MAPK Phosphatase AP2C3 Induces Ectopic Proliferation of Epidermal Cells Leading to Stomata Development in Arabidopsis

Abstract

In plant post-embryonic epidermis mitogen-activated protein kinase (MAPK) signaling promotes differentiation of pavement cells and inhibits initiation of stomata. Stomata are cells specialized to modulate gas exchange and water loss. Arabidopsis MAPKs MPK3 and MPK6 are at the core of the signaling cascade; however, it is not well understood how the activity of these pleiotropic MAPKs is constrained spatially so that pavement cell differentiation is promoted only outside the stomata lineage. Here we identified a PP2C-type phosphatase termed AP2C3 (Arabidopsis protein phosphatase 2C) that is expressed distinctively during stomata development as well as interacts and inactivates MPK3, MPK4 and MPK6. AP2C3 co-localizes with MAPKs within the nucleus and this localization depends on its N-terminal extension. We show that other closely related phosphatases AP2C2 and AP2C4 are also MAPK phosphatases acting on MPK6, but have a distinct expression pattern from AP2C3. In accordance with this, only AP2C3 ectopic expression is able to stimulate cell proliferation leading to excess stomata development. This function of AP2C3 relies on the domains required for MAPK docking and intracellular localization. Concomitantly, the constitutive and inducible AP2C3 expression deregulates E2F-RB pathway, promotes the abundance and activity of CDKA, as well as changes of CDKB1;1 forms. We suggest that AP2C3 downregulates the MAPK signaling activity to help maintain the balance between differentiation of stomata and pavement cells.

Stomatal Development in Arabidopsis

“…The modification of YDA, MKK4/5, and MPK3/6 activities through the action of phosphatases represents a regulation point to adjust the initiation of stomata during development. The Arabidopsis protein phosphatase 2C (AP2C3: At2g40180) is expressed specifically in the stomatal lineage from meristemoid onwards and results in an epidermis composed solely of stomata when ectopically expressed (Umbrasaite et al, 2010). This phenotype is similar to yda or mpk3/6 loss-of-function mutants and implicates AP2C3 as a suppressor of MAPK activity in epidermal cells.…”

“…Although direct connections have not been established, downstream signaling from ERf and TMM are likely transduced through a MAPK module, consisting of the MAP-KKK YODA (YDA: At1g63700), MKK4/5 (At1g51660/At3g21220), and MPK3/6 (At3g45640/At2g43790) (Bergmann et al, 2004;Lukowitz et al, 2004;Wang et al, 2007;Umbrasaite et al, 2010). These MAPKs are broadly expressed and have pleiotropic effects on development, but uniformly work to promote pavement cell differentiation and inhibit the initiation of stomata.…”

“…This predicts that AP2C3 deactivates MAPK3/6 specifically in the stomatal lineage, providing a mechanism to tightly adjust the signaling pathway controlling stomatal production (Figure 9). Further work needs to be initiated to investigate the endogenous or redundant role of AP2C3, because loss-of-function mutants have no detectable phenotype (Umbrasaite et al, 2010).…”

Protein phosphatase AP2C1 negatively regulates basal resistance and defense responses toPseudomonas syringae

“…The specificities for interactions between AP2Cs and MAPKs have been demonstrated, and extensive yeast two-hybrid screenings with AP2Cs repeatedly isolated MPK6 (Schweighofer et al , 2007; Umbrasaite et al , 2010). Interaction between AP2Cs and MPK3, 4 and 6 has been confirmed by bimolecular fluorescence complementation (BiFC) (Schweighofer et al , 2007; Brock et al , 2010; Umbrasaite et al , 2010) and co-immunoprecipitation (Schweighofer et al , 2007).…”

Lineage-specific stem cells, signals and asymmetries during stomatal development

“…In addition, a MAPK phosphatase, AP2C3 (also known as PP2C5), is expressed in stomatal lineage cells and when overexpressed confers an epidermis that is solely composed of stomata, suggesting that it might act as a negative regulator of stomatal signaling (Umbrasaite et al, 2010). However, ap2c3 loss-of-function mutants do not exhibit any phenotype (Umbrasaite et al, 2010), implying possible redundancy or, alternatively, that endogenous AP2C3 does not play a direct role in stomatal development. In animals, MAPK cascades play pivotal roles in the regulation of cell proliferation, differentiation, and tissue patterning (Qi and Elion, 2005).…”