Complex signals for simple cells: the expanding ranks of signals and receptors guiding stomatal development

Abstract

In development, pattern formation requires that cell proliferation and differentiation be precisely coordinated. Stomatal development has served as a useful model system for understanding how this is accomplished in plants. Although it has been known for some time that stomatal development is regulated by a family of receptor-like kinases (RLKs) and an accompanying receptor-like protein (RLP), only recently have putative ligands been identified. Despite the structural homology demonstrated by the genes that encode these small, secreted peptides, they convey different information, vary with one another in their relationship to common signaling components, control distinct aspects of stomatal development, and do so antagonistically. Their discovery has revealed the intricate network of interactions required upstream of RLK signal transduction for the patterning of complex tissues. However, at issue still is whether specific ligand-receptor combinations are responsible for the activation of discrete signaling pathways or spatiotemporal modulation of a common pathway. This review integrates the latest findings regarding RLK-mediated signaling in stomatal development with emerging paradigms in the field.

Figure 1. Progression through the stomatal lineage and expression patterns of EPF, ERf and TMM genes

Depiction of the origin and progression of cells in the stomatal lineage of the leaf epidermis. Examples of divisions creating specific precursor cell types are shown in bulk on the growing leaf. Above each leaf stage are depicted two cells, one stomatal lineage and one not, and their development over time. The expression pattern of ligands (color coded shading as indicated in the key) follows their published transcriptional reporter expression. Receptors (depicted with the extracellular portion as a V-shape) are placed with their intracellular domains in the cell type corresponding to published transcriptional reporter expression. MMC, meristemoid mother cell; M, meristemoid, GC, guard cell.

Figure 2. Structural conservation and phylogenetic relationships within the eleven-member EPF gene family

A. Protein sequence alignment of conserved C-termini generated using ClustalW2. The STOMAGEN propeptide is processed in vivo to yield the bioactive 45-amino-acid C-terminal fragment depicted above [18,19]. Among all EPFs, cysteine residues are strictly conserved at six positions. It was demonstrated with STOMAGEN that these residues form disulfide bridges and are essential for function [19]. B. A Neighbor-Joining phylogeny [36] of EPF family members generated using Kalignvu [37]. In contrast to CHALLAH and EPF1/2, which belong to distinct clades, STOMAGEN fails to cluster with any other EPF genes.

Figure 3. Genetic model for EPF signaling in stomatal development

The EPF family peptides regulate distinct phases of stomatal development and diverge in their relationship to TMM and the ERf. EPF2 (red) restricts acquisition of MMC identity in protodermal cells (1.) and activates ERf-signaling in the presence and absence of TMM. EPF2 may also function synergistically with EPF1 (orange) (and TMM-dependently) to inhibit amplifying asymmetric divisions (2.), the acquisition of MMC identity among SLGCs (3.), and the meristemoid to GMC transition (4.). EPF1 functions TMM-dependently to orient the production of satellite meristemoids such that they do not form in contact with stomata or precursors (5). STOMAGEN (green) may inhibit TMM-mediated signaling by binding TMM receptor complexes in competition with other EPFs. In a subset of stomata-producing tissues (such as stems and hypocotyls), TMM titrates CHAL (yellow) to prevent CHAL from activating ERf signaling complexes. Ectopic activation of ER-signaling by CHALLAH inhibits entry and progression through the stomatal lineage (6.). All depicted relationships are based on genetic data. In the above figure, arrows or T-bars indicate activation and inhibition, respectively and are colored to correspond with the factors responsible. Black T-bars correspond with roles that may be fulfilled collectively by EPF1 and EPF2. For the sake of simplicity, ERf members are depicted en masse, but it is important to recognize that ER, ERL1, and ERL2 function in distinct phases of stomatal development.