Rice Aroma: A Natural Gift Comes with Price and the Way Forward

Aromatic rice belongs to a small but important sub-group of rice, which is highly regarded for its excellent aroma and superior grain quality. Aromatic rice, especially Basmati- and Jasmine-type rice, is being traded at a high price in the local and global markets. Genetically, rice aroma is a phenotypical expression of spontaneous recessive mutations of the OsBadh2 gene (also known as fgr/badh2/osbadh2/os2AP gene). These mutations inhibit the flow of γ-aminobutyraldehyde (GAB-ald) to γ-aminobutyric acid (GABA), and consequently, the accumulated GAB-ald is diverted to a potent flavour component 2-acetyl-1-pyrroline (2AP) by a non-enzymatic reaction with methylglyoxal. The natural incidence of non-functional osbadh2 mutation along with selection and nursing by the farmer from the ancient time makes rice aroma as a prominent natural gift. As GABA and methylglyoxal play significant roles in stress tolerance, and their biosynthesis is strictly regulated in rice plants, the accumulation of 2AP in aromatic rice depends on the interaction of various genetic and environmental factors, and its production may come at some costs of sacrificing tolerance. This review focused on some potential underlying genes in the 2AP and GABA biosynthesis pathways, and analyzed most aspects of aroma formation in rice, and summarized the molecular mechanism of aroma production together with its genetic and non-genetic influencing factors. The present review also stated approaches to produce high-quality aromatic rice via developing novel cultivars and with good agronomic knowledge-based practice.