Composition analysis of lysine, tryptophan and provitamin-A during different stages of kernel development in biofortified sweet corn

Enrichment of Vitamin A and Vitamin E in Sweet Corn Kernels Through Genomics-Assisted Introgression of Mutant Version of crtRB1 and vte4 Genes

Recessive shrunken2 (sh2)-based sweet corn is preferred worldwide as it possesses higher sugar and extended shelf life. However, traditional sh2-based sweet corn is poor in vitamin A and vitamin E. Here, parental lines of two sh2-based sweet corn hybrids, viz. PSSC-2 and ASKH-2, were targeted for introgression of β-carotene hydroxylase 1 (crtRB1) and γ-tocopherol methyltransferase (vte4) genes through marker-assisted backcross breeding. Seeds with sh2sh2sh2 genotype in the endosperm were selected based on the shrunken phenotype in BC1F1, BC2F1 and BC2F2 generations. Gene-based markers, viz. 3'-TE-InDel and 118-InDel specific for crtRB1 and vte4, respectively, were successfully utilized for foreground selection in BC1F1, BC2F1 and BC2F2. Reconstituted hybrids showed high provitamin A (proA: 19.52 ± 0.52 µg/g) with a maximum of 7.8-fold increase over original hybrids (ASKH-2 and PSSC-2: 3.33 ± 0.28 µg/g). High α-tocopherol (20.75 ± 0.44 µg/g) and α/γ-tocopherol ratio (0.55 ± 0.02) with an average enhancement of 2.3- and 1.7-fold, respectively, was recorded among reconstituted hybrids over original versions (α-tocopherol: 9.21 ± 0.33 µg/g, α/γ-tocopherol ratio: 0.31 ± 0.01). The average yield of reconstituted hybrids (11.40 ± 0.22 t/ha) was at par with the original sweetcorn hybrids (11.60 ± 0.20 t/ha). This is the first report of stacking sh2, crtRB1 and vte4 genes to improve nutritional quality in sweet corn. These biofortified sweet corn hybrids hold immense significance to alleviate micronutrient malnutrition.

Multi-layer molecular analysis reveals distinctive metabolomic and transcriptomic profiles of different sweet corn varieties

In plants, sugar metabolism involves a complex interplay of genetic, molecular and environmental factors. To better understand the molecular mechanisms underlying these processes, we utilized a multi-layered approach that integrated transcriptomic and metabolomic datasets generated from multiple different varieties of sweet corn. Through this analysis, we found 2533 genes that were differentially expressed in the immature kernel tissues of sweet corn, including genes involved in transcriptional regulation, sugar metabolism, primary metabolism, and other processes associated with adaptability of sweet corn. We also detected 31 differential metabolites among the three types of sweet corn. Utilizing an integrated approach encompassing transcriptomics and eGWAS, we elucidated the transcriptional regulatory patterns governing these differential metabolites. Specifically, we delved into the transcriptional modulation of malate- and ubiquitin-associated genes across a range of sweet corn varieties, shedding new light on the molecular mechanisms underlying their regulation. This study provides a framework for future research aimed at improving the current understanding of sugar metabolism and regulatory gene networks in sweet corn, which could ultimately lead to the development of novel strategies for crop improvement.

Mutant crtRB1 gene negates the unfavourable effects of opaque2 gene on germination and seed vigour among shrunken2-based biofortified sweet corn genotypes

Sweet corn is one of the most popular vegetables worldwide. However, traditional shrunken2 (sh2 )-based sweet corn varieties are poor in nutritional quality. Here, we analysed the effect of (1) β-carotene hydroxylase1 (crtRB1 ), (2) opaque2 (o2 ) and (3) o2+crtRB1 genes on nutritional quality, germination, seed vigour and physico-biochemical traits in a set of 27 biofortified sh2 -based sweet corn inbreds. The biofortified sweet corn inbreds recorded significantly higher concentrations of proA (16.47μg g-1 ), lysine (0.36%) and tryptophan (0.09%) over original inbreds (proA: 3.14μg g-1 , lysine: 0.18%, tryptophan: 0.04%). The crtRB1 -based inbreds had the lowest electrical conductivity (EC), whereas o2 -based inbreds possessed the highest EC. The o2 +crtRB1 -based inbreds showed similar EC to the original inbreds. Interestingly, o2 -based inbreds also had the lowest germination and seed vigour compared to original inbreds, whereas crtRB1 and o2 +crtRB1 introgressed sweet corn inbreds showed similar germination and seed vigour traits to their original versions. This suggested that the negative effect of o2 on germination, seed vigour and EC is nullified by crtRB1 in the double mutant sweet corn. Overall, o2 +crtRB1 -based sweet corn inbreds were found the most desirable over crtRB1 - and o2 -based inbreds alone.

