The functional specificity of ERECTA-family receptors in Arabidopsis stomatal development is ensured by molecular chaperones in the endoplasmic reticulum.

Stomata are epidermal pores that control gas exchange between plants and the atmosphere. In Arabidopsis, the ERECTA family (ERECTAf) receptors, including ERECTA, ERECTA-LIKE 1 (ERL1) and ERL2, redundantly play pivotal roles in enforcing the 'one-cell-spacing' rule. Accumulating evidence has demonstrated that the functional specificities of receptors are likely associated with their differential subcellular dynamics. The endoplasmic reticulum (ER)-resident chaperone complex SDF2-ERdj3B-BiP functions in many aspects of plant development. We employed pharmacological treatments combined with cell biological and biochemical approaches to demonstrate that the abundance of ERECTA was reduced in the erdj3b-1 mutant, but the localization and dynamics of ERECTA were not noticeably affected. By contrast, the erdj3b mutation caused the retention of ERL1/ERL2 in the ER. Furthermore, we found that the function of SDF2-ERdj3B-BiP is implicated with the distinct roles of ERECTAf receptors. Our findings establish that the ERECTAf receptor-mediated signaling in stomatal development is ensured by the activities of the ER quality control system, which preferentially maintains the protein abundance of ERECTA and proper subcellular dynamics of ERL1/ERL2, prior to the receptors reaching their destination - the plasma membrane - to execute their functions.

Arabidopsis F-BOX STRESS INDUCED 4 is required to repress excessive divisions in stomatal development.

During the terminal stage of stomatal development, the R2R3-MYB transcription factors FOUR LIPS (FLP/MYB124) and MYB88 limit guard mother cell division by repressing the transcript levels of multiple cell-cycle genes. In Arabidopsis thaliana possessing the weak allele flp-1, an extra guard mother cell division results in two stomata having direct contact. Here, we identified an ethylmethane sulfonate-mutagenized mutant, flp-1 xs01c, which exhibited more severe defects than flp-1 alone, producing giant tumor-like cell clusters. XS01C, encoding F-BOX STRESS-INDUCED 4 (FBS4), is preferentially expressed in epidermal stomatal precursor cells. Overexpressing FBS4 rescued the defective stomatal phenotypes of flp-1 xs01c and flp-1 mutants. The deletion or substitution of a conserved residue (Proline166) within the F-box domain of FBS4 abolished or reduced, respectively, its interaction with Arabidopsis Skp1-Like1 (ASK1), the core subunit of the Skp1/Cullin/F-box E3 ubiquitin ligase complex. Furthermore, the FBS4 protein physically interacted with CYCA2;3 and induced its degradation through the ubiquitin-26S proteasome pathway. Thus, in addition to the known transcriptional pathway, the terminal symmetric division in stomatal development is ensured at the post-translational level, such as through the ubiquitination of target proteins recognized by the stomatal lineage F-box protein FBS4.