The PEAPOD repressor complex in Arabidopsis stomatal development

Abstract

Stomata comprise two guard cells that function as microscopic valves in the plant epidermis, connecting mesophyll interstices to the atmosphere. Stomata regulate gas exchange and evapotranspiration, directly impacting photosynthesis and leaf temperature regulation, and their function is thus crucial for plant adaptability and fitness. In Arabidopsis, stomatal development is primarily driven by three basic helix-loop-helix transcription factors: SPEECHLESS (SPCH), MUTE, and FAMA, and occurs within the broader context of leaf development. During leaf development, a characteristic division-to-differentiation transition zone, marked by the first cell cycle arrest front (1^st AF), progresses from the apex to the base of the leaf blade. The repeated division of meristemoids (M), self-renewing cells of stomatal lineages, is not halted during 1^st AF, requiring a second arrest front, which is associated with activity of the PEAPOD (PPD) proteins, PEAPOD1 (PPD1) and PEAPOD2 (PPD2), which form a transcriptional repressor complex that halts M stem cell-like activity; however, the relationship between PPDs and stomatal development has not been fully elucidated. Here, we review data on PPD-mediated regulation of light signaling and the cell cycle and the influence of these factors on stomatal development.

Keywords: PEAPOD; asymmetric cell division; cell cycle; leaf development; light signaling; stomatal development.

Figure 1

(A) Schematic overview of the proliferative zone, 1^st cell cycle arrest front (1^st arrest front), 2^nd cell cycle arrest front (2^nd arrest front), and non-division zone during development from leaf primordium to mature leaf in Arabidopsis. (B) Stomatal lineage cell types found in leaf epidermis: protodermal cell, meristemoid mother cell (MMC), meristemoid (M), guard mother cell (GMC) and guard cell (GC), stomatal lineage ground cell (SLGC) and pavement cell. Among them, only M exhibit proliferative activity. Cell types where 1^st arrest front (1^st AF) and 2^nd arrest front (2^nd AF) act are depicted with bold lines. Amplifying and spacing divisions in the stomatal lineages are indicated with dashed lines. (C) Comparison of proliferative activity zones during leaf development in wild type (above) and ppd (below) mutant plants. Extended M proliferative activity in ppd mutants is depicted in pale green bars below. Note that ppd mutation results in bigger leaves.

Figure 2

(A) Integration of PEAPODs (PPD) function with the control of light signaling networks comprising PHYB, CRY1, COP1, SPA1, HY5 and PIF4; and stomatal development SPCH-EPFL9 module. (B) Schematic overview showing interconnection of PPD complex, and its direct regulator SAP, with SPCH, SCRM/2 and the cell cycle machinery through their known targets, CYCD3;1 and CYCD3;2. Dashed lines: indirect activation/stabilization or repression/degradation. Solid lines: direct activation/stabilization or repression/degradation.