A PCR-based marker for a locus conferring aroma in vegetable soybean (Glycine max L.)

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.

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.

A multiple phenotype imputation method for genetic diversity and core collection in Taiwanese vegetable soybean

Establishment of vegetable soybean (edamame) [Glycine max (L.) Merr.] germplasms has been highly valued in Asia and the United States owing to the increasing market demand for edamame. The idea of core collection (CC) is to shorten the breeding program so as to improve the availability of germplasm resources. However, multidimensional phenotypes typically are highly correlated and have different levels of missing rate, often failing to capture the underlying pattern of germplasms and select CC precisely. These are commonly observed on correlated samples. To overcome such scenario, we introduced the "multiple imputation" (MI) method to iteratively impute missing phenotypes for 46 morphological traits and jointly analyzed high-dimensional imputed missing phenotypes (EC impu ) to explore population structure and relatedness among 200 Taiwanese vegetable soybean accessions. An advanced maximization strategy with a heuristic algorithm and PowerCore was used to evaluate the morphological diversity among the EC impu . In total, 36 accessions (denoted as CC impu ) were efficiently selected representing high diversity and the entire coverage of the EC impu . Only 4 (8.7%) traits showed slightly significant differences between the CC impu and EC impu . Compared to the EC impu , 96% traits retained all characteristics or had a slight diversity loss in the CC impu . The CC impu exhibited a small percentage of significant mean difference (4.51%), and large coincidence rate (98.1%), variable rate (138.76%), and coverage (close to 100%), indicating the representativeness of the EC impu . We noted that the CC impu outperformed the CC raw in evaluation properties, suggesting that the multiple phenotype imputation method has the potential to deal with missing phenotypes in correlated samples efficiently and reliably without re-phenotyping accessions. Our results illustrated a significant role of imputed missing phenotypes in support of the MI-based framework for plant-breeding programs.

A Sequence Variation in GmBADH2 Enhances Soybean Aroma and Is a Functional Marker for Improving Soybean Flavor

The vegetable soybean (Glycine max L. Merr.) plant is commonly consumed in Southeast Asian countries because of its nutritional value and desirable taste. A "pandan-like" aroma is an important value-added quality trait that is rarely found in commercial vegetable soybean varieties. In this study, three novel aromatic soybean cultivars with a fragrant volatile compound were isolated. We confirmed that the aroma of these cultivars is due to the potent volatile compound 2-acetyl-1-pyrroline (2AP) that was previously identified in soybean. A sequence comparison of GmBADH1/2 (encoding an aminoaldehyde dehydrogenase) between aromatic and non-aromatic soybean varieties revealed a mutation with 10 SNPs and an 11-nucleotide deletion in exon 1 of GmBADH2 in Quxian No. 1 and Xiangdou. Additionally, a 2-bp deletion was detected in exon 10 of GmBADH2 in ZK1754. The mutations resulted in a frame shift and the introduction of premature stop codons. Moreover, genetic analyses indicated that the aromatic trait in these three varieties was inherited according to a single recessive gene model. These results suggested that a mutated GmBADH2 may be responsible for the aroma of these three aromatic soybean cultivars. The expression and function of GmBADH2 in aromatic soybean seeds were confirmed by qRT-PCR and CRISPR/Cas9. A functional marker developed on the basis of the mutated GmBADH2 sequence in Quxian No. 1 and Xiangdou was validated in an F2 population. A perfect association between the marker genotypes and aroma phenotypes implied that GmBADH2 is a major aroma-conferring gene. The results of this study are potentially useful for an in-depth analysis of the molecular basis of 2-AP formation in soybean and the marker-assisted breeding of aromatic vegetable soybean cultivars.

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.

Comparative Analysis and Development of a Flavor Fingerprint for Volatile Compounds of Vegetable Soybean Seeds Based on Headspace-Gas Chromatography-Ion Mobility Spectrometry

Evaluating the volatile compounds and characteristic fingerprints of the core cultivars of vegetable soybean would provide useful data for improving their aroma in the breeding programs. The present study used headspace-gas chromatography-ion mobility spectrometry (HS-GC-IMS) to evaluate the volatile compounds of vegetable soybean seeds at a specific growth stage. In total, 93 signal peaks were identified, 63 compounds qualitatively, with 14 volatile flavor compounds providing multiple signals. The 63 volatile compounds consisted of 15 esters, 15 aldehydes, 13 alcohols, 15 ketones, one acid, and four other compounds. The peak intensity of most of the volatile compounds varied greatly between the core cultivars. The alcohols and aldehydes determined the basic volatile flavor of the vegetable soybean seeds. Volatile flavors were determined by their respective esters, ketones, or other components. Characteristic fingerprints were found in some core vegetable soybean cultivars. Four cultivars (Xiangdou, ZHE1754, Zhexian 65018-33, and Qvxian No. 1) had pleasant aromas, because of their higher content of 2-acetyl-1-pyrroline (2-AP). A principal component analysis (PCA) was used to distinguish the samples based on the signal intensity of their volatile components. The results showed that the composition and concentration of volatile compounds differed greatly between the core cultivars, with the volatile flavor compounds of soybeans being determined by the ecotype of the cultivar, the direction of breeding selection, and their geographical origin. Characteristic fingerprints of the cultivars were established by HS-GC-IMS, enabling them to be used to describe and distinguish cultivars and their offspring in future breeding studies.

