Thirty-three years of 2-acetyl-1-pyrroline, a principal basmati aroma compound in scented rice (Oryza sativa L.): a status review

Multiomics analysis reveals a substantial decrease in nanoplastics uptake and associated impacts by nano zinc oxide in fragrant rice (Oryza sativa L.)

Nanoplastics (NPs) have emerged as global environmental pollutants with concerning implications for sustainable agriculture. Understanding the underlying mechanisms of NPs toxicity and devising strategies to mitigate their impact is crucial for crop growth and development. Here, we investigated the nanoparticles of zinc oxide (nZnO) to mitigate the adverse effects of 80 nm NPs on fragrant rice. Our results showed that optimized nZnO (25 mg L-1) concentration rescued root length and structural deficits by improving oxidative stress response, antioxidant defense mechanism and balanced nutrient levels, compared to seedlings subjected only to NPs stress (50 mg L-1). Consequently, microscopy observations, Zeta potential and Fourier transform infrared (FTIR) results revealed that NPs were mainly accumulated on the initiation joints of secondary roots and between cortical cells that blocks the nutrients uptake, while the supplementation of nZnO led to the formation of aggregates with NPs, which effectively impedes the uptake of NPs by the roots of fragrant rice. Transcriptomic analysis identified a total of 3973, 3513 and 3380 differentially expressed genes (DEGs) in response to NPs, nZnO and NPs+nZnO, respectively, compared to the control. Moreover, DEGs were significantly enriched in multiple pathways including biosynthesis of secondary metabolite, phenylpropanoid biosynthesis, amino sugar and nucleotide sugar metabolism, carotenoid biosynthesis, plant-pathogen interactions, MAPK signaling pathway, starch and sucrose metabolism, and plant hormone signal transduction. These pathways could play a significant role in alleviating NPs toxicity and restoring fragrant rice roots. Furthermore, metabolomic analysis demonstrated that nZnO application restored 2-acetyl-1-pyrroline (2-AP) pathways genes expression, enzymatic activities, and the content of essential precursors related to 2-AP biosynthesis under NPs toxicity, which ultimately led to the restoration of 2-AP content in the leaves. In conclusion, this study shows that optimized nZnO application effectively alleviates NPs toxic effects and restores both root structure and aroma production in fragrant rice leaves. This research offers a sustainable and practical strategy to enhance crop production under NPs toxicity while emphasizing the pivotal role of essential micronutrient nanomaterials in agriculture.

Comprehensive multi-layered analyses of genotype-dependent proteo-metabolic networks reveal organellar crosstalk and biochemical pathways regulating aroma formation in rice

Molecular basis of rice aroma formation is sparsely known and developmental programs driving biochemical pathways towards aroma is in infancy. Here, discovery and targeted proteo-metabolome of non-aromatic and aromatic rice seeds across developmental stages identified a total of 442 aroma-responsive proteins (ARPs) and 824 aroma-responsive metabolites (ARMs) involved in metabolism, calcium and G-protein signaling. Biochemical examination revealed ARM/Ps were linked to 2-acetylpyrrolidine, γ-aminobutyrate, anthocyanin, tannins, flavonoids and related enzymes. Pairwise correlation and clustering showed positive correlation among ARM/Ps. Consistent with aroma-related QTLs, ARPs were mapped on chromosomes 3,4,5,8 and were mainly compartmentalized in cytoplasm and mitochondria. ARM/P-correlation network identified associations related to metabolism and signaling. Multiple reaction monitoring (MRM) confirmed role of catechins, quinic acid and quercetin in aroma formation. Pathway enrichment, multivariate analysis and qRT-PCR validated that calcium and G-protein signaling, aromatic/branched-chain aminoacid, 2-acetylpyrrolidine, oxylipin, melvonate and prenylpyrophosphate pathways, indole, phenylacetate, flavonoid, cinnamoic ester govern aroma formation in rice.

Genetic characterization and diversity analysis of indigenous aromatic rice

Gangetic old alluvial zone in India has conserved many locally adapted aromatic rice landraces. In order to determine the extent of genetic divergence of ten morphological characters, the study was conducted to examine forty-eight aromatic rice genotypes for six Kharif seasons (2016-2021) at the Instructional Farm of Regional Research Station (Old Alluvial Zone), Uttar Banga Krishi Viswavidyalaya, Majhian, West Bengal, India. The experiment was laid out in Randomized Complete Block Design (RCBD) with three replications. A considerable degree of variation was noted for all the traits being investigated. It was found that the total number of tillers per plant, panicle numbers per plant, number of grains per panicle, fertility percentage, test weight, and grain length/breadth ratio had significantly positive correlated with seed yield per plant. Based on D2 analysis values, all the genotypes were grouped into six clusters. Cluster III (Tulaipanji, Patnai, Basmati 1121, Jugal, and Bahurupi) and Cluster VI (Kanakchur), containing genotypes were found most divergent with maximum inter-cluster distance (6941.51). According to the cluster means, Cluster II had the largest intra-cluster distance (1937.52), and important attributes including test weight, number of grains per panicle, seed yield per plant, and fertility percentage made remarkably significant contributions to this cluster. In terms of principal component analysis, maximum variability was found in PC1 (23.88 %), with high positive loading values for tillers per plant (0.459), panicle number per plant (0.441), seed yield per plant (0.408), fertility percentage (0.364), test weight (0.264), and grain length/breadth (L/B) ratio (0.263). On the basis of biplot analysis, four genotypes, namely Shakbhati, Sugandhi, Bahurupi and Kanakchur, were identified as the most divergent types for the yield-attributing traits of aromatic rice. The diverse genotypes could be used as potential donors in future breeding programmes.

Absolute contents of aroma-affecting volatiles in cooked rice determined by one-step rice cooking and volatile extraction coupled with standard-addition calibration using HS-SPME/GC-MS

To quantify volatiles in cooked rice, analysis methods for one-step rice cooking and volatile extraction in a single headspace vial, combined with standard addition calibration using solid-phase microextraction and GC-MS were developed and applied to 41 rice varieties with various fragrances and palatability. The newly developed methods significantly improved the qualitative and quantitative recovery of volatiles compared with conventional methods. Among 29 aroma-affecting volatiles, the highest average contents (ng/g) were observed for nonanal (39.30), octanal (13.29), and 1-octen-3-ol (13.18); the total volatile contents of aldehyde, base, and alcohol groups were 4156, 2481, and 1739 ng/g, respectively. Fifteen rice varieties contained 2-acetyl-1-pyrroline in range of 41.37-421.70 ng/g. Although there were no linear correlations among volatiles and the Toyo taste-score, multivariate PLS-DA analysis of the volatile could discriminate between low- and high-palatability rice varieties. The results indicated the accuracy and practicality of the newly developed methods for quantifying volatiles in cooked rice.

Application of Silicon Influencing Grain Yield and Some Grain Quality Features in Thai Fragrant Rice

Silicon (Si) is a beneficial nutrient that has been shown to increase rice productivity and grain quality. Fragrant rice occupies the high end of the rice market with prices at twice to more than three times those of non-fragrant rice. Thus, this study evaluated the effects of increasing Si on the yield and quality of fragrant rice. Also measured were the content of proline and the expression of the genes associated with 2AP synthesis and Si transport. The fragrant rice varieties were found to differ markedly in the effect of Si on their quality, as measured by the grain 2AP concentration, while there were only slight differences in their yield response to Si. The varieties with low 2AP when the Si supply is limited are represented by either PTT1 or BNM4 with only slight increases in 2AP when Si was increased. Si affects the gene expression levels of the genes associated with 2AP synthesis, and the accumulation of 2AP in fragrant rice mainly occurred through the upregulation of Badh2, DAO, OAT, ProDH, and P5CS genes. The findings suggest that Si is a potential micronutrient that can be utilized for improving 2AP and grain yield in further aromatic rice breeding programs.

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.

Riboflavin improves grain yield, 2-acetyl-1-pyrroline accumulation, and antioxidative properties of fragrant rice

BACKGROUND

Riboflavin, a vital water-soluble vitamin with antioxidative activity, plays a critical role in maintaining overall bodily health and defense responses. However, its impact on fragrant rice yield and aroma remains unexplored.

