Delta1-pyrroline-5-carboxylic acid formed by proline dehydrogenase from the Bacillus subtilis ssp. natto expressed in Escherichia coli as a precursor for 2-acetyl-1-pyrroline

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.

Genome sequencing and protein modeling unraveled the 2AP biosynthesis in Bacillus cereus DB25

2-Acetyl-1-pyrroline (2AP) is an important and major flavor aroma compound responsible for the fragrance of basmati rice, cheese, wine, and several other food products. Biosynthesis of 2AP in aromatic rice and a few other plant species is associated with a recessive Betaine aldehyde dehydrogenase 2 (BADH2) gene. However, the literature is scant on the relationship between the functional BADH2 gene and 2AP biosynthesis in prokaryotic systems. Therefore, in the present study, we aimed to explore the functionality of the BADH2 gene for 2AP biosynthesis in 2AP synthesizing rice rhizobacterial isolate Bacillus cereus DB25 isolated from the rhizosphere of basmati rice (Oryza sativa L.). Full-length BcBADH2 sequence was obtained through whole genome sequencing (WGS) and further confirmed through traditional PCR and Sanger sequencing. Then the functionality of the BcBADH2 gene was evaluated in-silico through bioinformatics analysis and protein docking studies and further experimentally validated through enzyme assay. The sequencing and bioinformatics analysis results revealed a full-length 1485 bp BcBADH2 coding sequence without any deletion or premature stop codons. Full-length BcBADH2 was found to encode a fully functional protein of 54.08 kDa with pI of 5.22 and showed the presence of the conserved amino acids responsible for enzyme activity. The docking studies confirmed a good affinity between the protein and its substrate whereas the presence of BcBADH2 enzyme activity confirmed the functionality of BADH2 enzyme in B. cereus DB25. In conclusion, the findings of the present study suggest that B. cereus DB25 is able to synthesize 2AP despite a functional BADH2 gene and there may be a different molecular mechanism responsible for 2AP biosynthesis in bacterial systems, unlike that found in aromatic rice and other eukaryotic plant species.

Proline Dehydrogenase and Pyrroline 5 Carboxylate Dehydrogenase from Mycobacterium tuberculosis: Evidence for Substrate Channeling

In Mycobacterium tuberculosis, proline dehydrogenase (PruB) and ∆1-pyrroline-5-carboxylate (P5C) dehydrogenase (PruA) are monofunctional enzymes that catalyze proline oxidation to glutamate via the intermediates P5C and L-glutamate-γ-semialdehyde. Both enzymes are essential for the replication of pathogenic M. tuberculosis. Highly active enzymes were expressed and purified using a Mycobacterium smegmatis expression system. The purified enzymes were characterized using natural substrates and chemically synthesized analogs. The structural requirements of the quinone electron acceptor were examined. PruB displayed activity with all tested lipoquinone analogs (naphthoquinone or benzoquinone). In PruB assays utilizing analogs of the native naphthoquinone [MK-9 (II-H2)] specificity constants Kcat/Km were an order of magnitude greater for the menaquinone analogs than the benzoquinone analogs. In addition, mycobacterial PruA was enzymatically characterized for the first time using exogenous chemically synthesized P5C. A Km value of 120 ± 0.015 µM was determined for P5C, while the Km value for NAD+ was shown to be 33 ± 4.3 µM. Furthermore, proline competitively inhibited PruA activity and coupled enzyme assays, suggesting that the recombinant purified monofunctional PruB and PruA enzymes of M. tuberculosis channel substrate likely increase metabolic flux and protect the bacterium from methylglyoxal toxicity.

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.

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.

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.

Organocatalytic Allylic Alkylation of α-(Alkylideneamino)nitriles and Its Application in the Preparation of Multisubstituted 1-Pyrrolines

A synthetic approach for the construction of functionalized diverse 1-pyrrolines incorporating β-quaternary carbon centers under mild reaction conditions has been reported, in which α-allyl α-(alkylideneamino)nitriles generated from a Lewis base-catalyzed allylic alkylation reaction engaged in a Lewis base-mediated tandem intramolecular cyclization to deliver the targeted molecules in a catalytically atom-economic fashion.

