Promotes cell fate progression in stomatal development. In leaves, needed to correctly orient spacing divisions, to limit the number of asymmetric divisions in neighbor cells, and to promote the asymmetric (amplifying) divisions of meristemoids. In stems, promotes the conversion of meristemoids into guard mother cells (GMC) (PubMed:11090210, PubMed:12040198, PubMed:18979118). Positively regulates CAPRICE (CPC) expression in differentiating stomaless-forming cell files (PubMed:19513241). Forms constitutive complexes with ERECTA and ERL1 involved in the recognition of the stomatal regulatory peptides EPF1, EPF2 and EPFL9/STOMAGEN (PubMed:28536146). Modulates the activity of the ligand-receptor pairs EPF2-ERECTA and EPF1-ERL1 in stomatal development (PubMed:16002616, PubMed:22241782). Functions in a combinatorial specific manner with the ERECTA-family (ERf) receptor kinases in the regulation of the immune response (PubMed:27446127). Forms heterodimer with ERECTA or ERL1 through their extracellular domains (PubMed:22241782, PubMed:28536146). Not able to form homodimer (PubMed:22241782). Interacts with EPF2 but not with EPF1 (PubMed:22241782). Interacts with SERK1, SERK2, SERK3/BAK1 and SERK4 (PubMed:26320950). Interacts with EPFL9/STOMAGEN (PubMed:26083750). In epidermal cells of developing shoots and leaves, but not in roots. Expressed in the stomatal cell lineage in the developing epidermis. Accumulates strongly in meristemoid mother cells (MMC) and meristemoids, somewhat less in meristemoid sister cells (stomatal-lineage ground cells, SLGC), and is barely detected in pavement cells (PubMed:26203655). Expressed in the stomatal precursor cells and in immature stomata (PubMed:19513241). Decreases as leaves matured and no expression in fully expanded leaves (PubMed:18979118). Up-regulated by SPCH-SCRM module. Lack of stomata in stem, hypocotyl and on the adaxial side of the sepal. By contrast, cotyledons, anthers and abaxial side of the sepal have excess stomata, with many in direct contact and producing clusters. Altered responses to abscisic acid (ABA) (PubMed:18434605, PubMed:24553751). Enhanced susceptibility to the necrotrophic fungus Plectosphaerella cucumerina BMM (PcBMM) (PubMed:27446127). TMM lacks an intracellular kinase domain and may participate in active signal transduction only through physical interaction with other proteins (PubMed:12040198). Overexpression of TMM leads to altered formation of trichomes (PubMed:24553751). Belongs to the RLP family.