Effect of the U genome on grain hardness in nascent synthetic hexaploids derived from interspecific hybrids between durum wheat and Aegilops umbellulata

Grain hardness is an important trait for improvement of wheat grain quality, and is mainly controlled by two puroindoline genes, Pina and Pinb, on chromosome 5D. The presence of functional alleles for both PINA and PINB proteins results in a soft grain texture. Here, we report nucleotide sequence variation and novel alleles of Pina and Pinb in a diploid wild wheat relative, Aegilops umbellulata Zhuk. Despite the presence of various alleles, mature grains of all Ae. umbellulata accessions examined had a hard texture. The hard-textured grains could be due to nonsynonymous substitutions in the Pina and Pinb alleles or lack of either PINA or PINB protein accumulation. Synthetic hexaploids with the AABBUU genome, derived from interspecific crosses between durum wheat and the Ae. umbellulata accessions, showed hard-textured grain character due to absent transmission of functional PINA and PINB proteins from the U-genome donors. In addition, increased thickness of the cell wall in the endosperm might contribute to the hard-texture in synthetic hexaploids. The U-genome addition to cultivated tetraploid wheat generally generated hard grains, suggesting that the Ae. umbellulata variation in grain-related traits will be useful for enlargement of grain hardness diversity in hard-textured common wheat.