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.

Genomic diversity of Serratia proteamaculans and Serratia liquefaciens predominant in seafood products and spoilage potential analyses

Serratia sp. cause food losses and waste due to spoilage; it is noteworthy that they represent a dominant population in seafood. The main spoilage associated species comprise S. liquefaciens, S. grimesii, S. proteamaculans and S. quinivorans, also known as S. liquefaciens-like strains. These species are difficult to discriminate since classical 16S rRNA gene-based sequences do not possess sufficient resolution. In this study, a phylogeny based on the short-length luxS gene was able to speciate 47 Serratia isolates from seafood, with S. proteamaculans being the main species from fresh salmon and tuna, cold-smoked salmon, and cooked shrimp while S. liquefaciens was only found in cold-smoked salmon. The genome of the first S. proteamaculans strain isolated from the seafood matrix (CD3406 strain) was sequenced. Pangenome analyses of S. proteamaculans and S. liquefaciens indicated high adaptation potential. Biosynthetic pathways involved in antimicrobial compounds production and in the main seafood spoilage compounds were also identified. The genetic equipment highlighted in this study contributed to gain further insights into the predominance of Serratia in seafood products and their capacity to spoil.

RNAi-mediated down regulation of BADH2 gene for expression of 2-acetyl-1-pyrroline in non-scented indica rice IR-64 (Oryza sativa L.)

2-acetyl-1-pyrroline (2AP) is a principal aroma compound in scented rice and a mutation in betaine aldehyde dehydrogenase 2 (OsBADH2) is responsible aroma in scented rice. The present study was aimed at inducing 2AP production in non-scented indica rice cultivar IR-64 by silencing OsBADH2 via RNAi technique. A vector pBSK was used for the construction of RNAi cassette and pRI101ON as a binary vector. Agrobacterium (GV3101)-mediated transformation was done using embryogenic calli of IR-64. The resultant transgenic lines showed up to 14-fold reduction in expression of OsBADH2 gene and 50% inhibition in enzyme activity. Gas chromatography (GC-MS) analyses showed a significant amount of 2AP production in RNAi callus, leaves, and seeds of IR-64. A total 39 volatile compounds were identified from the control and RNAi seeds of IR-64. Among them, octanal and 2-pentylfuron were found to be increased (30-40%) in RNAi seeds of IR-64. The content of precursors, proline, and methylglyoxal increased substantially, whereas GABA content reduced up to 25% in transgenic IR-64 lines. The study demonstrated that RNAi approach could be successfully used for imparting pleasant aroma character in non-scented indica rice cultivars.

Nitrogen application and different water regimes at booting stage improved yield and 2-acetyl-1-pyrroline (2AP) formation in fragrant rice

BACKGROUND

Water (W) and nitrogen (N) management generally cause regulations in the 2-acetyl-1-pyrroline (2AP) accumulation in fragrant rice; nevertheless, the feasibility of such management strategies at booting stage in improving 2AP accumulation has not been examined in details.

METHODS

Field experiments were conducted in the early season (March-July) and repeated in the late season (July-November) in 2013. The treatments were applied urea (90 kg ha^- 1), calcium super phosphate (90 kg ha^- 1) and potassium chloride (195 kg ha^- 1) as basal fertilizer, and urea (65 kg ha^- 1) at tillering stage. Three N levels i.e., 0 kg N ha^- 1 (N1), 30 kg N ha^- 1 (N2), and 60 kg N ha^- 1 (N3) and three water levels i.e., W1 treatment (well-watered treatment with water layer of 2-4 cm), W2 treatment (soil water potential was - 15 ± 5 kPa), and W3 treatment (soil water potential was - 25 ± 5 kPa) at booting stage was set up for three rice varieties i.e., Nongxiang 18, Yungengyou 14 and Basmati. The grain yield, head milled rice yield, 2AP contents and the biochemical parameters related to 2AP formation were investigated.