RESULTS

In a 2022 pot experiment with Meixiangzhan and Yuxiangyouzhan fragrant rice cultivars, we applied riboflavin foliar treatments at concentrations of 0 (CK), 10 (R10), 20 (R20), and 40 (R40) mg L-1 during the initial heading stage. Riboflavin increased rice yield, 2-acetyl-1-pyrroline (2-AP) content, and antioxidative properties. It boosted 2-AP level by 13.1-50.1% for Meixiangzhan and 22.3-35.3% for Yuxiangyouzhan, with the highest levels in R20 and R10 treatments. This increase is significantly correlated with elevated levels of proline, pyrroline-5-carboxylic acid, pyrroline, and methylglyoxal, as well as heightened enzyme activities, including those of proline dehydrogenase, ornithine aminotransferase, and pyrroline-5-carboxylic acid synthetase (P5CS). The R20 treatment resulted in the highest yield due to an improved seed-setting rate. Importantly, a positive correlation emerged between 2-AP content and yield, both significantly linked to superoxide dismutase, proline, hydrogen peroxide, P5CS, catalase, and pyrroline.

CONCLUSION

Riboflavin maintained enzyme activities, regulated substance synthesis pathways, and increased 2-AP and yield, especially in the R20 treatment. These insights advance fragrant rice production theory by uncovering riboflavin's role in the development of fragrant rice. © 2023 Society of Chemical Industry.

Effects of paclobutrazol seed priming on seedling quality, photosynthesis, and physiological characteristics of fragrant rice

BACKGROUND

Paclobutrazol is widely used in the agricultural field. This study investigated the effects of seed priming with different concentrations of paclobutrazol on seedling quality, 2-acetyl-1-pyrroline (2-AP, a key aroma component of fragrant rice) biosynthesis, and related physiological and biochemical indicators in fragrant rice seedlings.

RESULTS

The experiment is being conducted at the College of Agriculture, South China Agricultural University. In the experiment, three concentrations of paclobutrazol (Pac 1: 20 mg·L-1; Pac 2: 40 mg·L-1; Pac 3: 80 mg·L-1) were used to initiate the treatment of fragrant rice seeds, while water treatment was used as a control (CK). The results showed that compared with CK, paclobutrazol treatment reduced plant height, increased stem diameter, and increased fresh and dry weight of aromatic rice seedlings. Moreover, paclobutrazol treatment also increased the seedlings' photosynthetic pigment content and net photosynthetic rate.

CONCLUSIONS

This study demonstrates that paclobutrazol primarily increases the content of proline by reducing the content of glutamate and down-regulating the expression of P5CS2, thereby promoting the conversion of proline to the aromatic substance 2-AP. Under the appropriate concentration of paclobutrazol (40 mg·L-1~80 mg·L-1), the seedling quality, stress resistance, and aroma of fragrant rice can be improved.

Multi-model genome-wide association studies for appearance quality in rice

Improving the quality of the appearance of rice is critical to meet market acceptance. Mining putative quality-related genes has been geared towards the development of effective breeding approaches for rice. In the present study, two SL-GWAS (CMLM and MLM) and three ML-GWAS (FASTmrEMMA, mrMLM, and FASTmrMLM) genome-wide association studies were conducted in a subset of 3K-RGP consisting of 198 rice accessions with 553,831 SNP markers. A total of 594 SNP markers were identified using the mixed linear model method for grain quality traits. Additionally, 70 quantitative trait nucleotides (QTNs) detected by the ML-GWAS models were strongly associated with grain aroma (AR), head rice recovery (HRR, %), and percentage of grains with chalkiness (PGC, %). Finally, 39 QTNs were identified using single- and multi-locus GWAS methods. Among the 39 reliable QTNs, 20 novel QTNs were identified for the above-mentioned three quality-related traits. Based on annotation and previous studies, four functional candidate genes (LOC_Os01g66110, LOC_Os01g66140, LOC_Os07g44910, and LOC_Os02g14120) were found to influence AR, HRR (%), and PGC (%), which could be utilized in rice breeding to improve grain quality traits.

Flavor profiling and gene expression studies of indigenous aromatic rice variety (Mushk Budiji) grown at different altitudes of Highland Himalayan regions

Mushk Budiji-an indigenous aromatic rice variety is usually grown at an altitude ranging from 5000 to 7000 ft above mean sea level in Highland Himalayas. This study was conducted to investigate the effects of altitude, soil nitrogen content and climatic conditions (temperature) of the selected locations on the flavor profile of Mushk Budiji using gas chromatography-mass spectroscopy (GC-MS) and electronic nose (E-nose). E-nose being rapid and non-destructive method was used to validate the results of volatile aromatic compounds obtained using GC-MS in Mushk Budiji. Around 35 aromatic compounds were identified in Mushk Budiji rice samples. Highest volatile peak area percentage (105.41%) was recorded for Mushk Budji grown at an altitude of 5216.53 ft. Highest E-nose score (2.52) was obtained at an altitude of 6299.21 ft. Over-expression of fatty acid degradation and linoleic acid metabolism genes was observed at higher altitudes, whereas lipid biosynthesis was negatively influenced by higher altitude. Fatty acid degradation and linoleic acid metabolism is responsible for the synthesis of volatile aromatic compounds in Mushk Budiji. This study will therefore be the path finder for investigating the intricate mechanism behind the role of altitude on aroma development in Mushk Budiji rice for future studies.

Generation of aroma in three-line hybrid rice through CRISPR/Cas9 editing of BETAINE ALDEHYDE DEHYDROGENASE2 (OsBADH2)

Aroma or fragrance in rice is a genetically controlled trait; Its high appreciation by consumers increases the rice market price. Previous studies have revealed that the rice aroma is controlled by a specific gene called BETAINE ALDEHYDE DEHYDROGENASE (OsBADH2), and mutation of this gene leads to the accumulation of an aromatic substance 2-acetyl-1-pyrroline (2-AP). The use of genetic engineering to produce aroma in commercial and cultivated hybrids is a contemporary need for molecular breeding. The current study reports the generation of aroma in the three-line hybrid restorer line Shu-Hui-313 (SH313). We created knock-out (KO) lines of OsBADH2 through the CRISPR/Cas9. The analysis of KO lines revealed a significantly increased content of 2AP in the grains compared with the control. However, other phenotypic traits (plant height, seed setting rate, and 1000-grain weight) were significantly decreased. These KO lines were crossed with a non-aromatic three-line hybrid rice male sterile line (Rong-7-A) to produce Rong-7-You-626 (R7Y626), R7Y627 and R7Y628. The measurement of 2-AP revealed significantly increased contents in these cross combinations. We compared the content of 2-AP in tissues at the booting stage. Data revealed that young spike stalk base contained the highest content of 2-AP and can be used for identification (by simple chewing) of aromatic lines under field conditions. In conclusion, our dataset offers a genetic source and illustrates the generation of aroma in non-aromatic hybrids, and outlines a straightforward identification under field conditions.

Transcription Factor OsbZIP60-like Regulating OsP5CS1 Gene and 2-Acetyl-1-pyrroline (2-AP) Biosynthesis in Aromatic Rice

The most important volatile in determining the aroma of fragrant rice is 2-Acetyl-1-pyrroline (2-AP); however, the transcriptional regulation mechanism of 2-AP biosynthesis in fragrant rice is still unclear. In this study, Osp5cs1 knockout mutant lines and OsP5CS1 over-expression lines were constructed by the genetic transformation of the Indica rice cultivar, i.e., 'Zhonghua11', which knocks out OsBADH2 to produce fragrance in aromatic rice. The OsP5CS1 gene was also identified as a key gene in the 2-AP biosynthesis pathway of aromatic rice. The OsP5CS1 promoter was used as bait, and the OsbZIP60-like transcription factor was screened by yeast one-hybrid assays. The OsbZIP60-like transcription factor specifically bound to the OsP5CS1 gene. The dual luciferase reporting system found that the OsbZIP60-like transcription factor promoted the transcriptional activation of OsP5CS1. Compared with the wild type, OsP5CS1 gene expression was significantly down-regulated in the Osbzip60-like mutant and resulted in a substantial reduction in 2-AP biosynthesis. Moreover, the OsP5CS1 gene expression was significantly up-regulated in OsbZIP60-like over-expressed plants, and the 2-AP concentrations were also increased, whereas the Osbzip60-like mutants were found to be sensitive to Zn deficiency. Overall, the OsbZIP60-like transcription factor promoted the 2-AP accumulation. This study provides a theoretical basis for the transcriptional regulation mechanism of 2-AP biosynthesis and explores the function of the OsbZIP transcription factor in fragrant rice.

