New pathways in stem cell regulation and impact on crop yields

Plants grow and develop in remarkably challenging and changing conditions, and are critical for our food supply and all life on earth. Plant growth depends upon meristems, pools of stem cells that are maintained in a number of ways, including a negative feedback loop between the CLAVATA pathway and a homeodomain transcription factor, WUSCHEL. CLAVATA signaling involves a secreted peptide, CLAVATA3 (CLV3), and its perception by cell surface leucine-rich repeat (LRR) receptors, including the CLV1 receptor kinase, and an LRR receptor-like protein, CLV2. We are interested in finding novel players in the CLV-WUS pathway, as well as new regulators that could affect meristem size in parallel pathways. Maize is an excellent model system for these studies, because there are many clavata- type mutants, and their isolation is becoming routine. For example, we isolated the COMPACT PLANT2 (CT2) gene, and found it encodes the predicted a subunit (Ga) of a heterotrimeric GTP binding protein. ct2 mutants have CLAVATA-like meristem proliferation phenotypes, and genetic, biochemical and functional assays indicate that CT2/Ga signaling transmits a stem cell restrictive signal from a maize CLAVATA LRR receptor, suggesting a new function for Ga signaling in plants. Recent studies have questioned the idea that plant heterotrimeric G proteins interact with canonical GPCRs, and our findings suggest that single pass LRR receptors act as GPCRs in plants, challenging the dogma that GPCRs are exclusively 7TM proteins. We also characterized a new CLV- related receptor, FASCIATED EAR3 (FEA3), that appears to function in a different pathway to control meristem size, in parallel to CLV-WUS. We found that FEA3 responds to a CLV3-related ligand that is expressed in differentiating cells in leaf primordia, so it could provide a feedback from these differentiating primordia to the meristem. This pathway is therefore complementary to the known CLV-WUS feedback, and computational modeling suggests the 2 pathways interact. I will discuss the functions of these new pathways, as well as their potential use in improvement of crop yields, for example through enhancing seed number.