).…”

“…However, because such a columella phenotype can also be explained by altered auxin distribution, levels, or response (11), the similarity between the pp2a-3 and acr4 mutants does not provide conclusive support for a genetic and/or physical interaction. This is especially relevant because PP2A-3 has been shown to play a role in auxin transport (36). We therefore tested to what extent acr4 and pp2a-3 are affected in their sensitivity to N-1-naphthylphthalamic acid (NPA) with respect to primary root length and columella differentiation.…”

“…However, although we could detect a peptide (NH 3 -GAGYTFGQDI-SEQFNHTNNLK-COOH) for PP2A-3 (or PP2A-4) in all samples, we did not observe any (differential) phosphorylation. Because PP2A holoenzymes act in various pathways (35,36,41,42) and because ACR4 acts in only a few cells, it is likely that subtle differences mediated by ACR4 could be masked. Therefore, we explored if PP2A-3 could be phosphorylated by ACR4 in vitro.…”

Atypical Protein Phosphatase 2A Gene Families Do Not Expand via Paleopolyploidization

“…It regulates multiple pathways in plants and animals, including processes such as mitotic entry and Wnt, ERK, MAPK, and TOR signaling (Janssens and Goris, 2001) in animals. In plants, PP2A plays roles including regulation of auxin transport (Garbers et al, 1996;Rashotte et al, 2001;Shin et al, 2005;Michniewicz et al, 2007;Blakeslee et al, 2008;Sukumar et al, 2009;Ballesteros et al, 2013), ethylene biosynthesis (Larsen and Chang, 2001;Muday et al, 2006;Skottke et al, 2011), abscisic acid and brassinosteroid signaling (Kwak et al, 2002;Pernas et al, 2007;Tang et al, 2011;Wu et al, 2011;Waadt et al, 2015;Wang et al, 2016), pathogen responses (Segonzac et al, 2014), and cytoskeletal regulation of cell division (Camilleri et al, 2002;Wright et al, 2009;Spinner et al, 2013). PP2A complexes are highly abundant, forming up to 1% of cellular protein mass in mammalian tissues (Shi, 2009).…”