Study on Differences in 2-AP Synthesis and Metabolism Among Fragrant Rice Varieties

Fragrant rice is regarded as a premium variety due to its distinctive aroma, delicate texture, and rich nutritional value. This aroma primarily originates from 2-acetyl-1-pyrroline (2-AP), but the metabolic basis of 2-AP remains elusive to this day, and the genetic basis for metabolite accumulation is largely unknown. While several researchers have investigated differences in 2-AP synthesis pathways between fragrant and non-fragrant rice, few studies have examined the 2-AP synthesis pathways in fragrant rice varieties exhibiting 2-AP differences. Therefore, after conducting gene similarity analyses on six fragrant rice varieties, we measured the expression levels of substances and related genes involved in multiple metabolic pathways within the 2-AP synthesis pathway, along with the specific enzyme activities associated with these pathways. Results indicate that XG12 (Guizhou Fragrant Rice Variety) exhibits the highest 2-AP content, yet its efficiency in synthesizing 2-AP is not the highest across any individual metabolic pathway. This finding reveals that among fragrant rice varieties, 2-AP content negatively correlates with OsBadh2 expression levels and GABA content, while showing no linear correlation with other related substances or metabolic genes. At this point, variations in the major effect gene OsBadh2 no longer dominate; instead, subtle differences in 2-AP content are jointly determined by numerous minor effect genes and environmental factors. This phenomenon not only resolves apparent contradictions but profoundly illuminates the complex regulatory mechanisms governing 2-AP biosynthesis. 2-AP synthesis represents a dynamic equilibrium process, with different fragrant rice varieties potentially accumulating 2-AP through distinct metabolic pathways. Additionally, this study analyzed the volatile organic compounds (VOCs) of six fragrant rice varieties through metabolomics. Results revealed that DLX, which exhibited the lowest 2-AP content, contained the richest array of aggregated VOCs, indicating no correlation between 2-AP and numerous VOCs. Our findings provide a clear research direction for elucidating the genetic regulatory mechanisms of 2-AP underlying fragrant rice and lay the foundation for technological research aimed at enhancing the aroma of fragrant rice varieties.