Evidence of polyamines mediated 2-acetyl-1-pyrroline biosynthesis in aromatic rice rhizospheric fungal species Aspergillus niger

Rhizosphere soil of aromatic rice inhabits different fungal species that produce many bioactive metabolites including 2-acetyl-1-pyrroline (2AP). The mechanism for the biosynthesis of 2AP in the fungal system is still elusive. Hence, the present study investigates the role of possible nitrogen (N) precursors such as some amino acids and polyamines as well as the enzymes involved in 2AP synthesis in the fungal species isolated from the rhizosphere of aromatic rice varieties. Three fungal isolates were found to synthesize 2AP (0.32-1.07 ppm) and maximum 2AP was synthesized by Aspergillus niger (1.07 ppm) isolated from rhizosphere of Dehradun Basmati (DB). To determine the N source for 2AP synthesis, various N sources such as proline, glutamate, ornithine putrescine, spermine, and spermidine were used in place of putrescine in the synthetic medium (Syn18). The results showed that maximum 2AP synthesis was found with putrescine (1.07 ppm) followed by spermidine (0.89 ppm) and spermine (0.84 ppm). Further, LC-QTOF-MS analysis revealed the mobilization of spermine and spermidine into the putrescine, indicating that putrescine is the key N source for 2AP synthesis. Moreover, higher enzyme activity of DAO, PAO, and ODC as well as higher content of methylglyoxal metabolite in the A. niger NFCCI 5060 as compared to A. niger NFCCI 4064 (control) suggests the prominent role of these enzymes in the synthesis of 2AP. In conclusion, this study showed evidence of the polyamines mediated 2AP biosynthesis in A. niger NFCCI 5060.

Review of technology advances to assess rice quality traits and consumer perception

The increase in rice consumption and demand for high-quality rice is impacted by the growth of socioeconomic status in developing countries and consumer awareness of the health benefits of rice consumption. The latter aspects drive the need for rapid, low-cost, and reliable quality assessment methods to produce high-quality rice according to consumer preference. This is important to ensure the sustainability of the rice value chain and, therefore, accelerate the rice industry toward digital agriculture. This review article focuses on the measurements of the physicochemical and sensory quality of rice, including new and emerging technology advances, particularly in the development of low-cost, non-destructive, and rapid digital sensing techniques to assess rice quality traits and consumer perceptions. In addition, the prospects for potential applications of emerging technologies (i.e., sensors, computer vision, machine learning, and artificial intelligence) to assess rice quality and consumer preferences are discussed. The integration of these technologies shows promising potential in the forthcoming to be adopted by the rice industry to assess rice quality traits and consumer preferences at a lower cost, shorter time, and more objectively compared to the traditional approaches.

Impact and mitigation of lead, cadmium and micro/nano plastics in fragrant rice

Heavy metals (HMs) and micro(nano)plastics (MNPs), represent a significant risk to global food supply as well as a potential risk to humankind. Over 50% of the worldwide population eat rice every day, and rice aroma is a significant qualitative trait that is highly valued by consumers and fetches premium prices in the global market. Despite the huge commercial importance of fragrant rice, limited studies were directed to investigate the influence of HMs and MNPs on yield related traits and 2-Acetyl-1-pyrroline (2-AP) compound, mainly responsible for aroma production in fragrant rice. In this review, we found that the interaction of HMs and MNPs in fragrant rice is complex and accumulation of HMs and MNPs was higher in root as compared to the grains. Nutrients and phytohormones mediated mitigation of HMs and MNPs were most effective sustainable strategies. In addition, monitoring the checkpoints of 2-AP biosynthesis and its interaction with HMs and MNPs is challenging. Finally, we explained the potential challenges that fragrant rice faces considering the continuous rise in environmental pollutants and discussed the future avenues of research to improve fragrant rice's yield and qualitative traits.

Zinc Oxide Nanoparticles Foliar Application Effectively Enhanced Zinc and Aroma Content in Rice (Oryza sativa L.) Grains

The search for an effective zinc fertilizer and its application method to effectively increase zinc content and enhance aroma in rice grains is a crucial objective. In this study, a 2-year field plot experiment was conducted to investigate the influence of ZnO NPs foliar spraying on rice quality, grain zinc and aroma content, along with exploring the physiological mechanisms underlying these effects. Our results demonstrated that the rice breakdown value and taste value of foliar spraying zinc oxide nanoparticles were improved by 31.0-41.7% and 8.2-13.0% compared with CK (control treatment involved spraying water), improving the tasting and steaming quality of rice. While Fe and Cu content in grains decreased for the application of zinc oxide nanoparticles, zinc oxide nanoparticles foliar spraying significantly increased the zinc content and accumulation of grains by 33.6-65.1% and 37.8-74.7%, respectively. Further analysis showed that the sprayed zinc oxide nanoparticles achieved effective enrichment of zinc in edible parts and increased the final bioavailability of Zn. In addition, foliar spraying of zinc oxide nanoparticles significantly increased activities of nitrate reductase and glutamine synthetase in leaves, which elevated nitrogen content in leaves and grains, and ultimately enhanced 2-acetyl-1-pyrroline (2-AP) content in grains at maturity by 6.1-21.4% compared to CK. Our findings indicated that zinc oxide nanoparticles can be practically applied as a foliar fertilizer at the gestation for quality improvement, zinc enrichment and aroma enhancement of rice grains.

Gene Genealogy-Based Mutation Analysis Reveals Emergence of Aus, Tropical japonica, and Aromatic of Oryza sativa during the Later Stage of Rice Domestication

Asian rice (Oryza sativa L.) has become a model for understanding gene functions and domestication in recent decades; however, its own diversification is still controversial. Although the division of indica and japonica and five subgroups (aus, indica (sensu stricto), japonica (sensu stricto), tropical japonica, and aromatic) are broadly accepted, how they are phylogenetically related is not transparent. To clarify their relationships, a sample of 121 diverse genes was chosen here from 12 Oryza genomes (two parental and ten O. sativa (Os)) in parallel to allow gene genealogy-based mutation (GGM) analysis. From the sample, 361 Os mutations were shared by two or more subgroups (referred to here as trans mutations) from 549 mutations identified at 51 Os loci. The GGM analysis and related tests indicates that aus diverged from indica at a time significantly earlier than when tropical japonica split from japonica. The results also indicate that aromatic was selected from hybrid progeny of aus and tropical japonica and that all five subgroups share a significant number of the early mutations identified previously. The results suggest that aus, tropical japonica, and aromatic emerged sequentially within the most recent 4-5 millennia of rice domestication after the split of indica and japonica.

Nitric oxide confers cadmium tolerance in fragrant rice by modulating physio-biochemical processes, yield attributes, and grain quality traits

Cadmium (Cd) stress causes serious disruptions in plant metabolism, physio-biochemical processes, crop yield, and quality characters. Nitric oxide (NO) improves the quality features and nutritional contents of fruit plants. However, how NO confers Cd toxicity in fragrant rice plants, is sparse. Hence, the present study investigated the effects of 50 µM NO donor sodium nitroprusside (SNP) on physio-biochemical processes, plant growth attributes, grain yield, and quality traits of fragrant rice under Cd stress (100 mg kg^-1 soil). The results revealed that Cd stress diminished rice plant growth, impaired photosynthetic apparatus and antioxidant defense system, and deteriorated the grain quality traits. However, foliar application of SNP mitigated Cd stress by improving plant growth and gas exchange attributes. Higher electrolyte leakage (EL) was accompanied with elevated levels of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) under Cd stress; however, exogenous application of SNP reduced them. The activities and relative expression levels of enzymatic antioxidants; superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) and non-enzymatic antioxidants, glutathione (GSH) contents were reduced by Cd stress, while SNP application regulated their activity and transcript abundances. SNP application improved fragrant rice grain yield and 2-acetyl-1-pyrroline content by 57.68 % and 75.54 % respectively, which is concomitant with higher biomass accumulation, photosynthetic efficiency, photosynthetic pigment contents, and an improved antioxidant defense system. Collectively, our results concluded that SNP application regulated the fragrant rice plant physio-biochemical processes, yield traits and grain quality characters under Cd-affected soil.

