Biosynthesis of 2-Acetyl-1-pyrroline in Fragrant Rice: Recent Insights into Agro-management, Environmental Factors, and Functional Genomics

Abstract

Rice is a staple food for more than half of the world's population, and rice fragrance is a key quality attribute which is highly desired by consumers and attracts premium prices in the international market. There are around 200 volatile compounds involved in rice fragrance, but 2-acetyl-1-pyrroline (2-AP) has been considered a master regulator of aroma in fragrant rice. Consequently, efforts were made to increase the 2-AP contents in the grain by managing agronomical practices or by using modern functional genomic tools, which successfully converted nonfragrant cultivars to fragrant rice. Furthermore, environmental factors were also reported to influence the 2-AP contents. However, a comprehensive analysis of 2-AP biosynthesis in response to agromanagement practices, environmental factors, and the application of functional genomic tools for fragrant rice production was missing. In this Review, we summarize how micro/macronutrients, cultivation practices, amino acid precursors, growth regulators, and environmental factors, such as drought, salinity, light, and temperature, influence the 2-AP biosynthesis to modulate the aroma of fragrant rice. Furthermore, we also summarized the successful conversion of nonfragrant rice cultivars to fragrant rice using modern gene editing tools, such as RNAi, TALENS, and CRISPR-Cas9. Finally, we discussed and highlighted the future perspective and challenges related to the aroma of fragrant rice.

l-Glutamate Seed Priming Enhances 2-Acetyl-1-pyrroline Formation in Fragrant Rice Seedlings in Response to Arsenite Stress

“…In the present study, Glu seed priming improved the germination rate by modulating the endogenous Glu content in seeds (SI, Figure S1), implying that Glu was absorbed into the seeds and supplied the amino acids and energy for rice growth and development. The fluctuation of endogenous Glu also reflected a complicated conversion of Glu and other amino acids and substances, which would supply the aroma formation of fragrant rice. , In addition, during the entire growth period of fragrant rice, 2-AP was found to be present in plant tissues above ground but not in the roots . This suggested that Glu possibly served as a supplementary source of substrates and energy source for plant metabolism, particularly in the biosynthesis of 2-AP.…”

“…The fluctuation of endogenous Glu also reflected a complicated conversion of Glu and other amino acids and substances, which would supply the aroma formation of fragrant rice. 7,21 In addition, during the entire growth period of fragrant rice, 2-AP was found to be present in plant tissues above ground but not in the roots. 34 This suggested that Glu possibly served as a supplementary source of substrates and energy source for plant metabolism, particularly in the biosynthesis of 2-AP.…”

“…This has led to the accumulation of As in crops which poses a serious risk to human health through the food chain . Contamination of As in the paddy field poses a critical challenge to growers because fragrant rice is sensitive to abiotic stresses that result in yield loss and 2-AP reduction . Arsenite (As­(III)) and arsenate (As­(V)) are two forms of inorganic As, in particular, As­(V) containing Fe­(III) (oxyhydr)­oxides can be dissolved by reduction into As­(III) becoming more bioavailable in paddy field .…”

“…Contamination of As in the paddy field poses a critical challenge to growers because fragrant rice is sensitive to abiotic stresses that result in yield loss and 2-AP reduction. 21 Arsenite (As(III)) and arsenate (As(V)) are two forms of inorganic As, in particular, As(V) containing Fe(III) (oxyhydr)oxides can be dissolved by reduction into As(III) becoming more bioavailable in paddy field. 22 A recent publication showed that the total As content in the rice grain increased by about 80% under continuous flooding conditions compared with aerobic conditions, 23 suggesting negative impact of As(III) rice.…”

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

“…The 2-AP biosynthesis in fragrant rice is complicated and is affected by the expression of many genes, such as BADH2 , PRODH , and P5CS2 . Some studies showed that 2-AP biosynthesis is closely associated with proline metabolism and there was always a positive and significant correlation between 2-AP content and proline content under different agronomic treatments such as water management, nitrogen fertilization, and zinc application. ,, Previous studies indicated that PRODH is an important gene in regulating 2-AP level by encoding proline dehydrogenase, which catalyzes the transformation from proline to 2-AP. , On the other hand, some research has indicated that the enhancement of 2-AP was not always through proline but through other metabolic pathways such as putrescine and γ-aminobutyric acid (GABA) metabolism .…”