RESULTS

Result indicated that W and N dynamics regulated the grain yield, head milled rice yield, and 2AP contents in brown rice across three varieties. The N2 and N3 treatment significantly increased the 2AP contents in brown rice by 9.54% and 11.95%, and 8.88% and 32.54% in the early and the late season, respectively; improved grain yield and head milled rice yield. The W3 treatment improved grain yield, head milled rice yield and 2AP content. Significant W and N interaction effect on 2AP content in brown rice was detected, where the W3 N3 treatment showed the strongest interaction regarding improvement of 2AP contents in brown rice. The 2AP accumulation and its related biochemical parameters and their relationships in different plant tissues at different growth stages under W and N treatments had also been assessed. The 2AP content, P5C content and DAO activity during grain filling periods was highly related to the 2AP content in brown rice.

CONCLUSION

This study revealed that the 60 kg N ha^- 1 coupled with - 25 ± 5 kPa treatment showed the best positive effects on yield and aroma in fragrant rice, suggested that water and nitrogen management at booting stage can improve grain yield and fragrance in fragrant rice. However, further study to evaluate the metabolic and molecular basis of 2AP accumulation in fragrant rice is needed.

Metabolic responses and their correlations with phytochelatins in Amaranthus hypochondriacus under cadmium stress

Phytochelatins (PCs) play a vital role in the tolerance and enrichment of cadmium (Cd) in higher plants by chelating with Cd²⁺. The aim of this study was to perform a full-scale metabolomics analysis of metabolic responses highly correlated with PCs generation. These metabolites and metabolic pathways were expected to promote PCs generation and further optimize Cd absorption in plants. In the current study, Amaranthus hypochondriacus, a potential species for phytoremediation, was first adopted to investigate physiological responses to Cd stress via LCMS/MS-based metabolomics and the HPLC based determination of thiol compounds. The results showed that the leaves of A. hypochondriacus under high Cd stress accumulated 40 times the amount of Cd compared to the leaves of the plants not under Cd stress and had an increased content of three types of PCs. Metabolomics qualitatively identified 12084 substances in total, among which 41 were significantly different metabolites (SDMs) between the two groups and involved in 7 metabolic pathways. Among the SDMs, 12 metabolites were highly linearly correlated with PCs involved in three pathways (Val, Leu and Ile biosynthesis; Ala, Asp and Glu metabolism; and Arg and Pro metabolism). These results provide an innovative method to promote PCs synthesis for the restoration of Cd-contaminated-soil.

Role of Proline in Pathogen and Host Interactions

SIGNIFICANCE

Proline metabolism has complex roles in a variety of biological processes, including cell signaling, stress protection, and energy production. Proline also contributes to the pathogenesis of various disease-causing organisms. Understanding the mechanisms of how pathogens utilize proline is important for developing new strategies against infectious diseases. Recent Advances: The ability of pathogens to acquire amino acids is critical during infection. Besides protein biosynthesis, some amino acids, such as proline, serve as a carbon, nitrogen, or energy source in bacterial and protozoa pathogens. The role of proline during infection depends on the physiology of the host/pathogen interactions. Some pathogens rely on proline as a critical respiratory substrate, whereas others exploit proline for stress protection.

CRITICAL ISSUES

Disruption of proline metabolism and uptake has been shown to significantly attenuate virulence of certain pathogens, whereas in other pathogens the importance of proline during infection is not known. Inhibiting proline metabolism and transport may be a useful therapeutic strategy against some pathogens. Developing specific inhibitors to avoid off-target effects in the host, however, will be challenging. Also, potential treatments that target proline metabolism should consider the impact on intracellular levels of Δ1-pyrroline-5-carboxylate, a metabolite intermediate that can have opposing effects on pathogenesis.

FUTURE DIRECTIONS

Further characterization of how proline metabolism is regulated during infection would provide new insights into the role of proline in pathogenesis. Biochemical and structural characterization of proline metabolic enzymes from different pathogens could lead to new tools for exploring proline metabolism during infection and possibly new therapeutic compounds.