Characteristic Flavor Compounds and Functional Components of Fragrant Rice with Different Flavor Types

Fragrant rice has various flavor types, mainly the popcorn flavor, corn flavor and lotus root flavor. Chinese fragrant rice from China and Thai fragrant rice from Thailand were analyzed. GC-MS was used to determine the volatile compounds of fragrant rice. It was found that there were 28 identical volatile compounds between Chinese and Thai fragrant rice. The key compounds of different flavor types of fragrant rice were obtained by comparing the common volatile compounds. The key compounds of the popcorn flavor were 2-butyl-2-octenal, 4-methylbenzaldehyde, ethyl 4-(ethyloxy)-2-oxobut-3-enoate and methoxy-phenyl-oxime. The key compounds of the corn flavor were 2,2',5,5'-tetramethyl-1,1'-biphenyl, 1-hexadecanol, 5-ethylcyclopent-1-enecarboxaldehyde and cis-muurola-4(14), 5-diene. By using a combination of GC-MS and GC-O, the flavor spectrogram of fragrant rice was constructed, and the characteristic flavor compounds of each flavor type were identified. It was found that the characteristic flavor compounds of the popcorn flavor were 2-butyl-2-octenal, 2-pentadecanone, 2-acetyl-1-pyrroline, 4-methylbenzaldehyde, 6,10,14-trimethyl-2-pentadecanone, phenol and methoxy-phenyl-oxime. The characteristic flavor compounds of the corn flavor were 1-octen-3-ol, 2-acetyl-1-pyrroline, 3-methylbutyl 2-ethylhexanoate, methylcarbamate, phenol, nonanal and cis-muurola-4(14), 5-diene. The characteristic flavor compounds of the lotus root flavor were 2-acetyl-1-pyrroline, 10-undecenal, 1-nonanol, 1-undecanol, phytol and 6,10,14-trimethyl-2-pentadecanone. The resistant starch content of lotus root flavor rice was relatively high (0.8%). The correlation between flavor volatiles and functional components was analyzed. It was found that the fat acidity of fragrant rice was highly correlated (R = 0.86) with the characteristic flavor compounds, such as 1-octen-3-ol, 2-butyl-2-octenal and 3-methylbutyl-2-ethylhexanoate. The characteristic flavor compounds had an interactive contribution to the production of the different flavor types of fragrant rice.

Zinc supplementation and light intensity affect 2-acetyl-1-pyrroline (2AP) formation in fragrant rice

BACKGROUND

Improving the yield and aroma content of fragrant rice is the focus of fragrant rice research. Light and Zinc (Zn) management generally cause regulations in the 2-acetyl-1-pyrroline (2AP) accumulation in fragrant rice. In addition, Zn promotes rice growth and improves rice yield, which has the potential to compensate for the negative impact of low light on fragrant rice yield. However, the potential of Zn to improve fragrant rice yield and 2AP content under shading conditions has not been verified.

METHODS

Field experiments were conducted in the rice season (May-September) in 2019 to 2021. Two light i.e., normal light (NL) and low light (LL) and four Zn levels i.e., 0 kg Zn ha^- 1 (N0), 1 kg Zn ha^- 1 (Zn1), 2 kg Zn ha^- 1(Zn2), and 3 kg Zn ha^- 1 (Zn3), which applied at booting stage was set up. The grain yield, 2AP contents, Zn content in polished rice, photosynthesis related indicators, MDA content, antioxidant enzyme activity and the biochemical parameters related to 2AP formation were investigated.

RESULTS

Shading reduced yield by 8.74% and increased 2AP content by 24.37%. In addition, shading reduced net photosynthetic rate (Pn), superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), and increased proline, γ-aminobutyric acid (GABA), and pyrroline-5-carboxylic acid (P5C), proline dehydrogenase (PDH), △1-pyrroline-5-carboxylic acid synthetase (P5CS), malondialdehyde (MDA). With increasing Zn application levels, yield, 2AP, Zn content in polished rice, Pn, proline, P5C, GABA, PDH, P5CS, SOD, CAT and POD increased, and MDA decreased. Significant Light and Zn interaction effect on 2AP content was detected, and both shading and increasing Zn application increased the 2AP content.

CONCLUSION

Shading can increase the 2AP content but reduce the yield of fragrant rice. Increasing Zn application under shading conditions can further promote the biosynthesis of 2AP, but the effect of improving yield is limited.

CRISPR/Cas9-mediated editing of Wx and BADH2 genes created glutinous and aromatic two-line hybrid rice

Amylose content (AC) is one of the physicochemical indexes of rice quality, which is largely determined by the Waxy (Wx) gene. Fragrance in rice is favored because it adds good flavor and a faint scent. Loss of function of the BADH2 (FGR) gene promotes the biosynthesis of 2-acetyl-1-pyrroline (2AP), which is the main compound responsible for aroma in rice. Here, we used a CRISPR/Cas9 system to simultaneously knock out Wx and FGR genes in 1892S and M858, which are the parents of an indica two-line hybrid rice, Huiliangyou 858 (HLY858). Four T-DNA-free homozygous mutants (1892Swxfgr-1, 1892Swxfgr-2, M858wxfgr-1, and M858wxfgr-2) were obtained. The 1892Swxfgr and M858wxfgr were crossed to generate double mutant hybrid lines HLY858wxfgr-1 and HLY858wxfgr-2. Size-exclusion chromatography (SEC) data indicated that true AC of the wx mutant starches ranged from 0.22 to 1.63%, much lower than those of the wild types (12.93 to 13.76%). However, the gelatinization temperature (GT) of the wx mutants in backgrounds of 1892S, M858, and HLY858 were still high, and showed no significant differences with the wild type controls. The aroma compounds 2AP content in grains of HLY858wxfgr-1 and HLY858wxfgr-2 were 153.0 μg/kg and 151.0 μg/kg, respectively. In contrast, 2AP was not detected in grains of HLY858. There were no significant differences in major agronomic traits between the mutants and HLY858. This study provides guidelines for cultivation of ideal glutinous and aromatic hybrid rice by gene editing.

Foliar methyl jasmonate (MeJA) application increased 2-acetyl-1-Pyrroline (2-AP) content and modulated antioxidant attributes and yield formation in fragrant rice

Endogenous methyl jasmonate (MeJA) mediates abiotic and biotic stresses in plants. Exogenous MeJA application can stimulate and defend plant gene expression and induce plant chemical defense. The effects of foliar MeJA application on yield and 2-acetyl-1-pyrroline (2-AP) biosynthesis of fragrant rice are scarcely investigated. The pot experiment was conducted by spraying different concentrations of MeJA (0, 1, and 2 μM; denoted as CK, MeJA-1, and MeJA-2) at the initial heading stage of two fragrant rice cultivars, Meixiangzhan and Yuxiangyouzhan. The results showed that foliar MeJA application significantly increased the grain 2-AP content by 32.1% and 49.7%, respectively, following MeJA-1 and MeJA-2 treatments, and the two cultivars showed the highest 2-AP content upon MeJA-2 treatment. However, the grain yield was increased in MeJA-1 as compared with MeJA-2 treatment for all rice cultivars and no significant differences were observed in yield and yield-related traits compared with CK. The aroma was improved by foliar MeJA application which was strongly associated with the regulation of the precursors and enzymes involved in 2-AP biosynthesis. In particular, the contents of proline, pyrroline-5-carboxylic acid, and pyrroline at maturity, as well as the activities of proline dehydrogenase, ornithine aminotransferase, and pyrroline-5-carboxylic acid synthetase, were positively correlated with grain 2-AP content. On the other hand, foliar MeJA application improved the contents of soluble protein, chlorophyll a and b, and carotenoid, and increased the activity of antioxidant enzymes. Moreover, peroxidase activity and leaf chlorophyll contents were significantly positively correlated to 2-AP content following foliar MeJA application. Therefore, our results implied that foliar MeJA application increased aroma and influenced yield by regulating the physio-biochemistry characters and resistance, and suggested that the optimal concentration of MeJA for the best positive effect on the yield and aroma was 1 μM. However, further study is required to evaluate the metabolic level and molecular basis of the regulatory mechanism of foliar MeJA application on 2-AP in fragrant rice.

