Background Procyanidins is a polyphenolic compound with multiple properties. However, the application of exogenous procyanidins in crops has not been reported. Aromatic rice is a high-quality rice with a special aroma and popular with consumers. The 2-acetyl-1-pyrroline (2-AP) is a key compound of aromatic rice aroma. In the current study, aromatic rice plants were sprayed with procyanidins solutions at 0.25 (Pr0.25), 0.50 (Pr0.50), 1.00 (Pr1.00), 2.00 (Pr2.00) g L−1, respectively and treatment sprayed with distilled water was taken as control (CK). The effects of exogenous procyanidins on growth and 2-AP biosynthesis of aromatic rice plants were explored. Results Compared with CK, Pr1.00 and Pr2.00 treatments significantly increased 2-AP content by 16.67% and 37.68%, respectively. Higher proline, 1-pyrroline-5-carboxylic acid (P5C), 1-pyrroline, methylglyoxal contents, and lower γ- aminobutyric acid (GABA) content were recorded in Pr1.00 and Pr2.00 treatments than CK. Compared with CK, Pr1.00 and Pr2.00 treatments significantly improved the activities of P5CS and OAT and diminished the activity of BADH. Furthermore, compared with CK, Pr1.00 and Pr2.00 treatments significantly up-regulated the transcript levels of P5CS2, P5CR, OAT, DAO4 and down-regulated the transcript levels of BADH2. Exogenous procyanidins had no substantial effects on plant height, stem diameter, fresh weight, and dry weight of aromatic rice plants. Conclusions In conclusion, our findings reported the increment of 2-AP content in aromatic rice under exogenous procyanidins. Our results indicated that the application of exogenous procyanidins enhanced 2-AP biosynthesis by improving proline biosynthesis and inhibiting GABA formation.

Proline is one of the precursors of the biosynthesis of 2-acetyl-1-pyrroline (2-AP) which is the key and characteristic volatile component of fragrant rice aroma. In order to study the effects of exogenous proline on 2-AP biosynthesis and other grain quality attributes in fragrant rice, two indica fragrant rice cultivars, "Meixiangzhan-2" and "Xiangyaxiangzhan", and one japonica fragrant rice, "Yunjingyou", were used in present study. At initial heading stage, proline solutions at 0 (CK), 0.10 (Pro1), 0.20 (Pro2) and 0.50 (Pro3) g L-1 were applied as foliar spray solution to fragrant rice plants. Compared with CK, Pro1, Pro2 and Pro3 treatments significantly increased the grain 2-AP content. The significant up-regulation effects due to proline treatments were observed in the contents of proline, △1-pyrrolidine-5-carboxylic acid (P5C) and △1-pyrroline which involved in 2-AP formation. Exogenous proline application also significantly decreased the grain γ-aminobutyric acid (GABA) content. Furthermore, proline treatments enhanced the activity of proline dehydrogenase (ProDH) as well as transcript level of gene PRODH. On the other hand, the transcript level of gene BADH2 and activity of betaine aldehyde dehydrogenase (BADH) decreased under proline treatments. Proline treatments (Pro2 and Pro3) also increased the grain protein content by 3.57-6.51%. Moreover, 32.03-34.25% lower chalky rice rate and 30.80-48.88% lower chalkiness were recorded in proline treatments (Pro2 and Pro3) for both Meixiangzhan and Xiangyaxiangzhan whilst for Yunjingyou, foliar application of proline had no significant effect on chalky rice rate and chalkiness. There was no remarkable difference observed in grain milled quality (brown rice rate, milled rice rate and head rice rate) and amylose content between CK and proline treatments. In conclusion, exogenous proline enhanced the 2-AP biosynthesis and promoted some grain quality characters of fragrant rice. Fragrant rice is famous for possessing a characteristic aroma and also fetches a high price in the international market because of the good grain quality 1,2. In the past two decades, many studies have conducted to investigate the compound of the aroma of fragrant rice. For example, the study of Widaja et al. 3 showed that the number of volatile compounds detected in the aroma exceeds 300 in both fragrant and non-fragrant rice varieties. Hashemi et al. 4 demonstrated there were more than 100 volatile compounds have been detected in the aroma of fragrant rice varieties. In recent years, with the development of many researches, it is established that 2-acetyl-1-pyrroline (2-AP) is the key compound in fragrant rice aroma 1,5,6. The process of 2-AP biosynthesis in fragrant rice is very complicated which involved many biochemical reactions while numerous studies have been conducted to understand the mechanism of 2-AP biosynthesis. An early study has evidenced that the expression of gene BADH2 which related to the betaine aldehyde dehydrogenase