The genetics and biosynthesis of 2-acetyl-1-pyrroline in fragrant rice

Rice (Oryza sativa L.) is a staple food for the majority of the world's population. Rice fragrance, aroma, or scent is not just a cooking quality, but also an eating quality of rice. Rice fragrance is a trait that is widely desired among rice consumers. Consequently, rice producers are sorting for rice cultivars with strong fragrance. High demand for fragrant rice cultivars has prompted rice breeders and researchers to investigate the genetics and the ways to improve fragrance in rice. It has been established by many researches that fgr gene on the chromosome 8 of rice controls its fragrance. As with other plants, rice contains BADH but because rice does not accumulate GB, a catalyst for BADH coding, BADH1 on chromosome 4 of rice and BADH2 on chromosome 8 of rice have been widely reported to be responsible for encoding BADH. badh2, a recessive allele of BADH2 has been confirmed to be responsible for fragrance in rice. badh2 and its alleles have been associated with the accumulation and synthesis of 2AP. Proline, ornithine, glutamate, methylglyoxal, Δ1-pyrroline-5-carboxylate synthetase and glyceraldehyde-3-phosphate dehydrogenase have all been identified as the precursors for the synthesis and accumulation of 2AP. By reviewing and summarising the main results of various researchers, we have been able to elucidate how various genes and metabolites influence 2AP accumulation in fragrant rice. It is our hope that this paper will be beneficial to researchers, who are working on the improvement of rice fragrance.

Differential levels of metabolites and enzymes related to aroma formation in aromatic indica rice varieties: comparison with non-aromatic varieties

Accounting for aroma production in different aromatic indica rice varieties based on variations in the levels of concerned metabolites and enzymes is poorly explored. The present investigation was, therefore, focused on unraveling the differential levels of metabolites and activities of enzymes related to aroma formation in eleven indigenous aromatic rice varieties, as compared with four non-aromatic varieties. The levels of metabolites such as proline (Pro) and Δ¹-pyrroline-5-carboxylate (P5C), and the activity of related enzymes such as proline dehydrogenase (PDH), Δ¹-pyrroline-5-carboxylate synthetase (P5CS), and ornithine aminotransferase (OAT) were comparatively higher in the aromatic varieties, with Kalonunia and Tulaipanji registering the highest Pro, Kalonunia the highest P5C content, Gobindobhog with the highest PDH activity, Gobindobhog and Tulaipanji with the highest P5CS, and Pusa Basmati-1 with the highest OAT activity. The levels of putrescine (Put) and γ-aminobutyric acid (GABA) were comparatively lower in aromatic varieties, with concomitant higher diamine oxidase (DAO) activity, especially in the varieties Gobindobhog and Tulaipanji. The betaine-aldehyde dehydrogenase 2 (BADH2) enzyme activity was remarkably lesser in aromatic varieties, especially Radhunipagal and Gobindobhog. Though the metabolites such as glycine-betaine and higher polyamines such as spermidine and spermine showed no specific trend with respect to their quantitative level in either aromatic or non-aromatic varieties, they were notably lower in the aromatic varieties such as Gobindobhog, Kalonunia, and Tulaipanji, indicating a possibility of their involvement in aroma formation. Therefore, the levels of metabolites such as Pro, P5C and methylglyoxal (MG), and the activity of enzymes such as PDH, P5CS, OAT, and DAO were comparatively higher in the aromatic rice varieties than the non-aromatic ones, whereas the levels of Put, GABA, and BADH2 were lower. Overall, the present study showed that there exist variations in the accumulations of such metabolites as well as differential activity of enzymes controlling their production, which altogether regulate generation of aroma in aromatic varieties.