Development and validation of HS-SPME-GCMS/MS method for quantification of 2-acetyl-1-pyrroline in rice cultivars

The commercial significance of accurate and simple quantification of 2-Acetyl-1-pyrroline (2-AP) cannot be overstated. Present study was carried out to standardize a method for extraction and accurate quantitation of 2-AP from rice grain using GC-MS/MS equipped with HS-SPME auto sampler. The effect of sample quantity, addition of solvent, grinding process, sample particle size, head space parameters and SPME fiber incubation parameters, were optimized in the developed method. Dehusked rice powder (2 g) prepared under liquid nitrogen, and passed through the 80-mesh sieve, incubated for 40 min at 80 °C in headspace, followed by fiber (DVB/Carbon WR/PDMS) saturation time of 15 min, could produce the maximum response. The recovery of 2-AP from fortified sample ranged between 7.02 and 9.02% at 50-200 ng g-1 fortification irrespective of the grain matrices used. Standard addition method was appropriate to overcome the matrix effect and recovery of 2-AP was more than 90% using this method. The developed method was further utilized for quantification of 2-AP in four Basmati and two non-Basmati aromatic rice samples. The content of 2-AP ranged between 57.17 and 147.10 ng g-1 of rice and varied with geographical location. This fully automated method could improve the work efficiency and reduce error during the volatile extraction and adsorption phase.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-023-05674-7.

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

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.

A non-synonymous SNP in homolog of BADH2 gene is associated with fresh pod fragrance in dolichos bean (Lablab purpureus var. lignosus (Prain) Kumari)

Fresh pods are harvestable and marketable economic product in dolichos bean. Fresh pod fragrance is one of the 'farmers' and 'consumers' preferred traits in dolichos bean varieties. The pods with high fragrance fetch a premium price in the market. In breeding programmes, pod fragrance is routinely assessed by organoleptic (sensory) means, which is highly relative and subjective. Identification of linked DNA markers not only offer an objective means but also enable selection of fragment genotypes at seedling stage itself. Betaine aldehyde dehydrogenase (BADH) is known to be the key gene responsible for fragrance in other legumes such as vegetable soybean and mung bean. Taking cues from highly conserved domains in proteins coded by BADH genes, we isolated dolichos bean homolog (LpBADH2) of soybean GmBADH2 gene using reported degenerate primers designed to conserved domains. Analysis of the translated amino acid sequence of LpBADH2 showed high degree of similarity (97.30%) with those of soybean homolog (GmBADH2). Conserved amino acid sequence of aldehyde dehydrogenase-super family were also identified in LpBADH2. Multiple sequence alignment of nucleotide sequences of LpBADH2 with those of related legumes using "ClustalW" revealed the presence of a single non-synonymous single nucleotide polymorphic (SNPs) and three synonymous SNP sites in LpBADH2. The substitution of the amino acid tyrosine in (fragrant genotypes) with phenyl alanine (non-fragrant genotypes) in protein coded by LpBADH2 appeared to be the cause for switch over from fragrance to non-fragrance in dolichos bean. These results are discussed in relation to strategies to breed dolichos bean cultivars with desired level of pod fragrance.

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

Aroma is an important economic trait of vegetable soybeans, which greatly influences their market value. The 2-acetyl-1-pyrroline (2AP) is considered as an important substance affecting the aroma of plants. Although the 2AP synthesis pathway has been resolved, the differences of the 2AP synthesis in the aromatic and non-aromatic vegetable soybeans are unknown. In this study, a broad targeted metabolome analysis including measurement of metabolites levels and gene expression levels was performed to reveal pathways of aroma formation in the two developmental stages of vegetable soybean grains [35 (S5) and 40 (S6) days after anthesis] of the 'Zhexian No. 8' (ZX8, non-aromatic) and ZK1754 (aromatic). The results showed that the differentially accumulated metabolites (DAMs) of the two varieties can be classified into nine main categories including flavonoids, lipids, amino acids and derivatives, saccharides and alcohols, organic acids, nucleotides and derivatives, phenolic acids, alkaloids and vitamin, which mainly contributed to their phenotypic differences. Furthermore, in combination with the 2AP synthesis pathway, the differences of amino acids and derivatives were mainly involved in the 2AP synthesis. Furthermore, 2AP precursors' analysis revealed that the accumulation of 2AP mainly occurred from 1-pyrroline-5-carboxylate (P5C), not 4-aminobutyraldehyde (GABald). The quantitative RT-PCR showed that the associated synthetic genes were 1-pyrroline-5-carboxylate dehydrogenase (P5CDH), ∆¹-pyrroline-5-carboxylate synthetase (P5CS), proline dehydrogenase (PRODH) and pyrroline-5-carboxylate reductase (P5CR), which further verified the synthetic pathway of 2AP. Furthermore, the betaine aldehyde dehydrogenase 2 (GmBADH2) mutant was not only vital for the occurrence of 2AP, but also for the synthesis of 4-aminobutyric acid (GABA) in vegetable soybean. Therefore, the differences of 2AP accumulation in aromatic and non-aromatic vegetable soybeans have been revealed, and it also provides an important theoretical basis for aromatic vegetable soybean breeding.

Insights of Improved Aroma under Additional Nitrogen Application at Booting Stage in Fragrant Rice

Plant mineral nutrition substantially affects the growth, yield and quality of rice, whereas nitrogen (N) application contributes significantly in this regard. Undoubtedly, N application improves rice aroma biosynthesis; however, the molecular mechanism underlying the regulation of grain 2-acetyl-1-pyrroline (2-AP) biosynthesis in the presence of nitrogen application at the booting stage has remained largely unexplored. The present study examined the effects of three N levels, i.e., 0 g per pot (N0), 0.43 g per pot (N1) and 0.86 g per pot (N2) on intermediates, enzymes and genes involved in 2-AP biosynthesis, as well as on the yield of two fragrant rice cultivars viz, Meixiangzhan2 and Xiangyaxiangzhan. N was additionally applied at the booting stage. The results depicted that the levels of precursor, such as proline, and the activity of enzymes involved in 2-AP biosynthesis, such as Δ1-pyrroline-5-carboxylate synthetase (P5CS) and diamine oxidase (DAO), and P5CS1 gene expression were comparatively higher under N1 than N0 in both fragrant rice cultivars. Moreover, the N2 treatment increased the grain panicle-1, filled grain percentage and grain yield of both rice cultivars, while the grain yield of Meixiangzhan2 and Xiangyaxiangzhan was increased by 15.87% and 12.09%, respectively, under N2 compared to N1 treatment. Hence, 0.43 g per pot of N showed positive performances in yield and aroma accumulation in fragrant rice and should be further employed in the practice and production for better cultivation in the rice market.

Volatile Organic Compounds, Evaluation Methods and Processing Properties for Cooked Rice Flavor

Rice (Oryza sativa L.), as the main refined grain in China, has attracted much attention in terms of quality. Rice is usually consumed after cooking, and it is a commonly staple food. Nowdays, people's requirements for cooked rice focus more on the taste characteristics and quality. Furthermore, aroma is one of the primary sensory reference points, which is the most intuitive way for people to judge cooked rice. By integrating and analyzing the researches of cooked rice aroma identification in recent five years, this paper expounds the extraction and identification methods (sensory evaluation method, GC-MS, SPME, MOS sensors, electronic nose, etc.) of the flavor substances in cooked rice, as the processing methods and properties of cooked rice, and the volatile organic compounds of cooked rice under different conditions are summarized as well.

