Flower development is an important determinant of grain yield in crops. In wheat (Triticum spp.), natural variation for the size of spikelet and floral organs is particularly evident in Triticum turgidum ssp. polonicum (also termed Triticum polonicum), a tetraploid subspecies of wheat with long glumes, lemmas, and grains. Using map-based cloning, we identified VEGETATIVE TO REPRODUCTIVE TRANSITION 2 (VRT2), which encodes a MADS-box transcription factor belonging to the SHORT VEGETATIVE PHASE family, as the gene underlying the T. polonicum long-glume (P1) locus. The causal P1 mutation is a sequence rearrangement in intron-1 that results in ectopic expression of the T. polonicum VRT-A2 allele. Based on allelic variation studies, we propose that the intron-1 mutation in VRT-A2 is the unique T. polonicum subspecies-defining polymorphism, which was later introduced into hexaploid wheat via natural hybridizations. Near-isogenic lines differing for the P1 locus revealed a gradient effect of P1 across spikelets and within florets. Transgenic lines of hexaploid wheat carrying the T. polonicum VRT-A2 allele show that expression levels of VRT-A2 are highly correlated with spike, glume, grain, and floral organ length. These results highlight how changes in expression profiles, through variation in cis-regulation, can affect agronomic traits in a dosage-dependent manner in polyploid crops.

The spatial and temporal patterns by which starch granules initiate vary greatly between species and organs, but molecular factors that contribute to these diverse patterns are poorly understood. In wheat, the endosperm of grains contains two distinct types of starch granules, where the synthesis of large A-type granules is initiated during early grain development, followed by small B-type granules that initiate c. 10-15 days after the A-type granules, whereas the leaves contain uniformly-sized starch granules. Here, we reveal distinct organ-specific roles of the MYOSIN-RESEMBLING CHLOROPLAST PROTEIN (MRC) in regulating granule initiation in the endosperm and leaves of wheat. We isolated three independent TILLING mutants of tetraploid wheat (Triticum turgidum cv. Kronos) with premature stop or missense mutations in the A genome homeolog, which is the only active homeolog in tetraploid wheat due to a disruption of the B genome homeolog that occurred shortly after wheat hybridisation. The mutants had significantly smaller A-type granules and a higher relative volume of B-type granules than the wild type. Quantification of starch granule size distributions and microscopy during grain development showed that B-type granules initiated 15 - 20 dpa in the wild-type, but as early as 10 dpa in the mrc-1 mutant, suggesting a novel role of MRC in suppressing B-type granule initiation during early grain development. By contrast, MRC has a role in promoting granule initiation in leaves, as the mutants carrying premature stop mutations in MRC had fewer granules per chloroplast in comparison to the wild type. Overall, these contrasting roles of MRC among wheat organs provide new insight into functional diversification of granule initiation proteins to facilitate the diverse patterns of granule initiation observed across species and organs.

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