J. Dubcovsky is the presenting author. Email jdubcovsky@ucdavis.edu

In wheat, the transition of the inflorescence meristem to a terminal spikelet (IM->TS) determines spikelet number per spike (SNS), an important grain yield component. Simultaneous loss-of-function mutations in the wheat SQUAMOSA MADS-box genes VRN1, FUL2 and FUL3 result in a reversion of spikelets to tillers subtended by leaves. Mutations in individual vrn1 or ful2 genes increase SNS, whereas vrn1 ful2 combined mutants have an indeterminate IM.

These increases are associated with a combination of a delay in the IM->TS transition and changes in the rate of lateral spikelet development. By contrast, loss-of-function mutations in LEAFY or WAPO1 reduce SNS, mainly by reducing the rate of spikelet formation. LFY and WAPO1 interact physically and genetically with each other, with lfy wapo1 showing a similar reduction in SNS as the single mutants, indicating that they jointly regulate the number of spikelets per spike. Single-molecule fluorescence in-situ hybridization revealed that LFY and WAPO1 are expressed together at the early stages of spike development but not at later stages.

This result suggests that the formation of the LFY-WAPO1 complex at the initiation of spike development is sufficient to alter the rate of spikelet formation. The spatial transcriptomics also showed that he down-regulation of LFY and up-regulation of the SQUAMOSA MADS-box genes in the distal part of the developing spike during the IM->TS transition, supporting their opposite roles on spikelet development. LFY and WAPO1 also play an important role in spikelet development.

Loss-of-function mutations in either gene result in the downregulation of similar floral organ identity genes and result in similar floral defects in lodicules, stamens and pistils. At the lemma primordia stage, LFY is expressed at high levels in a nest-shaped region distal to the lemma primordia and proximal to the WAPO1 expression domain.

We hypothesize that the overlapping expression domains of LFY and WAPO1 facilitate interactions between their encoded proteins, providing important spatial information for the developing florets. Finally, we describe LFY and WAPO1 natural alleles with significant effects on SNS and identify allele combination that maximize SNS for wheat improvement.