The two guard cells of a stoma are produced by a single symmetric division just before terminal differentiation. Recessive mutations in the FOUR LIPS (FLP) gene abnormally induce at least four guard cells in contact with one another. These pattern defects result from a persistence of precursor cell identity that leads to extra symmetric divisions at the end of the cell lineage. FLP is likely to be required for the correct timing of the transition from cell cycling to terminal differentiation. FLP encodes a two-repeat (R2R3) MYB protein whose expression accumulates just before the symmetric division. A paralogous gene, MYB88, overlaps with FLP function in generating normal stomatal patterning. Plants homozygous for mutations in both genes exhibit more severe defects than flp alone, and transformation of flp plants with a genomic MYB88 construct restores a wild-type phenotype. Both genes compose a distinct and relatively basal clade of atypical R2R3 MYB proteins that possess an unusual pattern of amino acid substitutions in their putative DNA binding domains. Our results suggest that two related transcription factors jointly restrict divisions late in the Arabidopsis thaliana stomatal cell lineage.

Eukaryotic endoplasmic reticulum (ER)-plasma membrane (PM) contact sites are evolutionarily conserved microdomains that have important roles in specialized metabolic functions such as ER-PM communication, lipid homeostasis, and Ca 2+ influx. Despite recent advances in knowledge about ER-PM contact site components and functions in yeast (Saccharomyces cerevisiae) and mammals, relatively little is known about the functional significance of these structures in plants. In this report, we characterize the Arabidopsis (Arabidopsis thaliana) phospholipid binding Synaptotagmin1 (SYT1) as a plant ortholog of the mammal extended synaptotagmins and yeast tricalbins families of ER-PM anchors. We propose that SYT1 functions at ER-PM contact sites because it displays a dual ER-PM localization, it is enriched in microtubule-depleted regions at the cell cortex, and it colocalizes with Vesicle-Associated Protein27-1, a known ER-PM marker. Furthermore, biochemical and physiological analyses indicate that SYT1 might function as an electrostatic phospholipid anchor conferring mechanical stability in plant cells. Together, the subcellular localization and functional characterization of SYT1 highlights a putative role of plant ER-PM contact site components in the cellular adaptation to environmental stresses.

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