Edible fungi efficiently degrade aflatoxin B1 in cereals and improve their nutritional composition by solid-state fermentation

Aflatoxin B₁ (AFB₁) is extremely harmful to human and livestock. Laccase, a green catalyst, has been shown to effectively degrade AFB₁ and can be obtained from edible fungi. The objective of this study was to screen edible fungi with high laccase activity and determine their effects on the degradation of AFB₁ in cereals and the nutritional composition of the cereals through solid-state fermentation. Results from plate assays confirmed that 51 of the 55 tested edible fungi could secrete laccase. Submerged fermentation results showed that 17 of the 51 edible fungi had maximum laccase activity exceeding 100 U/L. The growth of different edible fungi varied significantly in corn, rice and wheat. More importantly, 6 edible fungi with high laccase activity and good growth could efficiently degrade AFB₁ in cereals. We found for the first time that Ganoderma sinense could not only secrete highly active laccase and efficiently degrade AFB₁ in corn by 92.91%, but also improve the nutritional quality of corn. These findings reveal that solid-state fermentation of cereals with edible fungi is an environmentally friendly and efficient approach for degrading AFB₁ in cereals and improving the nutritional composition of cereals.

Expression Dynamics of lpa1 Gene and Accumulation Pattern of Phytate in Maize Genotypes Possessing opaque2 and crtRB1 Genes at Different Stages of Kernel Development

Phytic acid (PA) acts as a storehouse for the majority of the mineral phosphorous (P) in maize; ~80% of the total P stored as phytate P is not available to monogastric animals and thereby causes eutrophication. In addition, phytic acid chelates positively charged minerals making them unavailable in the diet. The mutant lpa1-1 allele reduces PA more than the wild-type LPA1 allele. Further, mutant gene opaque2 (o2) enhances lysine and tryptophan and crtRB1 enhances provitamin-A (proA) more than wild-type O2 and CRTRB1 alleles, respectively. So far, the expression pattern of the mutant lpa1-1 allele has not been analysed in maize genotypes rich in lysine, tryptophan and proA. Here, we analysed the expression pattern of wild and mutant alleles of LPA1, O2 and CRTRB1 genes in inbreds with (i) mutant lpa1-1, o2 and crtRB1 alleles, (ii) wild-type LPA1 allele and mutant o2 and crtRB1 alleles and (iii) wild-type LPA1, O2 and CRTRB1 alleles at 15, 30 and 45 days after pollination (DAP). The average reduction of PA/total phosphorous (TP) in lpa1-1 mutant inbreds was 29.30% over wild-type LPA1 allele. The o2 and crtRB1-based inbreds possessed ~two-fold higher amounts of lysine and tryptophan, and four-fold higher amounts of proA compared to wild-type alleles. The transcript levels of lpa1-1, o2 and crtRB1 genes in lpa1-1-based inbreds were significantly lower than their wild-type versions across kernel development. The lpa1-1, o2 and crtRB1 genes reached their highest peak at 15 DAP. The correlation of transcript levels of lpa1-1 was positive for PA/TP (r = 0.980), whereas it was negative with inorganic phosphorous (iP) (r = -0.950). The o2 and crtRB1 transcripts showed negative correlations with lysine (r = -0.887) and tryptophan (r = -0.893), and proA (r = -0.940), respectively. This is the first comprehensive study on lpa1-1 expression in the maize inbreds during different kernel development stages. The information generated here offers great potential for comprehending the dynamics of phytic acid regulation in maize.

Provitamin A, lysine and tryptophan enrichment in shrunken2-based sweet corn genotypes through genomics-assisted breeding for crtRB1 and opaque2 genes

BACKGROUND

Malnutrition affects large section of population worldwide. Vitamin A and protein deficiencies have emerged as the major global health-issue. Traditional shrunken2 (sh2)-based sweet corn is deficient in provitamin A (proA), lysine and tryptophan. Natural variant of β-carotene hydroxylase1 (crtRB1) and opaque2 (o2) enhances proA, lysine and tryptophan in maize. So far, no sweet corn hybrid rich in these nutrients has been released elsewhere. Development of biofortified sweet corn hybrids would help in providing the balanced nutrition.

