Regulation of 2-Acetyl-1-pyrroline Content in Fragrant Rice under Different Temperatures at the Grain-Filling Stage.

2-Acetyl-1-pyrroline (2-AP) is a key volatile organic compound in fragrant rice aroma. However, the effects of temperature on 2-AP biosynthesis in fragrant rice and its regulation mechanism have been rarely reported. In the present study, three fragrant rice varieties were used as plant materials, and four temperature treatments during the grain-filling stage, i.e., (T1) 22/17 °C, (T2) 27/22 °C, (T3) 32/27 °C, and (T4) 37/32 °C, were adopted. The results showed that grain contents of 2-AP, proline, and γ-aminobutyric acid (GABA) significantly (P < 0.05) increased with decreased temperature, while the lowest and highest 2-AP contents were recorded in the T4 and T1 treatments, respectively. Higher pyrroline-5-carboxylic acid (P5C) content was recorded in low-temperature treatments (T1 and T2) than in high-temperature treatments (T3 and T4). The transcript levels of genes BADH2, PRODH, and OAT significantly (P < 0.05) decreased with decreased temperature. Lower transcript levels of genes P5CR, P5CS2, DAO2, DAO4, and DAO5 were recorded in low-temperature treatments (T1 and T2) than in high-temperature treatments (T3 and T4). In conclusion, low temperature increased 2-AP content and high temperature decreased 2-AP content in fragrant rice. We deduced that temperature regulated 2-AP biosynthesis through the metabolism of proline and GABA.

Organic cultivation induced regulation in yield formation, grain quality attributes, and volatile organic compounds of fragrant rice.

Organic cultivation is an eco-friendly planting strategy. To explore the effects of organic cultivation on yield, grain quality, and volatile organic compounds of fragrant rice, the present study conducted a field experiment with organic cultivation and inorganic cultivation treatments over two subsequent growing seasons. The late season rice was sent for the determination of untargeted flavoromics using GC × GC-ToF-MS. Meanwhile, the GC × MS were used for the quantification of 2-acetyl-1-pyrroline. Our results showed that compared with inorganic cultivation, organic cultivation increased grain 2-AP and amylose contents by 21.19-26.10 % and 4.51-7.69 %, respectively. 5.89 %-15.55 % lower crude protein contents were recorded in organic cultivation. The results of untargeted flavoromics showed that organic cultivation caused the absence of 56 volatiles and the exclusive presence of 10 new volatiles compared with inorganic cultivation. In conclusion, organic cultivation produced fragrant rice with lower yield and higher 2-AP content as well as different flavor profiles.

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

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 performances in response to continuous zero-tillage in machine-transplanted double-cropped rice system.

Zero-tillage is one of conservation tillage techniques. In order to investigate the effects of continuous zero-tillage on yield formation and grain 2-acetyl-1-pyrroline (2-AP, key component of fragrant rice aroma) content of fragrant rice, present study was conducted with a six-season field experiment from 2017 to 2019. The conventional tillage (twice puddling with rotary cultivator before transplanting) was set as control (CK) and zero-tillage was set as treatment (ZT). At the first year after applying zero-tillage, yield loss was observed in the ZT treatment which was attributed to the lower effective panicle number per area and grain number per panicle. However, from late season in 2018 to late season in 2019, significant higher grain yield was recorded in ZT than CK. ZT increased the net photosynthetic rate and chlorophyll content (SPAD value) by 6.81-20.77% and 7.23-23.80% in the last three cropping seasons compared with CK. Higher nitrogen, potassium and phosphorus accumulations in plant tissues were also recorded in ZT than CK from late season in 2018 to late season in 2019. Other hand, higher grain 2-AP content was recorded in ZT than CT in the whole six cropping seasons which might be related to the grain proline content. Furthermore, compared with CK, ZT significantly increased the soil organic matter content and the number of bacteria, fungi and actinomycetes in the last three cropping seasons. In conclusion, continuous zero-tillage could improve soil and increase the photosynthesis and nutrient accumulation and finally achieve the improvement of fragrant rice yield.