Date of Award
12-1-2024
Degree Name
Doctor of Philosophy
Department
Agricultural Sciences
First Advisor
Meksem, Khalid
Abstract
Soybean [Glycine max (L.) Merr.] is a valuable leguminous crop providing protein and oil for food, feed, and industry. Carbohydrates comprise 30-35% of the soybean seed dry weight. The market for soybean-based products is increasing significantly with the increase of consumer awareness about the benefits of plant-based products. The soybean sugar contains different molecules, including sucrose and polysaccharides: raffinose and stachyose. Sucrose is desirable whereas high contents of raffinose and stachyose are undesirable because they are not digested in humans and monogastric animals due to the lack of α-galactosidases. This causes a reduced gastrointestinal performance. The improvement of the soybean seed sugar profile could considerably improve the soybean seed taste and value. In this dissertation, we aim to improve soybean seed sugar content and composition in soybean seeds using genetic and genomic approaches. In the current study, we used QTL (Quantitative Trait Loci) mapping to identify genomic regions associated with soybean seed sugar content. Then we identified soybean candidate genes that are involved in the sugar biosynthesis pathway. Additionally, we produced mutagenized soybean populations and used TILLING (Targeting Induced Local Lesions IN Genomes) by sequencing approach to validate the function of the candidate genes. 26 QTL controlling seed sugar contents were identified and mapped on 16 soybean chromosomes, among them five QTL regions were found in both locations, Illinois and North Carolina, in this study on chrs. 2, 5, 13, 17, and 20. Furthermore, novel QTL regions were identified including qSUC-4, qSUC-8, and qSUC-11 mapped on chrs. 4, 10, and 18 respectively, that were identified for the first time in this study. Additionally, a total of 57 candidate genes were identified, among them 16 genes located within 10 MB of the identified QTL. Amongst these genes a cluster of four involved in the sugars’ pathway was collocated within 6 MB with two QTL that were detected in this study on chromosome17. Furthermore, five candidate genes InDels between Forrest and Williams 82 sequences, including Glyma.09G073600, Glyma.08G143500, Glyma.17G111400, Glyma.05G003900, and Glyma.09G016600 have SNPs. These SNPs could potentially explain the difference in sugar content between Forrest and Williams 82 cultivars. The soybean sucrose synthase and invertase candidate genes were characterized through a comprehensive analysis of phylogenetic tree, synteny analysis, gene structure, and conserved domains variations. Additionally, TILLING (Targeting Induced Local Lesions IN Genomes) by Target Sequencing (TbyS) technology was used to identify mutants with improved sugar content. The results have shown that many mutations have been identified in soybean sucrose synthase and invertase genes that resulted in high sucrose content including the sucrose synthase mutants SL446 (R582W) and F1115 (G249E) on Glyma.02G240400 that have a sucrose content of 9.5% and 9.1%, respectively. Remarkably, two invertase mutants including F1291 (G632E) on Glyma.20G177200 and F1384 (D184V) on Glyma.10G145600 have shown an increased sucrose content and reduced raffinose and stachyose contents without altering the agronomic performance of the soybean plant. The obtained soybean mutants with enhanced sugar content can be useful in breeding programs to improve the soybean sugar content and composition.
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