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Background In plants, cytokinin is activated into trans-zeatin to fight abiotic stresses. However, the mechanism of the effect of trans-zeatin on 2-acetyl-1-pyrroline (2-AP) biosynthesis in fragrant rice has yet to be studied. The present study was conducted to explore the effects of exogenous trans-zeatin on enzymes activities, genes expression, and precursors involved in 2-AP biosynthesis and 2-AP contents as well as the seedling quality of a fragrant rice cultivar viz., Meixiangzhan2. Four concentrations of trans-zeatin solutions at 20, 40, and 80 μmol L− 1 (ZT1, ZT2, and ZT3) were sprayed onto rice seedlings. Results Compared to the control, trans-zeatin treatments showed significantly higher 2-AP contents of fragrant rice seedlings. Increased plant height and stem width were observed due to trans-zeatin treatments. The trans-zeatin application increased 1-pyrroline, methylglyoxal, proline, and P5C contents, enhanced P5CS and OAT activities, and reduced glutamic acid contents. In addition, expressions of ProDH, P5CS2, and DAO4 were comparatively higher under trans-zeatin treatments than CK in fragrant rice seedlings. Conclusions Overall, up-regulation of P5C, 1-pyrroline, and proline and down-regulation of glutamic acid under appropriate trans-zeatin concentrations (20 and 40 μmol L− 1) resulted in enhanced 2-AP biosynthesis in fragrant rice seedlings and 20–40 μmol L− 1 was considered as the suggested concentrations of trans-zeatin application in fragrant rice seedling.

Selenium (Se) is a beneficial element for plants and a necessary element for humans. Previous studies showed Se could increase 2‐acetyl‐1‐pyrroline (2‐AP) content in fragrant rice (Oryza sativa L.), but the underlying molecular mechanism remains largely unexplored. There are several methods for Se application in rice production, and it is not clear which is better. This study investigated the effects of three different Se application methods on 2‐AP biosynthesis, yield formation, and grain Se concentration of fragrant rice through a four‐season field experiment with two fragrant rice cultivars. For soil application, Se was applied as Se mineral powder as basal application to soil (36 g Se ha−1); for foliar application (FA), sodium selenate solution (40 μmol L−1) was applied as a foliar spray at heading stage; and for seed priming, fragrant rice seeds were primed with sodium selenate at 80 mg kg−1 concentration. The treatment without Se was taken as control (CK). The results showed Se applications increased grain 2‐AP content and grain yield by 3.95–22.32% and 1.03–8.05%, respectively. Compared with CK, Se applications regulated transcript levels of genes PRODH, BADH2, DAO, OAT, and P5CS2, which are involved in 2‐AP biosynthesis. Selenium applications also increased the contents of precursors (Δ1‐pyrroline, proline and pyrroline‐5‐carboxylic acid) and enhanced the activities of enzymes (proline dehydrogenase, △1‐pyrroline‐5‐carboxylic acid synthetase, ornithine aminotransferase, diamine oxidase), which are related to 2‐AP. Furthermore, Se applications increased grain Se concentration by 14.12–63.25%. The highest grain yield, grain 2‐AP content, and grain Se concentration were all recorded in the FA treatment.