Ectopic expression of VRT-A2 underlies the origin of Triticum polonicum and Triticum petropavlovskyi with long outer glumes and grains.

Polish wheat (Triticum polonicum) is a unique tetraploid wheat species characterized by an elongated outer glume. The genetic control of the long-glume trait by a single semi-dominant locus, P1 (from P olish wheat), was established more than 100 years ago, but the underlying causal gene and molecular nature remain elusive. Here, we report the isolation of VRT-A2 , encoding an SVP-clade MADS-box transcription factor, as the P1 candidate gene. Genetic evidence suggests that in T. polonicum , a naturally occurring sequence rearrangement in the intron-1 region of VRT-A2 leads to ectopic expression of VRT-A2 in floral organs where the long-glume phenotype appears. Interestingly, we found that the intron-1 region is a key ON/OFF molecular switch for VRT-A2 expression, not only because it recruits transcriptional repressors, but also because it confers intron-mediated transcriptional enhancement. Genotypic analyses using wheat accessions indicated that the P1 locus is likely derived from a single natural mutation in tetraploid wheat, which was subsequently inherited by hexaploid T. petropavlovskyi. Taken together, our findings highlight the promoter-proximal intron variation as a molecular basis for phenotypic differentiation, and thus species formation in Triticum plants. A naturally occurring sequence rearrangement in the intron-1 region of the MADS-box gene VRT-A2 , featuring a 560-bp deletion coupled with a 157-bp sequence substitution, leads to ectopic expression of VRT-A2 in floral organs and underlies the origin of Polish wheat (Triticum polonicum) and 'Daosuimai' (T. petropavlovskyi), species with elongated outer glumes and grains. [ABSTRACT FROM AUTHOR]