The Soybean ( L.) Cytokinin Oxidase/dehydrogenase Multigene Family; Identification of Natural Variations for Altered Cytokinin Content and Seed Yield

Cytokinins (CKs) play a fundamental role in regulating dynamics of organ source/sink relationships during plant development, including flowering and seed formation stages. As a result, CKs are key drivers of seed yield. The cytokinin oxidase/dehydrogenase (CKX) is one of the critical enzymes responsible for regulating plant CK levels by causing their irreversible degradation. Variation of activity is significantly correlated with seed yield in many crop species while in soybean ( L.), the possible associations between gene family members (GFMs) and yield parameters have not yet been assessed. In this study, 17 GFMs were identified, and natural variations among genes were probed among soybean cultivars with varying yield characteristics. The key genes responsible for regulating CK content during seed filling stages of reproductive development were highlighted using comparative phylogenetics, gene expression analysis and CK metabolite profiling. Five of the seventeen identified GFMs, showed natural variations in the form of single nucleotide polymorphisms (SNPs). The gene , with high expression during critical seed filling stages, was found to have a non-synonymous mutation (H105Q), on one of the active site residues, Histidine 105, previously reported to be essential for co-factor binding to maintain structural integrity of the enzyme. Soybean lines with this mutation had higher CK content and desired yield characteristics. The potential for marker-assisted selection based on the identified natural variation within , is discussed in the context of hormonal control that can result in higher soybean yield.