Volatile Compounds Governed by Single Recessive Gene Impart Aroma in Sponge Gourd (Luffa cylindrica L. Roem)

As a vegetable crop, sponge gourd is widely consumed worldwide due to its health promoting and nutraceutical value. This study describes genetics of an aromatic genotype VRSG-7-17 and deciphers the genetic control and volatile compound composition of sponge gourd. To study the inheritance of this trait, a cross was made between aromatic light-green-fruited VRSG-7-17 and non-aromatic dark-green-fruited VRSG-194 genotypes. The F₁s were found to be non-aromatic and have a green fruit colour. Chi-square (χ²) analysis of backcross and F₂ population segregating for aroma suggested that the inheritance of aroma in VRSG-7-17 is governed by a single recessive gene in a simple Mendelian fashion. The SPME-GC/MS analysis of the volatile compounds suggested that the compounds responsible for Basmati rice-like aroma were mainly hexanal, 1-octen-3-ol, 3-octanone and limonene. The aroma persists in the cooked VRSG-7-17 fruits, that did not lose fragrance traits at high temperatures. The inheritance of fruit colour was found to be controlled by a single gene with incomplete dominance. The segregation analysis showed that the aroma and fruit colour were not linked, and they segregated independently. The findings will lead to understanding the inheritance of the aromatic compounds in the sponge gourd and may be utilised in the breeding programmes for developing improved aromatic varieties.

Impact of OsBadh2 Mutations on Salt Stress Response in Rice

Mutations in the Betaine aldehyde dehydrogenase 2 (OsBadh2) gene resulted in aroma, which is a highly preferred grain quality attribute in rice. However, research on naturally occurring aromatic rice has revealed ambiguity and controversy regarding aroma emission, stress tolerance, and response to salinity. In this study, mutant lines of two non-aromatic varieties, Huaidao#5 (WT_HD) and Jiahua#1 (WT_JH), were generated by targeted mutagenesis of OsBadh2 using CRISPR/Cas9 technology. The mutant lines of both varieties became aromatic; however, WT_HD mutants exhibited an improved tolerance, while those of WT_JH showed a reduced tolerance to salt stress. To gain insight into the molecular mechanism leading to the opposite effects, comparative analyses of the physiological activities and expressions of aroma- and salinity-related genes were investigated. The WT_HD mutants had a lower mean increment rate of malondialdehyde, superoxide dismutase, glutamate, and proline content, with a higher mean increment rate of γ-aminobutyric acid, hydrogen peroxide, and catalase than the WT_JH mutants. Fluctuations were also detected in the salinity-related gene expression. Thus, the response mechanism of OsBadh2 mutants is complicated where the genetic makeup of the rice variety and interactions of several genes are involved, which requires more in-depth research to explore the possibility of producing highly tolerant aromatic rice genotypes.

Post-transcriptional regulation of 2-acetyl-1-pyrroline (2-AP) biosynthesis pathway, silicon, and heavy metal transporters in response to Zn in fragrant rice

Fragrant rice (Oryza sativa L.) has a high economic and nutritional value, and the application of micronutrients regulates 2-acetyl-1-pyrroline (2-AP) production, which is responsible for aroma in fragrant rice. Alternative splicing (AS) is an important post-transcriptional regulatory mechanism to generate transcript variability and proteome diversity in plants. However, no systematic investigation of AS events in response to micronutrients (Zn) has been performed in fragrant rice. Furthermore, the post-transcriptional regulation of genes involved in 2-AP biosynthesis is also not known. In this study, a comprehensive analysis of AS events under two gradients of Zn treatment in two different fragrant rice cultivars (Meixiangzhan-2 and Xiangyaxiangzhan) was performed based on RNA-seq analysis. A total of 386 and 598 significant AS events were found in Meixiangzhan-2 treated with low and high doses of Zn, respectively. In Xiangyaxiangzhan, a total of 449 and 598 significant AS events were found in low and high doses of Zn, respectively. Go analysis indicated that these genes were highly enriched in physiological processes, metabolism, and cellular processes in both cultivars. However, genotype and dose-dependent AS events were also detected in both cultivars. By comparing differential AS (DAS) events with differentially expressed genes (DEGs), we found a weak overlap among DAS and DEGs in both fragrant rice cultivars indicating that only a few genes are post-transcriptionally regulated in response to Zn treatment. We further report that Zn differentially regulates the expression of 2-AP biosynthesis-related genes in both cultivars and Zn treatment altered the editing frequency of single nucleotide polymorphism (SNPs) in the genes involved in 2-AP biosynthesis. Finally, we showed that epigenetic modifications associated with active gene transcription are generally enriched over 2-AP biosynthesis-related genes. Similar to the 2-AP pathway, we found that heavy metal transporters (genes related to silicon, iron, Zn and other metal transport) are also regulated at transcriptional and post-transcriptional levels in response to Zn in fragrant rice. Taken together, our results provide evidence of the post-transcriptional gene regulation in fragrant rice in response to Zn treatment and highlight that the 2-AP biosynthesis pathway and heavy metal transporters may also be regulated through epigenetic modifications. These findings will serve as a cornerstone for further investigation to understand the molecular mechanisms of 2-AP biosynthesis and regulation of heavy metal transporters in fragrant rice.

Foliar application of procyanidins enhanced the biosynthesis of 2-acetyl-1-pyrroline in aromatic rice (Oryza sativa L.)

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.

Streptomyces spinosus sp. nov. and Streptomyces shenzhenensis subsp. oryzicola subsp. nov. endophytic actinobacteria isolated from Jasmine rice and their genome mining for potential as antibiotic producers and plant growth promoters

Two endophytic actinobacteria, strains SBTS01^T and W18L9^T, were isolated from leaf sheath and leaf tissue, respectively, of Jasmine rice (Oryza sativa KDML 105) grown in a rice paddy field in Roi Et Province, Thailand. A polyphasic taxonomic study showed that both strains belong to the genus Streptomyces; they are aerobic, forming well-developed substrate mycelia and aerial mycelia with long chains of spores. Strain SBTS01^T shares high 16S rRNA gene sequence similarity with Streptomyces rochei NRRL B-2410 ^T (99.0%) and Streptomyces naganishii NRRL ISP-5282 ^T (99.0%). Strain W18L9^T shares high 16S rRNA gene sequence similarity with Streptomyces shenzhenensis DSM 42034 ^T (99.7%). The genotypic and phenotypic properties of strains SBTS01^T and W18L9^T distinguish these two strains from the closely related species with validly published names. The genome analysis showed the dDDH, ANIb and ANIm values of the draft genome between strain SBTS01^T and its close neighbour in the phylogenomic tree, Streptomyces corchorusii DSM 40340^T to be 54.1, 92.6, and 94.3%, respectively; similarly for strain W18L9^T and the closely related species S. shenzhenensis DSM 42034 ^T values were 72.5, 95.1 and 97.0%. The name proposed for the new species represented by the type strain SBTS01^T is Streptomyces spinosus (= NRRL B-65636 ^T = TBRC 15052^T). The name proposed for the novel subspecies of strain W18L9^T is Streptomyces shenzhenensis subsp. oryzicola (= NRRL B-65635 ^T = TBRC 15051^T). Recognition of this subspecies also permits the description of Streptomyces shenzhenensis subsp. shenzhenensis. Strains SBTS01^T and W18L9^T can produce antibiotic against rice and human pathogens and showed plant growth promoting properties such as production of indole acetic acid, cytokinin, 1-aminocyclopropane-1-carboxylate (ACC) deaminase, siderophores and cellulase. Genomic data mining of these two strains confirmed their potential as antibiotic producers and plant growth promoters. Their genomes contain multiple biosynthetic gene clusters including those for terpene, type 1, 2 and 3 polyketide synthase, Non-ribosomal peptide synthetase and lanthipeptides. Genes encoding plant growth promoting traits such; nitrogen fixation, ACC deaminase, siderophore production and stress-related adaption may have ecological significance.