Capturing and Explaining Sensory Differences among Organically Grown Vegetable-Soybean Varieties Grown in Northern California

Vegetable soybean (VS), known as edamame, is appreciated for its robust flavor and exceptional nutrient content. Although the United States is one of the world's leading producers of soy, only a fraction is harvested as edamame. Research involving varietal differences is very limited, particularly among U.S. consumers. Sensory attributes of three VS varieties grown in Northern California (with organic methods) were assessed: Giant Midori (GM), ButterBean (BB), and Kuroshinja (KU). Participants (n = 74) rated four characteristics: flavor, texture, appearance, and overall liking on unlabeled, 11-point hedonic scales and were asked if they would purchase the variety. A small portion of these participants would again evaluate the varieties, using free choice profiling (FCP) methodology. After these evaluations, texture analyzer, colorimeter, and high-performance liquid chromatography (free amino acids, sugars, and isoflavones) testing were performed. Among tested varieties, distinct differences in quality were found, with GM preferred first, followed by KU, and BB. GM was significantly harder, a more intense green, had more free sugars and isoflavones and fewer free amino acids, whereas KU and BB trended to be the opposite. Analyzing FCP data with a General Procrustes Analysis, varieties were separated in different factors for flavor and texture, but not appearance. Although repeated, and larger trials are needed, our study signifies that consumers are able to detect differences among VS varieties. Further research may influence producers to grow the variety likely to be most profitable and marketable.

A Dual Repeat Cis-Element Determines Expression of GERANYL DIPHOSPHATE SYNTHASE for Monoterpene Production in Phalaenopsis Orchids

Phalaenopsis bellina is a scented orchid emitting large amount of monoterpenes. GERANYL DIPHOSPHATE SYNTHASE (PbGDPS) is the key enzyme for monoterpene biosynthesis, and shows concomitant expression with the emission of monoterpenes during flower development in P. bellina. Here, we identified a dual repeat cis-element in the GDPS promoter that is critical for monoterpene biosynthesis in Phalaenopsis orchids. A strong correlation between the dual repeat and the monoterpene production was revealed by examination of the GDPS promoter fragments over 12 Phalaenopsis species. Serial-deletion of the 2-kb GDPS promoter fragments demonstrated that the integrity of the dual repeat was crucial for its promoter activities. By screening the Arabidopsis transcription factors (TFs) cDNA library using yeast one-hybrid assay, AtbZIP18, a member of group I of bZIP TFs, was identified to be able to bind the dual repeat. We then identified PbbZIP4 in the transcriptome of P. bellina, showing 83% identity in the DNA binding region with that of AtbZIP18, and the expression level of PbbZIP4 was higher in the scented orchids. In addition, PbbZIP4 transactivated the GDPS promoter fragment containing the dual repeat in dual luciferase assay. Furthermore, transient ectopic expression of PbbZIP4 induced a 10-fold production of monoterpenoids in the scentless orchid. In conclusion, these results indicate that the dual repeat is a real TF-bound cis-element significant for GDPS gene expression, and thus subsequent monoterpene biosynthesis in the scented Phalaenopsis orchids.

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.

Determination of the genetic diversity of vegetable soybean [Glycine max (L.) Merr.] using EST-SSR markers

The development of expressed sequence tag-derived simple sequence repeats (EST-SSRs) provided a useful tool for investigating plant genetic diversity. In the present study, 22 polymorphic EST-SSRs from grain soybean were identified and used to assess the genetic diversity in 48 vegetable soybean accessions. Among the 22 EST-SSR loci, tri-nucleotides were the most abundant repeats, accounting for 50.00% of the total motifs. GAA was the most common motif among tri-nucleotide repeats, with a frequency of 18.18%. Polymorphic analysis identified a total of 71 alleles, with an average of 3.23 per locus. The polymorphism information content (PIC) values ranged from 0.144 to 0.630, with a mean of 0.386. Observed heterozygosity (Ho) values varied from 0.0196 to 1.0000, with an average of 0.6092, while the expected heterozygosity (He) values ranged from 0.1502 to 0.6840, with a mean value of 0.4616. Principal coordinate analysis and phylogenetic tree analysis indicated that the accessions could be assigned to different groups based to a large extent on their geographic distribution, and most accessions from China were clustered into the same groups. These results suggest that Chinese vegetable soybean accessions have a narrow genetic base. The results of this study indicate that EST-SSRs from grain soybean have high transferability to vegetable soybean, and that these new markers would be helpful in taxonomy, molecular breeding, and comparative mapping studies of vegetable soybean in the future.