Metabolomics and genomics combine to unravel the pathway for the presence of fragrance in rice

Since it was first characterised in 1983, 2-acetyl-1-pyrroline (2AP) has been considered to be the most important aroma compound in rice. In this study, we show four other amine heterocycles: 6-methyl, 5-oxo-2,3,4,5-tetrahydropyridine (6M5OTP), 2-acetylpyrrole, pyrrole and 1-pyrroline, that correlate strongly with the production of 2AP, and are present in consistent proportions in a set of elite aromatic rice varieties from South East Asia and Australia as well as in a collection of recombinant inbred lines (RILs) derived from indica Jasmine-type varieties, Australian long grain varieties (temperate japonica) and Basmati-type rice (Grp V). These compounds were detected through untargeted metabolite profiling by two-dimensional gas chromatography-time-of-flight mass spectrometry (GC × GC-TOF-MS), and their identity were confirmed by comparison with authentic standards analysed using gas chromatography mass spectrometry (GC-MS) and High Resolution GC × GC-TOF-MS (GC × GC HRT-4D). Genome-wide association analysis indicates that all compounds co-localised with a single quantitative trait locus (QTL) that harbours the FGR gene responsible for the production of GABA. Together, these data provide new insights into the production of 2AP, and evidence for understanding the pathway leading to the accumulation of aroma in fragrant rice.

Identification of aroma volatiles and understanding 2-acetyl-1-pyrroline biosynthetic mechanism in aromatic mung bean (Vigna radiata (L.) Wilczek)

Mung bean having high food value and easily digestible proteins, is one of the socioeconomically important crop of India. Among the varied cultivars, Sona mung is having aroma and hence popularly cultivated in the pockets of Ganga river basin at Bhutnir char village of Malda District in the West Bengal state. In the present study, aroma volatiles with special reference to 2-acetyl-1-pyrroline (2AP) were analyzed using HS-SPME-GCMS from Sona mung bean and compared with non-scented mung bean (PHULE M-9339). 26 volatiles in seeds of Sona mung and 20 in non-scented mung bean were identified, in which 3,7-dimethyl-6-octenal, (2E)-2-decen-1-ol, 2-ethyl-1-dodecanol and 3,5,5-trimethyl-2-cyclohexene-1-one are first time reported. 0.19 ± 0.001 ppm 2AP was recorded in Sona mung seeds whereas it was not detected in non-scented mung bean. PCA analysis indicated that 2AP, octanal, 1 pentanol, decanal, phenylmethanol and 2-nonen-1-ol were the major contributors in the aroma of Sona mung bean. The significantly higher level proline, methylglyoxal and lower level of BADH2 transcript were detected in Sona mung than non-scented mung, suggesting similar 2AP biosynthesis mechanism in Sona mung bean as reported in scented rice, sorghum and soybean.

Supplementation of 2-Ap, Zn and La Improves 2-Acetyl-1-Pyrroline Concentrations in Detached Aromatic Rice Panicles In Vitro

Aromatic rice is highly prized by consumers worldwide due to its special aromatic character. 2-acetyl-1-pyrroline (2-AP) is considered to be the single most important volatile compound responsible for aroma in aromatic rice. The present study demonstrated the effects of 2-AP, zinc (Zn) and lanthanum (La) on the 2-AP concentration of detached aromatic rice panicles in vitro. Detached panicles from three well-known aromatic cultivars, Guixiangzhan, Pin14, and Pin 15, were cultured separately in basic culture medium supplemented with 2-AP, Zn and La, and 2-AP concentrations were assessed at 7 and 14 days after culture (DAC). The results show that supplementation of 2-AP, Zn and La in the basic culture medium significantly increases the accumulation of proline. 2-AP concentration and the activity of proline dehydrogenase (ProDH) were also increased in rice grains. Zn concentrations were also found to be higher when Zn was added to the basic culture medium, and La concentrations in grains were too low to be measured. Additionally, grain 2-AP concentrations were significantly and positively correlated with proline concentrations, ProDH activities in grains and 2-AP in culture medium. In summary, higher grain 2-AP concentrations might be due to Zn- and La-induced increases in proline concentrations and ProDH activities, as well as the direct uptake and transportation of 2-AP from the culture medium. Furthermore, application of both Zn and La might be helpful for improving aroma formation in rice. However, interactions of both these elements with the complex process of 2-AP formation remain to be explored.

