Molecular basis for increased 2-acetyl-1-pyrroline contents under alternate wetting and drying (AWD) conditions in fragrant rice

Factors affecting rice aroma biosynthesis have been well documented previously, however the molecular mechanism lies behind the regulations in grain 2-acetyl-1-pyrroline (2AP) biosynthesis under alternate wetting and drying (AWD) remained largely unexplored. Present study investigated the effects of three irrigation regimes i.e., conventional irrigation (CI), alternate wetting and moderate drying (WMD), and alternate wetting and severe drying (WSD) on the yield, quality traits, intermediates, enzyme activities and genes involved in 2-acetyl-1-pyrroline biosynthesis in two fragrant rice cultivars viz, Meixiangzhan2 and Xiangyaxiangzhan. Results revealed that the levels of intermediates such as Δ1-pyrroline-5-carboxylate (P5C) and Δ1-pyrroline, and the activity of enzymes such as proline dehydrogenase (PRODH), Δ1-pyrroline-5-carboxylate synthetase (P5CS), diamine oxidase (DAO), and gene expressions of PRODH, P5CS2 and DAO were comparatively higher under AWD than CI in both aromatic rice cultivars. The levels of gamma-aminobutyric acid (GABA), betaine aldehyde dehydrogenase 2 (BADH2) and BADH2 gene were lower that together led to enhanced 2-AP contents in rice grains. Moreover, WMD irrigation improved yield and yield characters, while WSD irrigation reduced yield and quality traits of rice. Overall, up-regulation of P5C and Δ1-pyrroline and down-regulation of GABA under AWD treatments resulted in enhanced 2AP biosynthesis in both rice cultivars. Evaluation and adoption of AWD (within safe limits) at field level could be an alternative option to conventional flooded rice to get better yield and quality.