Transcriptional cascades in the regulation of 2-AP biosynthesis under Zn supply in fragrant rice

Transcription factors (TFs) regulate gene expression to control certain genetic programs, such as growth and development, phytohormone regulation, and environmental stresses. 2-acetyl-1-pyrroline (2-AP) is the key element involved in aroma biosynthesis pathway, and the application of micronutrients can increase the 2-AP levels. However, little is known about the micronutrient-induced TFs involved in 2-AP biosynthesis. Here, we identify a number of TF families in two fragrant rice varieties, "Meixiangzhan-2" (M) and "Xiangyaxiangzhan" (X), in response to Zinc (Zn) application through transcriptomic analysis. A total of ~678 TFs were identified and grouped into 26 TF families, each of which was found to be involved in numerous signaling pathways. The WRKY TF family was found to be the most abundant, followed by bHLH and MYB. Furthermore, members of the WRKY, bHLH, MYB, ERF, HSF, MADS-box, NFY, and AP2 TF families were significantly upregulated and may be involved in the transcriptional regulation of aroma biosynthesis. In brief, this study enhances our understanding of the molecular mechanism of 2-AP biosynthesis and highlights the key TFs potentially involved in the production of aroma in fragrant rice.

A SNP of betaine aldehyde dehydrogenase (BADH) enhances an aroma (2-acetyl-1-pyrroline) in sponge gourd (Luffa cylindrica) and ridge gourd (Luffa acutangula)

Luffa is a genus of tropical and subtropical vines belonging to the Cucurbitaceae family. Sponge gourd (Luffa cylindrica) and ridge gourd (Luffa acutangula) are two important species of the genus Luffa and are good sources of human nutrition and herbal medicines. As a vegetable, aromatic luffa is more preferred by consumers than nonaromatic luffa. While the aroma trait is present in the sponge gourd, the trait is not present in the ridge gourd. In this study, we identified Luffa cylindrica’s betaine aldehyde dehydrogenase (LcBADH) as a gene associated with aroma in the sponge gourd based on a de novo assembly of public transcriptome data. A single nucleotide polymorphism (SNP: A > G) was identified in exon 5 of LcBADH, causing an amino acid change from tyrosine to cysteine at position 163, which is important for the formation of the substrate binding pocket of the BADH enzyme. Based on the identified SNP, a TaqMan marker, named AroLuff, was developed and validated in 370 F₂ progenies of the sponge gourd. The marker genotypes were perfectly associated with the aroma phenotypes, and the segregation ratios supported Mendelian’s simple recessive inheritance. In addition, we demonstrated the use of the AroLuff marker in the introgression of LcBADH from the aromatic sponge gourd to the ridge gourd to improve aroma through interspecific hybridization. The marker proved to be useful in improving the aroma characteristics of both Luffa species.

Multi-omics analysis reveals the genetic basis of rice fragrance mediated by betaine aldehyde dehydrogenase 2

INTRODUCTION

Fragrance is an important economic and quality trait in rice. The trait is controlled by the recessive gene betaine aldehyde dehydrogenase 2 (BADH2) via the production of 2-acetyl-1-pyrroline (2AP).

OBJECTIVES

Variation in BADH2 was evaluated at the population, genetic, transcriptional, and metabolic levels to obtain insights into fragrance regulation in rice.

METHODS

Whole-genome resequencing of the Korean World Rice Collection of 475 rice accessions, including 421 breeding lines and 54 wild accessions, was performed. Transcriptome analyses of a subset of 279 accessions, proteome analyses of 64 accessions, and volatile profiling of 421 breeding lines were also performed.

RESULTS

We identified over 3.1 million high-quality single nucleotide polymorphisms (SNPs) in Korean rice collection. Most SNPs were present in intergenic regions (79%), and 190,148 SNPs (6%) were located in the coding sequence, of which 53% were nonsynonymous. In total, 38 haplotypes were identified in the BADH2 coding region, including four novel haplotypes (one in cultivated and three in wild accessions). Tajima's D values suggested that BADH2 was under balancing selection in japonica rice. Furthermore, we identified 316 expression quantitative trait loci (eQTL), including 185 cis-eQTLs and 131 trans-eQTLs, involved in BADH2 regulation. A protein quantitative trait loci (pQTL) analysis revealed the presence of trans-pQTLs; 13 pQTLs were mapped 1 Mbp from the BADH2 region. Based on variable importance in projection (VIP) scores, 15 volatile compounds, including 2AP, discriminated haplotypes and were potential biomarkers for rice fragrance.

CONCLUSION

We generated a catalog of haplotypes based on a resequencing analysis of a large number of rice accessions. eQTLs and pQTLs associated with BADH2 gene expression and protein accumulation are likely involved in the regulation of 2AP variation in fragrant rice. These data improve our understanding of fragrance and provide valuable information for rice breeding.

The Regulatory Mechanism of 2-Acetyl-1-Pyrroline Biosynthesis in Fragrant Rice (Oryza sativa L.) Under Different Soil Moisture Contents

2-acetyl-1-pyrroline (2-AP) is the key compound of rice aroma. However, the responses of 2-AP biosynthesis in fragrant rice under different soil moisture and the corresponding mechanism are little known. The present study evaluated the effects of different soil moisture on 2-AP biosynthesis through a pot experiment. Four soil moisture contents, that is, 50% (SM50), 40% (SM40), 30% (SM30), and 20% (SM20), were adopted, and SM50 treatment was taken as control. The pots were weighed and watered to maintain the corresponding soil moisture content. The results showed no significant difference in growth parameters (plant height, stem diameter, and plant dry weight) among all treatments. Compared with SM50, SM40, SM30, and SM20 treatments significantly (p<0.05) increased 2-AP content by 32.81, 23.18, and 53.12%, respectively. Between 20 to 90% higher proline content was observed in SM40, SM30, and SM20 treatments than in SM50. Enzymes including proline dehydrogenase, ornithine transaminase, and 1-pyrroline-5-carboxylate synthetase exhibited lower activities with soil moisture declined. Higher diamine oxidase activity was observed in SM40, SM30, and SM20 treatments compared with SM50, and real-time PCR analyses showed that transcript level of DAO1 was greatly increased under low soil moisture treatments, especially in SM20 treatment. Transcript levels of PRODH, DAO2, DAO4, DAO5, OAT, P5CS1, and P5CS2 decreased or maintained in SM40, SM30, and SM20 treatments compared with SM50. We deduced that low soil moisture content enhanced 2-AP biosynthesis mainly by upregulating the expression of DAO1 to promote the conversion from putrescine to 2-AP.

Enhancement of the Aroma Compound 2-Acetyl-1-pyrroline in Thai Jasmine Rice (Oryza sativa) by Rhizobacteria under Salt Stress

Simple Summary

The major aroma compound (2-acetyl-1-pyrroline) of the world-famous Thai jasmine rice, variety KDML105, has declined due to high soil salinity and agrochemical input. In this work, the rhizobacteria from rice were investigated for the aroma compound’s production, as well as their potential for increasing the compound content in Thai jasmine rice seedlings under saline conditions. Our results provide evidence that the addition of aroma compound-producing rhizobacteria increases the aroma content in the rice seedlings under salt stress. Sinomonas sp. strain ORF15-23 which colonize the rice roots, is a promising rhizobacteria in promoting the aroma level of the Thai jasmine rice grown under salt stress and could be developed as a bioinoculant for Thai jasmine rice cultivation in a salt-affected area.

Abstract

Thai jasmine rice (Oryza sativa L. KDML105), particularly from inland salt-affected areas in Thailand, is both domestically and globally valued for its unique aroma and high grain quality. The key aroma compound, 2-acetyl-1-pyrroline (2AP), has undergone a gradual degradation due to anthropogenic soil salinization driven by excessive chemical input and climate change. Here, we propose a cheaper and an ecofriendly solution to improve the 2AP levels, based on the application of plant growth-promoting rhizobacteria (PGPR). In the present study, nine PGPR isolates from rice rhizosphere were investigated for the 2AP production in liquid culture and the promotion potential for 2AP content in KDML105 rice seedlings under four NaCl concentrations (0, 50, 100, and 150 mM NaCl). The inoculation of 2AP-producing rhizobacteria resulted in an increase in 2AP content in rice seedling leaves with the maximum enhancement from Sinomonas sp. ORF15-23 at 50 mM NaCl (19.6 µg·kg⁻¹), corresponding to a 90.2% increase as compared to the control. Scanning electron microscopy confirmed the colonization of Sinomonas sp. ORF15-23 in the roots of salinity-stressed KDML105 seedlings. Our results provide evidence that Sinomonas sp. ORF15-23 could be a promising PGPR isolate in promoting aroma level of Thai jasmine rice KDML105 under salt stress.