2-Acetyl-1-pyrroline augmentation in scented indica rice (Oryza sativa L.) varieties through Δ(1)-pyrroline-5-carboxylate synthetase (P5CS) gene transformation

2-Acetyl-1-pyrroline (2AP) has been identified as a principal aroma compound in scented rice varieties. Δ(1)-Pyrroline-5-carboxylate synthetase (P5CS) gene is reported to regulate the proline synthesis in plants and acts as the precursor of 2AP. Two scented indica rice varieties, namely Ambemohar 157 and Indrayani, were subjected to Agrobacterium tumefaciens-mediated genetic transformation containing P5CS gene. Overexpression of P5CS led to a significant increase in proline, P5CS enzyme activity and 2AP levels in transgenic calli, vegetative plant parts, and seeds over control in both the varieties. 2AP level increased more than twofold in transgenic seeds in both varieties. This is the first report of enhancement in 2AP content through overexpression of using P5CS gene, indicating the role of proline as a precursor amino acid in the biosynthesis of 2AP in scented rice.

Virus evolution toward limited dependence on nonessential functions of the host: the case of bacteriophage SPP1

All viruses are obligate intracellular parasites and depend on certain host cell functions for multiplication. However, the extent of such dependence and the exact nature of the functions provided by the host cell remain poorly understood. Here, we investigated if nonessential Bacillus subtilis genes are necessary for multiplication of bacteriophage SPP1. Screening of a collection of 2,514 single-gene knockouts of nonessential B. subtilis genes yielded only a few genes necessary for efficient SPP1 propagation. Among these were genes belonging to the yuk operon, which codes for the Esat-6-like secretion system, including the SPP1 receptor protein YueB. In addition, we found that SPP1 multiplication was negatively affected by the absence of two other genes, putB and efp. The gene efp encodes elongation factor P, which enhances ribosome activity by alleviating translational stalling during the synthesis of polyproline-containing proteins. PutB is an enzyme involved in the proline degradation pathway that is required for infection in the post-exponential growth phase of B. subtilis, when the bacterium undergoes a complex genetic reprogramming. The putB knockout shortens significantly the window of opportunity for SPP1 infection during the host cell life cycle. This window is a critical parameter for competitive phage multiplication in the soil environment, where B. subtilis rarely meets conditions for exponential growth. Our results in combination with those reported for other virus-host systems suggest that bacterial viruses have evolved toward limited dependence on nonessential host functions.

IMPORTANCE

A successful viral infection largely depends on the ability of the virus to hijack cellular machineries and to redirect the flow of building blocks and energy resources toward viral progeny production. However, the specific virus-host interactions underlying this fundamental transformation are poorly understood. Here, we report on the first systematic analysis of virus-host cross talk during bacteriophage infection in Gram-positive bacteria. We show that lytic bacteriophage SPP1 is remarkably independent of nonessential genes of its host, Bacillus subtilis, with only a few cellular genes being necessary for efficient phage propagation. We hypothesize that such limited dependence of the virus on its host results from a constant "evolutionary arms race" and might be much more widespread than currently thought.

Characterization of the proline-utilization pathway in Mycobacterium tuberculosis through structural and functional studies

The proline-utilization pathway in Mycobacterium tuberculosis (Mtb) has recently been identified as an important factor in Mtb persistence in vivo, suggesting that this pathway could be a valuable therapeutic target against tuberculosis (TB). In Mtb, two distinct enzymes perform the conversion of proline into glutamate: the first step is the oxidation of proline into Δ(1)-pyrroline-5-carboxylic acid (P5C) by the flavoenzyme proline dehydrogenase (PruB), and the second reaction involves converting the tautomeric form of P5C (glutamate-γ-semialdehyde) into glutamate using the NAD(+)-dependent Δ(1)-pyrroline-5-carboxylic dehydrogenase (PruA). Here, the three-dimensional structures of Mtb-PruA, determined by X-ray crystallography, in the apo state and in complex with NAD(+) are described at 2.5 and 2.1 Å resolution, respectively. The structure reveals a conserved NAD(+)-binding mode, common to other related enzymes. Species-specific conformational differences in the active site, however, linked to changes in the dimer interface, suggest possibilities for selective inhibition of Mtb-PruA despite its reasonably high sequence identity to other PruA enzymes. Using recombinant PruA and PruB, the proline-utilization pathway in Mtb has also been reconstituted in vitro. Functional validation using a novel NMR approach has demonstrated that the PruA and PruB enzymes are together sufficient to convert proline to glutamate, the first such demonstration for monofunctional proline-utilization enzymes.