METHODS AND RESULTS

We targeted three sh2-based sweet corn inbreds (SWT-19, SWT-20 and SWT-21) for introgression of mutant crtRB1 and o2 genes using molecular breeding. The gene-based 3'TE-InDel and simple sequence repeat (SSR) (umc1066) markers specific to crtRB1 and o2, respectively were utilized in foreground selection in BC₁F₁, BC₂F₁ and BC₂F₂. Segregation distortion was observed for crtRB1 and o2 genes in majority of populations. Background selection using 91-100 SSRs revealed recovery of recurrent parent genome (RPG) up to 96%. The introgressed progenies possessed significantly higher proA (13.56 µg/g) as compared to the original versions (proA: 2.70 µg/g). Further, the introgressed progenies had accumulated moderately higher level of lysine (0.336%) and tryptophan (0.082%) over original versions (lysine: 0.154% and tryptophan: 0.038%). Kernel sweetness among introgressed progenies (17.3%) was comparable to original sweet corn (17.4%). The introgressed inbreds exhibited higher resemblance with their recurrent parents for yield and morphological characters.

CONCLUSION

These newly developed biofortified sweet corn genotypes hold immense promise to alleviate malnutrition.

Allelic variation in shrunken2 gene affecting kernel sweetness in exotic-and indigenous-maize inbreds

Sweet corn has become a popular food worldwide. It possesses six-times more sugar than field corn due to the presence of recessive shrunken2 (sh2) gene. Despite availability of diverse sweet corn germplasm, comprehensive characterization of sh2 has not been undertaken so far. Here, entire Sh2 gene (7320 bp) among five field corn-(Sh2Sh2) and six sweet corn-(sh2sh2) inbreds was sequenced. A total of 686 SNPs and 372 InDels were identified, of which three SNPs differentiated the wild-(Sh2) and mutant-(sh2) allele. Ten InDel markers were developed to assess sh2 gene-based diversity among 23 sweet corn and 25 field corn lines. Twenty-five alleles and 47 haplotypes of sh2 were identified among 48 inbreds. Among markers, MGU-InDel-2, MGU-InDel-3, MGU-InDel-5 and MGU-InDel-8 had PIC>0.5. Major allele frequency varied from 0.458–0.958. The gene sequence of these maize inbreds was compared with 25 orthologues of monocots. Sh2 gene possessed 15–18 exons with 6-225bp among maize, while it was 6–21 exons with 30-441bp among orthologues. While intron length across maize genotypes varied between 67-2069bp, the same among orthologues was 57–2713 bp. Sh2-encoded AGPase domain was more conserved than NTP transferase domain. Nucleotide and protein sequences of sh2 in maize and orthologues revealed that rice orthologue was closer to maize than other monocots. The study also provided details of motifs and domains present in sh2 gene, physicochemical properties and secondary structure of SH2 protein in maize inbreds and orthologues. This study reports detailed characterization and diversity analysis in sh2 gene of maize and related orthologues in various monocots.

Characterization of crtRB1- and vte4-based biofortified sweet corn inbreds for seed vigour and physico-biochemical traits

Sweet corn possessing recessive shrunken2 (sh2) gene is popular worldwide. Traditional sweet corn is poor in vitamin A and vitamin E. Plant breeders during the selection of sweet corn genotypes mainly emphasize on plant architecture and yield. Seed germination and seedling vigour play important role for early establishment in field, thereby increasing yield and income. Here, we analysed a set of 15 sh2-based biofortified sweet corn inbreds with crtRB1 (β-carotene hydroxylase1) and vte4 (γ-tocopherol methyltransferase) genes and three traditional sh2-based sweet corn inbreds for nutritional quality, seed vigour and various physico-biochemical traits. The newly developed inbreds possessed significantly higher provitamin A (proA: 15.60 µg/g) and vitamin E [α-tocopherol (α-T): 20.42 µg/g] than the traditional sweet corn inbreds (proA: 2.51 µg/g, α-T: 11.16 µg/g). The biofortified sweet corn inbreds showed wide variation for germination (80.67-87.33%), vigour index-I (2097.17-2925.28 cm), vigour index-II (134.27-216.19 mg) and electrical conductivity (10.19-13.21 μS cm^-1 g^-1). Wide variation was also observed for dehydrogenase (1.29-1.59 OD g^-1 ml^-1), super oxide dismutase (4.01-9.82 g^-1), peroxidase (11.66-16.47 μM min^-1 g^-1), esterase (22.98-34.76 nM min^-1 g^-1) and α-amylase (5.91-8.15 OD g^-1 ml^-1). Enrichment of proA and vitamin E in sweet corn did not affect seed vigour and physico-biochemical traits. Correlation analysis revealed that electrical conductivity and α-amylase activity was the reliable indicator for assessing seed germination and vigour. The study identified superior biofortified sweet corn genotypes that would contribute to better vigour and establishment in field. This is the first report of analysis of biofortified sweet corn genotypes for seed vigour and physico-biochemical traits.

