Variations in endosperm structure facilitate the formation of high protein quality in tetraploid rice

Rice (Oryza sativa L.) is an important food source and a primary source of high-quality protein. Polyploid breeding is an effective approach to improving the nutritional quality of crops. Several stable tetraploid rice lines with both high seed setting rates and high protein content have been bred. In the present study, the protein quality of two tetraploid rice lines GD2-4x and GD4-4x with high protein content was analyzed in detail, and mechanisms associated with the high protein quality were explored from the perspective of endosperm structure. The results showed that the total protein content of GD2-4x and GD4-4x increased significantly by 40.27% and 35.15%, respectively, when compared to that of 9311-2x (control). The content of each protein component also increased significantly, with a major increase being observed in glutelin content. The contents of 16 types of amino acids and total amino acids, as well as contents of nutrient limiting essential amino acids, such as lysine, threonine, and methionine, in tetraploid rice lines increased significantly when compared to those in the control. The thickness of the aleurone layer on the dorsal, lateral, and ventral positions of GD2-4x and GD4-4x seeds during different developmental days increased significantly when compared to that of 9311-2x seeds. Amyloplasts were more regular and loosely arranged in GD2-4x and GD4-4x seeds. The tetraploid rice lines had higher total protein and amino acid contents, with glutelin accounting for the highest proportion of the increase. The tetraploid rice lines with high protein content had higher nutritional quality and value than the diploid rice line. The high protein quality of tetraploid rice lines could be associated with an increase in aleurone layer thickness, as well as changes in amyloplast morphology and distribution. This study demonstrates that polyploidization is an effective breeding approach to improving the nutritional quality and value of rice seeds. The results provide a theoretical basis for the utilization of tetraploid high protein rice lines and a reference for improving the nutritional quality of other crops.