Kinetic and isotopic characterization of L-proline dehydrogenase from Mycobacterium tuberculosis

The monofunctional proline dehydrogenase (ProDH) from Mycobacterium tuberculosis performs the flavin-dependent oxidation of l-proline to Δ(1)-pyrroline-5-carboxylate in the proline catabolic pathway. The ProDH gene, prub, was cloned into the pYUB1062 vector, and the C-terminal His-tagged 37 kDa protein was expressed and purified by nickel affinity chromatography. A steady-state kinetic analysis revealed a ping-pong mechanism with an overall kcat of 33 ± 2 s(-1) and Km values of 5.7 ± 0.8 mM and 3.4 ± 0.3 μM for l-proline and 2,6-dichlorophenolindophenol (DCPIP), respectively. The pH dependence of kcat revealed that one enzyme group exhibiting a pK value of 6.8 must be deprotonated for optimal catalytic activity. Site-directed mutagenesis suggests that this group is Lys110. The primary kinetic isotope effects on V/KPro and V of 5.5 and 1.1, respectively, suggest that the transfer of hydride from l-proline to FAD is rate-limiting for the reductive half-reaction, but that FAD reoxidation is the rate-limiting step in the overall reaction. Solvent and multiple kinetic isotope effects suggest that l-proline oxidation occurs in a stepwise rather than concerted mechanism. Pre-steady-state kinetics reveal an overall kred of 88.5 ± 0.7 s(-1), and this rate is subject to a primary kinetic isotope effect of 5.2. These data confirm that the overall reaction is limited by reduced flavin reoxidation in the second half-reaction.

Regulation of proline metabolism in mycobacteria and its role in carbon metabolism under hypoxia

Genes with a role in proline metabolism are strongly expressed when mycobacterial cells are exposed to nutrient starvation and hypoxia. Here we show that proline metabolism in mycobacteria is mediated by the monofunctional enzymes Δ(1) -pyrroline-5-carboxylate dehydrogenase (PruA) and proline dehydrogenase (PruB). Proline metabolism was controlled by a unique membrane-associated DNA-binding protein PruC. Under hypoxia, addition of proline led to higher biomass production than in the absence of proline despite excess carbon and nitrogen. To identify the mechanism responsible for this enhanced growth, microarray analysis of wild-type Mycobacterium smegmatis versus pruC mutant was performed. Expression of the DNA repair machinery and glyoxalases was increased in the pruC mutant. Glyoxalases are proposed to degrade methylglyoxal, a toxic metabolite produced by various bacteria due to an imbalance in intermediary metabolism, suggesting the pruC mutant was under methylglyoxal stress. Consistent with this notion, pruB and pruC mutants were hypersensitive to methylglyoxal. Δ(1) -pyrroline-5-carboxylate is reported to react with methylglyoxal to form non-toxic 2-acetyl-1-pyrroline, thus providing a link between proline metabolism and methylglyoxal detoxification. In support of this mechanism, we show that proline metabolism protects mycobacterial cells from methylglyoxal toxicity and that functional proline dehydrogenase, but not Δ(1) -pyrroline-5-carboxylate dehydrogenase, is essential for this protective effect.

