A cascade of arabinosyltransferases controls shoot meristem size in tomato

Zachary Lippman and colleagues report mutations in the tomato ortholog of CLV1 and a gene encoding a hydroxyproline O-arabinosyltransferase enzyme that modifies CLV3, both of which cause fasciated flowers and fruits owing to increased meristem size. They also find that a natural mutation in CLV3 was a major target of selection during tomato domestication. Shoot meristems of plants are composed of stem cells that are continuously replenished through a classical feedback circuit involving the homeobox WUSCHEL ( WUS ) gene and the CLAVATA ( CLV ) gene signaling pathway. In CLV signaling, the CLV1 receptor complex is bound by CLV3, a secreted peptide modified with sugars. However, the pathway responsible for modifying CLV3 and its relevance for CLV signaling are unknown. Here we show that tomato inflorescence branching mutants with extra flower and fruit organs due to enlarged meristems are defective in arabinosyltransferase genes. The most extreme mutant is disrupted in a hydroxyproline O-arabinosyltransferase and can be rescued with arabinosylated CLV3. Weaker mutants are defective in arabinosyltransferases that extend arabinose chains, indicating that CLV3 must be fully arabinosylated to maintain meristem size. Finally, we show that a mutation in CLV3 increased fruit size during domestication. Our findings uncover a new layer of complexity in the control of plant stem cell proliferation.