Development generates complex morphologies in a remarkably reproducible manner, and is characterized by tightly controlled gene expression driven by the activity of transcription factors (TFs). TF activity is highly regulated, integrating inputs across multiple regulatory levels to impact developmental outcomes. In plants, this is exemplified by CLASS III HOMEODOMAIN-LEUCINE ZIPPER (HD-ZIPIII) proteins, a >700 million-year-old family that arose before the common ancestor of Chlorokybus algae and land plants. HD-ZIPIII TFs were then repeatedly coopted to regulate pivotal developmental innovations including stem cell niches, lateral organs, and vasculature. HD-ZIPIII activity is regulated by multiple mechanisms, including the miRNA mir166 and the LITTLE ZIPPER (ZPR) family of microProteins. HD-ZIPIII proteins also contain a START domain. Initially identified in animals, START domains adopt an α/β helix-grip fold, creating a hydrophobic pocket which accommodates lipophilic ligands ranging from long-chain fatty acids to sterols to isoprenoids. We recently discovered HD-ZIPIII TFs also contain a second, new type of START domain. Remarkably, the impact of both HD-ZIPIII START domains remains unknown. Using PHABULOSA (PHB) as a representative HD-ZIPIII protein, we demonstrate that the first START domain promotes dimerization, DNA-binding competence, and transcriptional potency, while the second START domain appears to be required for HD-ZIPIII DNA-binding activity. The developmental and evolutionary implications of these findings will be discussed. I will also briefly discuss our efforts to use flat leaf production as a model to identify determinants and relationships that lend robustness to the complex process of development.