Increased and ectopic expression of Triticum polonicum VRT-A2 underlies elongated glumes and grains in hexaploid wheat in a dosage-dependent manner

Abstract

Flower development is a major determinant of yield in crops. In wheat, natural variation for the size of spikelet and floral organs is particularly evident in Triticum polonicum, a tetraploid subspecies of wheat with long glumes, lemmas, and grains. Using map-based cloning, we identified VRT2, a MADS-box transcription factor belonging to the SVP family, as the gene underlying the T. polonicum long-glume (P1) locus. The causal P1 mutation is a sequence re-arrangement in intron-1 that results in both increased and 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 species 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 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 impact on agronomic traits in a dosage-dependent manner in polyploid crops. One-sentence summaryAn intron-1 rearrangement in the MADS-box transcription factor VRT-A2 leads to its misexpression and defines the long-glume phenotype of Polish wheat (T. polonicum).