Indirect repression by Bacillus subtilis CodY via displacement of the activator of the proline utilization operon

Proline is an efficient source of both carbon and nitrogen for many bacterial species. In Bacillus subtilis, the proline utilization pathway, encoded by the putBCP operon, is inducible by proline. Here, we show that this induction is mediated by PutR, a proline-responsive transcriptional activator of the PucR family. When other amino acids are present in the medium, proline utilization is prioritized through transient repression by CodY, a global transcriptional regulator in Gram-positive bacteria that responds to amino acid availability. CodY-mediated repression of the putBCP operon has two novel features. First, repression requires the cooperative binding of CodY to at least two adjacent motifs. Second, though CodY binds to the region that overlaps the putB promoter, repression is due to displacement of PutR rather than competition with RNA polymerase.

Mechanism of 2-acetyl-1-pyrroline biosynthesis in Bassia latifolia Roxb. flowers

The flowers of Bassia latifolia are known to contain 2-acetyl-1-pyrroline (2AP), the compound responsible for pleasant aroma in basmati and other scented rice. Four growth stages of Bassia flowers were identified and 2AP contents were analysed in each stage. It was found that 2AP (3.30 ppm) gets synthesized only in fleshy corolla of mature flowers (fourth stage). The activity of γ-aminobutyraldehyde dehydrogenase (AADH); an enzyme responsible for synthesis of γ-aminobutyricacid (GABA) from γ-aminobutyraldehyde (GABald) was assessed in these four stages. The AADH activity was absent in the fourth stage. It was concluded that ceased activity of AADH in fourth stage flowers leads to the accumulation of γ-aminobutyraldehyde which is cyclised spontaneously to Δ(1)-pyrroline, the key precursor of 2AP. Δ(1)-pyrroline further reacts unenzymatically with methylglyoxal to form 2AP.

Characterization and the possible formation mechanism of 2-acetyl-1-pyrroline in aromatic vegetable soybean (Glycine max L.)

2-Acetyl-1-pyrroline (2-AP) was identified as an important aroma compound of aromatic vegetable soybean. The level of 2-AP in 6 aromatic vegetable soybean lines was found to be positively correlated with popcorn-like aroma score. Comparison between aromatic and nonaromatic vegetable soybeans found that aromatic vegetable soybean contains higher concentration of methylglyoxal (MG) and Delta(1)-pyrroline-5-carboxylate (P5C) than a nonaromatic one. For MG formation-related genes, GapC was down-regulated and TPI was up-regulated in aromatic cultivar (Aromatic 7) as compared to nonaromatic control, which may contribute to the increase of MG level. Based on the data presented, a formation mechanism for 2-AP via interaction between MG and P5C in aromatic vegetable soybean was proposed.

Biosynthetic mechanism of 2-acetyl-1-pyrroline and its relationship with Delta1-pyrroline-5-carboxylic acid and methylglyoxal in aromatic rice (Oryza sativa L.) callus

2-Acetyl-1-pyrroline (2-AP) was identified as the major flavor compound in aromatic rice varieties Tainung 71 and 72. In order to understand the mechanism of 2-AP biosynthesis in aromatic rice, we studied the formation of putative precursors, Delta(1)-pyrroline-5-carboxylic acid and methylglyoxal. The endogenous Delta(1)-pyrroline-5-carboxylic acid contents of Tainung 71 and 72 calli reached 191 to 276%, compared to nonaromatic rice Tainung 67. In addition, calli of Tainung 71 and 72 contained 1.30- and 1.36-fold, respectively, higher methylglyoxal levels than that of Tainung 67. Specific enzyme activities of Delta(1)-pyrroline-5-carboxylic acid-synthetic enzyme including Delta(1)-pyrolline-5-carboxylic acid synthetase (P5CS) and ornithine aminotransferase (OAT) increased significantly in aromatic rice varieties. The expression levels of P5CS1 and P5CS2 genes were found to be significantly higher in aromatic rice than nonaromatic rice. Results of a tracer experiment with (15)N-labeled glutamic acid revealed that the nitrogen atom of 2-acetyl-1-pyrroline was derived from glutamic acid. Upregulation of P5CS in aromatic rice Tainung 72 may contribute to the increase of Delta(1)-pyrroline-5-carboxylic acid level and thus leads to the accumulation of an extra amount of 2-acetyl-1-pyrroline.