Research Advances of Haplotype Variation at Badh2 Gene and 2-Acetyl-1-pyrroline Bi osynthetic Pathway in Aromatic Rice

Aromatic rice has long been favored by the world rice trade because of its pleasant scent. China has a long history of aromatic rice cultivation and diverse aromatic rice accessions. However, there is a real lack of top aromatic rice brands. Nowadays, there is an increasing demand for good-quality aromatic rice, leading to speed up the breeding process of high quality of aromatic rice varieties. The Badh2, encoding betaine aldehyde dehydrogenase gene, is the major genetic basis of fragrance in rice. The loss-of-function alleles of Badh2 accounts for accumulation of 2-acetyl-1-pyrroline(2AP), which is widely regarded as the main compound responsible for the characteristic aroma. With the development of rice functional genomics, the Badh2 gene haplotypes in aromatic rice resources have been deeply explored and the biosynthesis pathway of 2AP has been preliminarily analyzed, which will speed up the process of aromatic rice breeding. This review focused on 19 Badh2 gene haplotypes which were found in aromatic rice accessions. The geographic locations of different types of haplotypes were mainly analyzed and the correlation between these haplotypes and 2AP concentrations were compared. The research advances of glutamate-proline metabolic pathway and polyamine metabolic pathway involved in 2AP biosynthesis pathway were demonstrated. Meanwhile, the 2AP distribution characteristics in aromatic rice grains were summarized. Two theoretical mechanisms of 2AP accumulation in grains were compared and a strategy to study the 2AP biosynthesis pathway changes at various grain developmental stages by multi-omics methods was proposed, with a view to serve as an inspiration for genetic utilization of aromatic rice resources and breeding of rice varieties with 2AP specific enrichment.