Exogenous proline induces regulation in 2-acetyl-1-pyrroline (2-AP) biosynthesis and quality characters in fragrant rice (Oryza sativa L.)

Abstract

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

Trans-Zeatin induce regulation the biosynthesis of 2-acetyl-1-pyrroline in fragrant rice (Oryza sativa L.) seedlings

“…Proline was recognized as the precursor for 2AP biosynthesis [18]. Although PDH plays a crucial part in proline degradation [19,20], its activity presented an opposite trend with increased proline contents. In contradiction to earlier findings [20], the increase in proline contents reduced the activity of PDH and the transcript level of ProDH.…”

“…Although PDH plays a crucial part in proline degradation [19,20], its activity presented an opposite trend with increased proline contents. In contradiction to earlier findings [20], the increase in proline contents reduced the activity of PDH and the transcript level of ProDH. The decline in PDH activity could be attributed to transcript reduction.…”

Alternate wetting and drying modulated physio‐biochemical attributes, grain yield, quality, and aroma volatile in fragrant rice

“…In the present study, lower levels of leaf proline contents at MS represent the higher aroma volatile (i.e., 2‐AP) contents in rice grains due to its conversion to 2‐AP via the above‐mentioned phenomena. Recently, Luo et al (2020) reported that exogenous proline application substantially improved the grain 2‐AP contents by up‐regulating the endogenous P5C and △1‐pyrroline contents, proline dehydrogenase (ProDH) activity and transcription level of PRODH gene. Yoshihashi et al (2002) also found increased 2AP and/or aroma in seedlings and developing callus of fragrant rice.…”

Combined Metabolomic and Quantitative RT-PCR Analyses Revealed the Synthetic Differences of 2-Acetyl-1-pyrroline in Aromatic and Non-Aromatic Vegetable Soybeans

“…Moreover, the synthesis of 2AP might also be affected by the expression of the GAPDH (Glyceraldehyde-3-phosphate dehydrogenase), TPI (Triosephosphate isomerase), P5CR (Pyrroline-5-carboxylate reductase), PRODH (Proline dehydrogenase), P5CS (Δ 1 -pyrroline-5-carboxylate synthetase), and OAT (Ornithine aminotransferase). The low expression level of GAPDH and the high expression levels of TPI , P5CR , PRODH , and P5CS in aromatic rice were found to be related to the synthesis of 2AP [ 9 , 25 , 26 , 27 ]. Similar expression patterns were also observed for GAPDH and TPI in aromatic soybeans, but the expression levels of PRODH , P5CS and OAT did not differ between aromatic and non-aromatic soybeans [ 5 ].…”