Effects of cultivars and fertilization levels on the quality of rice milling: A diagnosis using near-infrared spectroscopy, X-ray diffraction, and scanning electron microscopy

Cultivars and fertilization levels influence rice productivity and can be associated with grain quality. Thus, it is possible to make decisions regarding the choice of cultivars and application of fertilizer levels based on the type of milling, a necessary post-harvest process that may minimize the nutrient load in the grains and result in loss in quality. This study relates the physicochemical composition and morphological quality of brown and polished milled rice grains, cultivar types, and different levels of soil fertilization using near-infrared spectroscopy analysis, X-ray diffraction and scanning electron microscopy. Statistical tools were used to test the various treatments and identify the relationship between factors and variables. A high fertilization level is related to increasing crude protein composition and starch for cultivar IRGA 431 CL associated with polished rice. However, the combination of cultivar IRGA 424 RI and brown rice demonstrated a higher grain resistance, and different percentages of whole, chalky, and damaged rice. The correlation between ash × crude protein and starch × crude fiber was found to be positive for brown rice and negative for the polished rice. Further, an increase in starch content was inversely proportional to the ash content, whereas an increase in crude protein was inversely proportional to the low-fat content in milled rice. The crystalline characteristics of rice starch were preserved at high fertilization levels associated with polished grains that demonstrated high starch content. Polished grains, however, showed more pores and cavities, and consequently greater permeabilities in the surface. It is recommended that batches of grains produced from cultivar IRGA 431 CL with high levels of fertilization be subjected to polished rice milling to achieve high protein and starch quality. However, grains from cultivar IRGA 424 RI with high levels of fertilization are recommended for brown rice milling owing to the high percentage of physical defects observed.

Lanthanum (La) improves growth, yield formation and 2-acetyl-1-pyrroline biosynthesis in aromatic rice (Oryza sativa L.)

BACKGROUND

Lanthanum (La) is a rare earth element that can influence plant growth and development. However, the effect of La on growth, yield formation and 2-acetyl-1-pyrroline (2-AP, a key compound responsible for the aroma of rice) biosynthesis in aromatic rice (Oryza sativa L. subsp. japonica Kato) has not been reported. Therefore, the present study investigated the effects of La on growth, photosynthesis, yield formation and 2-AP biosynthesis in aromatic rice through three experiments.

RESULTS

Two pot experiments and a two-year field trial were conducted with different rates of La application (20-120 LaCl3 mg kg-1 and 12 kg ha-1 LaCl3), and treatments without La application were used as controls. The results showed that the application of LaCl3 at 80 and 100 mg kg-1 and at 12 kg ha-1 greatly increased the 2-AP content (by 6.45-43.03%) in aromatic rice seedlings and mature grains compared with the control. The La treatments also increased the chlorophyll content, net photosynthetic rate and total aboveground biomass of rice seedlings. Higher antioxidant enzyme (superoxide, peroxidase, and catalase) activity was detected in the La treatments than in the control. The La treatments also increased the grain yield, grain number per panicle and seed-setting rate of aromatic rice relative to the control. Moreover, the grain proline and γ-aminobutyric acid contents and the activity of betaine aldehyde dehydrogenase significantly decreased under the La treatment. The application of La to soil enhanced the activity of proline dehydrogenase by 20.62-56.95%.

CONCLUSIONS

La improved the growth, yield formation and 2-AP content of aromatic rice and enhanced 2-AP biosynthesis by increasing the conversion of proline to 2-AP and decreasing the conversion of GABald to GABA.

Volatile and sensory profiles of different black rice (Oryza sativa L.) cultivars varying in milling degree

In this study, we compared the volatile and sensory profiles of various black rice (Oryza sativa L.) cultivars varying in milling degree. In total, 51 volatiles were identified. 2-Acetyl-1-pyrroline (2-AP) was only detected in aromatic cultivars, and the amount varied depending on the cultivar. Additionally, a nonaromatic black rice cultivar (cv. Sinnongheukchal) contained high amounts of guaiacol. Descriptive analyses revealed significant differences in 13 sensory attributes between aromatic and nonaromatic black rice samples. After milling, the intensities of most volatile compound concentrations and sensory attributes decreased. However, the intensities of the "floral," "sweet aroma," and "sweet taste" sensory attributes increased, regardless of the cultivar. Partial least squares regression analysis showed that 2-AP correlated with the "popcorn" sensory attribute of aromatic black rice cultivars, whereas guaiacol was related to the "smoky" sensory attribute of cooked black rice. This information aids in the selection of black rice cultivars for various purposes in the food industry. For example, Heukhyangchal can be used as sources of food additives due to its smoky and popcorn-like aroma.

Rapid, High-Throughput Quantitation of Odor-Active 2-Acetyl Azaheterocycles in Food Products by UHPLC-MS/MS

A rapid, high-throughput method for the quantitation of the 2-acetyl azaheterocycles, 2-acetyl-1-pyrroline, 2-acetyl-1,4,5,6-tetrahydropyridine, 2-acetylpyrazine, and 2-acetyl-2-thiazoline, in different food products, by liquid chromatography-tandem mass spectrometry (LC-MS/MS), was developed. The quick extraction by bead beater homogenization, fast derivatization by 3-nitrophenylhydrazine (40 °C, 2 h), and efficient LC separation make this method suitable for high-throughput analysis. As established in this study, the highly precise LC-MS/MS method applies to different food products or beverages without requiring further adjustment. The analysis was performed with sample amounts of 0.2-0.5 g, and limit of quantitation values of 0.6, 0.5, 0.6, and 1.0 μg/kg were obtained for 2-acetyl-1-pyrroline, 2-acetyl-1,4,5,6-tetrahydropyridine, 2-acetylpyrazine, and 2-acetyl-2-thiazoline, respectively. Thus, it was possible to quantitate the analytes in the range of their odor thresholds.

Light and water treatment during the early grain filling stage regulates yield and aroma formation in aromatic rice

The effect of light and water on aromatic rice remain largely unclear. A pot experiment was conducted to investigate the influences of light-water treatments (CK: natural light and well-watered conditions, WS: natural light and water-stressed conditions, LL: low light and well-watered conditions, LL-WS: low light and water-stressed treatment) on yield and 2-acetyl-1-pyrroline (2AP) formation in aromatic rice. Compared with CK, the light-water treatments decreased grain yield (10.32–39.19%) due to reductions in the filled grain percentage and total dry weight, in the regulation of biomass distribution, and in the attributes of gas exchange and antioxidant response parameters. The 2AP content in grains increased in the LL treatment (5.08–16.32%) but decreased in the WS treatment compared with that in CK. The changes in 2AP were associated with changes in 2AP formation-related traits and element content. Low light and water stress led to yield declines in aromatic rice, but low light alleviated the decrease in 2AP content caused by water stress.

Identification of key odorants in complex mixtures occurring in nature

Covering: up to 2019Soon after the birth of gas chromatography, mass spectrometry and olfactometry were used as detectors, which allowed impressive development to be achieved in the area of odorant determinations. Since the mid-80s, structured methods of gas chromatography-olfactometry have appeared, allowing the determination of which odor constituents play a key role in materials. Progressively, numerous strategies have been proposed for sample preparation from raw materials, the representativeness evaluation of extracts, the identification of odor constituents, their quantification, and subsequently, the recombination of the key odorants to mimic the initial odor. However, the multiplicity of options at each stage of the analysis leads to a confusing landscape in this field, and thus, the present review aims at critically presenting the available options. For each step, the most frequently used alternatives are described, together with their strengths and weaknesses based on theoretical and experimental justifications according to the literature. These techniques are exemplified by many applications in the literature on aromas, fragrances and essential oils, with the initial focus on wine odorants, followed by a short overview on the molecular diversity of key odorants, which illustrates most of the facets and complexities of odor studies, including the issues raised by odorant interactions such as synergies.