Expression analysis of β-carotene hydroxylase1 and opaque2 genes governing accumulation of provitamin-A, lysine and tryptophan during kernel development in biofortified sweet corn

Traditional sweet corn possesses low levels of provitamin-A (proA), lysine and tryptophan. Mutant version of β-carotene hydroxylase1 (crtRB1) gene affecting the accumulation of β-carotene (BC), β-cryptoxanthin (BCX) and proA, and opaque2 (o2) gene governing the enhancement of lysine and tryptophan were introgressed together into elite sweet corn inbreds through marker-assisted selection. Here, we analyzed the expression pattern of crtRB1 and o2 genes among introgressed and traditional sweet corn inbreds at 20-, 24- and 28-days after pollination (DAP). The introgressed inbreds possessed two- to sevenfolds higher BC, BCX, proA, lysine and tryptophan compared to their original inbreds. However, all the nutrients attained the peak at 20-DAP (BC: 9.95 µg/g, BCX: 8.21 µg/g, proA: 14.05 µg/g, lysine: 0.301%, tryptophan: 0.074%), which gradually reduced through 24-DAP (BC: 8.24 µg/g, BCX: 7.53 µg/g, proA: 12.01 µg/g, lysine: 0.273%, tryptophan: 0.057%) and 28-DAP (BC: 5.84 µg/g, BCX: 5.82 µg/g, proA: 8.75 µg/g, lysine: 0.202%, tryptophan: 0.037%). Biofortified sweet corn inbreds possessed significantly lower expression levels of crtRB1 (4.1-fold) and o2 (2.2-fold) compared to their wild type alleles in traditional sweet corn inbreds across DAPs. The expression of crtRB1 and o2 increased from 20-DAP to attain the highest peak at 24-DAP, and further decreased by 28-DAP. The transcript levels of crtRB1 were negatively correlated with BC (r = - 0.83), BCX (r = - 0.79) and proA (r = - 0.83) across dates of harvest. Lysine (r = - 0.83) and tryptophan (r = - 0.73) were also inversely associated with o2 transcript levels. This is the first report on expression of crtRB1 and o2 genes during kernel development in biofortified sweet corn. This information holds immense promise in understanding the dynamics of gene-regulation during kernel development in sweet corn.

Development of multinutrient-rich biofortified sweet corn hybrids through genomics-assisted selection of shrunken2, opaque2, lcyE and crtRB1 genes

Sweet corn has gained worldwide popularity. Traditional sweet corn possesses low concentration of essential nutrients such as lysine (0.15-0.25%), tryptophan (0.03-0.04%) and provitamin-A (proA 3-4 ppm), and deficiency leads to serious health problems in humans. Here, stacking of shrunken2 (sh2), opaque2 (o2), lycopene epsilon cyclase (lcyE) and β-carotene hydroxylase (crtRB1) genes  were undertaken in the parents of four hybrids viz., APQH1, APHQ4, APHQ5 and APHQ7 using marker-assisted backcross breeding (MABB). Gene-linked markers (umc2276 and umc1320) for sh2, while gene-based markers for o2 (umc1066 and phi057), lcyE (5'TE-InDel) and crtRB1 (3'TE-InDel), were used for genotyping in BC₁F₁, BC₂F₁ and BC₂F₂. Selected backcross progenies showed high recovery of recurrent parent genome (92.4-97.7%). The reconstituted sweet corn hybrids possessed significantly high lysine (0.390%), tryptophan (0.082%) and proA (21.14 ppm), coupled with high kernel sweetness (brix 18.96%). The improved sweet corn hybrids had high cob yield (12.22-15.33 t/ha) across three environments. These newly developed biofortified sweet corn hybrids possess great significance in providing balanced nutrition. This is the first report of combining sh2, o2, lcyE and crtRB1 genes for enrichment of sweet corn hybrids with multiple essential nutrients.