The biosynthesis of 2-acetyl-1-pyrroline is physiologically driven by carbon-nitrogen metabolism in fragrant rice

Abstract

Carbon and nitrogen are important energy sources and nutrients, and carbon metabolism and nitrogen metabolism play key roles in the growth and development of fragrant rice. However, little is known about how carbon-nitrogen treatments regulate the biosynthesis of 2-acetyl-1-pyrroline (2-AP), which imparts a special flavor and makes fragrant rice a preferred rice variety, in fragrant rice. The aim of this study was to explore the effects of carbon-nitrogen treatments on 2-AP biosynthesis, carbon metabolism, and nitrogen metabolism and their relationships in fragrant rice. A two-year field experiment was conducted at the experimental farm of South China Agricultural University. Two fragrant rice varieties, Meixiangzhan2 and Xiangyaxiangzhan, were used as materials, three nitrogen levels (urea sources at concentrations of 0, 0.5 %, and 1.0 %) and three carbon levels (glucose sources at concentrations of 0, 1.5 %, and 3 %) were used at the booting stage, and treatment without the application of glucose and urea was used as the control treatment. The 2-AP accumulation, carbon metabolism, and nitrogen metabolism parameters were investigated. The results showed that the carbon-nitrogen treatments differed from the control treatment. The 2-AP content in grains was significantly influenced by carbon, nitrogen, and their interactions, yet a significant impact of nitrogen on head rice yield was detected only. Treatment with 0.5 % urea + 1.5 % glucose resulted in the highest 2-AP content in the grains, which was 39.83–138.27 % and 56.45–123.77 % greater than that in the control treatment in Meixiangzhan2 and Xiangyaxiangzhan, respectively. Furthermore, a structural equation model indicated that nitrogen metabolism and sugar metabolism benefit 2-AP formation-related attributes in grain and ultimately lead to 2-AP accumulation. Sugar metabolism can directly lead to the accumulation of sugar and starch in stems and sheaths and 2-AP accumulation in grains. Overall, our results indicate that optimized carbon-nitrogen levels and/or carbon-nitrogen metabolism in fragrant rice plants benefit head rice yield and 2-AP accumulation in fragrant rice. Treatment with 0.5 % urea + 1.5 % glucose effectively increased the 2-AP content while maintaining a relatively high head rice yield in fragrant rice.