Paraiso, F.; Zhang, J.; Lin, H.; Li, C.; Woods, D. P.; Lan, T.; Debernardi, J. M.; Joe, A.; Dubcovsky, J.

In wheat, the transition of the inflorescence meristem (IM) to a terminal spikelet determines the spikelet number per spike (SNS) and the maximum number of grains. In this study, we demonstrate that the plant-specific transcription factor LEAFY (LFY) physically interacts with WHEAT ORTHOLOG OF APO1 (WAPO1), a protein encoded by a gene underlying a major QTL for SNS in wheat. Loss-of-function mutations in LFY, WAPO1, or both result in significant and similar reductions in SNS, which indicates that both genes are required to maintain IM indeterminacy and prevent a precocious transition to a terminal spikelet. We also identify a significant genetic interaction between LFY and VERNALIZATION1 (VRN1) and show that these genes play opposite roles in the regulation of SNS. To understand these interactions, we characterized the spatio-temporal distribution of these and other flowering genes during wheat spike development using single-molecule fluorescence in-situ hybridization. Finally, we identify natural LFY-B alleles that affect SNS, determine their interactions with natural alleles of WAPO-A1, and select an allele combination that maximizes SNS for wheat improvement.