Seminars

Efficient gas exchange between a plant and the atmosphere requires coordinated spacing of stomata, turgor-driven valves on the plant epidermis. Recent progress by our group and others has led to the discovery of key molecules and pathways controlling stomatal patterning and differentiation. Stomatal differentiation is specified by sequential and combinatorial actions of 5 bHLH (basic-helix-loop-helix) transcription factors. Heterodimeric partner bHLH proteins, SCREAM/2, integrate the three cell-state transitions mediated by the sister bHLHs, SPEECHLESS (SPCH), MUTE, and FAMA. In order to generate proper stomatal patterning, activities of these transcription factors must be precisely regulated, both spatially and temporarily.  Molecular signals enforcing proper stomatal patterning have recently been identified as the EPIDERMAL PATTERNING FACTOR-LIKE (EPFL) family of small, secreted cysteine-rich peptides. Two founding members, EPF1 and EPF2, are expressed in the late and early stages of stomatal cell-lineages, respectively. EPF2 restricts the specification of cells undergoing the initial asymmetric cell division that creates stomatal precursor cells. On the other hand, EPF1 enforces stomatal spacing. Putative receptors, TOO MANY MOUTHS (TMM) and three ERECTA-family receptor-like kinases, ERECTA, ERL1, and ERL2, exhibit synergistic as well as contrasting effects on stomatal patterning. For example, ER is expressed in the entire epidermal tissues while ERLs and TMM are expressed in the stomatal-lineage cells. Therefore, important questions are whether these putative receptors physically associate with each other, whether they comprise the receptor complex for EPFLs, and how combinatorial activities of the ligands and receptors create diversity of outputs. Furthermore, regulatory relationships between signaling components and key transcription factors (five bHLH proteins; SPEECHLESS, MUTE, FAMA, SCREAM and SCREAM2) still remain unclear. I will present our latest research findings and discuss about the complexity, challenges, and new insight into our understanding of receptor-mediated cell signaling during plant tissue patterning.