Arabidopsis guard cell integrity involves the epigenetic stabilization of the FLP and FAMA transcription factor genes.

Arabidopsis guard cell (GC) fate is conferred via a transient pulse of expression of FAMA that encodes a bHLH transcription factor. Stomata often function for years, suggesting that the FAMA expression window stabilizes long-term GC identity or that additional factors operate. Transgenic lines harboring a copy of a FAMA transgene were found to induce the fate resetting of mature GCs to that of lineage-specific stem cells causing new stomata to arise within shells of the old, a Stoma-in-Stoma (SIS) phenotype. These lines disrupt the normal trimethylation on lysine 27 of histone3 (H3K27me3) on stomatal stem cell genes, a phenotype rescued by constitutive expression of the Polycomb Group (PcG) gene CURLY LEAF. Thus the stability of stomatal fate is enforced by a PcG-mediated reduction in the transcriptional accessibility of stem cell genes and by the endogenous FAMA gene itself. Moreover, a transgenic FOUR LIPS gene, which encodes a MYB protein that is not required for GC fate, also induces a SIS phenotype and disrupts H3K27 trimethylation. Thus FLP might indirectly enforce GC fate as well.

Deep functional redundancy between FAMA and FOUR LIPS in stomatal development.

Functional redundancy arises between gene paralogs as well as non-homologous genes that play a common role at a shared node. The bHLH transcription factor FAMA, along with the paralogous MYB genes, FOUR LIPS (FLP) and MYB88 all ensure that Arabidopsis stomata contain just two guard cells (GCs) by enforcing a single symmetric precursor cell division before stomatal maturity. Consistent with this function, FLP and FAMA exhibit the same expression pattern in which both translational GFP fusions emit fluorescence just before and after symmetric division; however, FAMA but not FLP is required to confer GC fate. Strikingly, swapping the genes and promoters of the FLP and FAMA genes results in the reciprocal complementation of respective loss-of-function mutants. Thus, an FLP transgene can restore GC fate to a fama mutant background. FAMA, FLP and the FLP paralog MYB88 were previously shown to influence higher order functions in stomatal development, including maintaining and stabilizing stomatal fate. Here we show that these overlapping functions are likely to also involve interactions between FLP and FAMA with the RETINOBLASTOMA-RELATED (RBR) protein.

Polar development of preprophase bands and cell plates in the Arabidopsis leaf epidermis.

Preprophase bands are belts of cortical microtubules that appear at the end of interphase and predict where cell plates will fuse with parental walls during division. Phragmoplasts are microtubule-rich arrays that orchestrate the growth and guidance of cell plates during cytokinesis. Descriptions of the development of these arrays often assume non-polar formation, with preprophase bands developing more or less simultaneously around the cell circumference. Phragmoplasts are often described as initiating at the cell center and then expanding evenly outwards until fusion with parent cell walls. We analyzed the spatio-temporal development of both arrays because initial observations of array growth in the Arabidopsis leaf epidermis revealed directional variability. Almost all preprophase bands formed in a polar fashion, with initiation and maturation occurring first in the cell cortex near the inside of the leaf, and later in the outer cell cortex. A similar polarity developed in phragmoplasts and cell plates, raising the possibility that polarized division is common in plants. Together, these findings identify additional polar features of the epidermis, and thereby provide a visually accessible system for identifying new proteins and subcellular components involved in the development of cell division and the previously formed division site.

Tip-growing cells of the moss Ceratodon purpureus Are gravitropic in high-density media.

Gravity sensing in plants and algae is hypothesized to rely upon either the mass of the entire cell or that of sedimenting organelles (statoliths). Protonemata of the moss Ceratodon purpureus show upward gravitropism and contain amyloplasts that sediment. If moss sensing were whole-cell based, then media denser than the cell should prevent gravitropism or reverse its direction. Cells that were inverted or reoriented to the horizontal displayed distinct negative gravitropism in solutions of iodixanol with densities of 1.052 to 1.320 as well as in bovine serum albumin solutions with densities of 1.037 to 1.184 g cm(-3). Studies using tagged molecules of different sizes and calculations of diffusion times suggest that both types of media penetrate through the apical cell wall. Estimates of the density of the apical cell range from 1.004 to 1.085. Because protonemata grow upward when the cells have a density that is lower than the surrounding medium, gravitropic sensing probably utilizes an intracellular mass in moss protonemata. These data provide additional support for the idea that sedimenting amyloplasts function as statoliths in gravitropism.