The soybean (Glycine max 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 dehydrogenase/oxidase (CKX) gene family is of critical importance for regulating plant CK levels by causing their irreversible degradation. Variation of CKX gene activity is significantly correlated with seed yield in many crop species while in soybean (Glycine max L.), the possible associations between CKX gene family members (GFMs) and yield parameters have not yet been assessed. In this study, seventeen GmCKX GFMs were identified, and natural variations among GmCKX genes were probed among soybean cultivars with varying yield characteristics. The key CKX 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 GmCKX GFMs, showed natural variations in the form of single nucleotide polymorphisms (SNPs). Three non-synonymous variations, T69A, H105Q and A159G, in the gene GmCKX14 with high expression during critical seed filling stages, were predicted to affect protein stability, resulting in a higher CK accumulation in soybean lines with desired yield characteristics. The potential for marker-assisted selection based on these natural variations within GmCKX14, is discussed in the context of hormonal control that can result in higher soybean yield.