Divergent routes to specialization: Guard cells, myrosin cells, and beyond.

Plant cell type diversity often arises from transcriptional regulatory programs that co-opt conserved transcription factors (TFs) involved in other developmental programs. For example, the basic helix-loop-helix (bHLH) TF FAMA directs the differentiation of guard cells (GCs), and part of the FAMA regulatory network was co-opted to direct the differentiation of Brassicales-specific idioblasts known as myrosin cells (MCs). In this review, we explore how new cell types and lineage-specific innovations can be specified by the same conserved bHLH TFs through different sets of downstream targets. We discuss how two direct targets of FAMA, WASABI MAKER (WSB) and STOMATAL CARPENTER 1 (SCAP1), have different effects on cell differentiation: sequential WSB-SCAP1 activation ensures GC maturation, whereas sustained WSB activity suppresses GC identity and establishes MC identity by activating the WSB target gene CELL CYCLE SWITCH PROTEIN 52 A1. We summarize the results of a single-cell transcriptome deep sequencing analysis that uncovered unexpected MCs in Arabidopsis thaliana roots that appear to be derived from phloem lineages rather than ground meristem in a process potentially regulated by the FAMA-WSB module, highlighting the developmental flexibility of these cell types. We discuss the finding that a FAMA-like regulator in the liverwort Marchantia polymorpha was co-opted for seta development, suggesting that FAMA-like factors were independently recruited multiple times across land plants. These examples collectively illustrate how conserved TFs diversify cell fates through co-option, providing a framework for addressing broader questions about cellular specialization in plants.