Yield associated traits correlate with cytokinin profiles in developing pods and seeds of field-grown soybean cultivars

While lab and greenhouse based studies have long indicated that cytokinins (CK) promote yield increases in soybean (Glycine max L.), it is not known if the relationship would be valid under more complex field conditions. Thus, an ambitious CK metabolite analysis was undertaken involving long-term field trials of commercial and historical soybean varieties to determine if differences between CK profiles are related to variation in yield performance. Twenty-seven cultivars were evaluated in this study representing a wide range of performance and assessed for 12 agronomically important plant and seed parameters under field conditions. Identification and quantification of 14 forms of CK was undertaken by high-performance liquid chromatography tandem mass spectrometry (HPLC–MS/MS) at three stages of reproductive development that are critical for yield determination (R4–R6). This revealed a substantial increase in CK levels, especially the highly metabolically active free bases, in the high yielding cultivars of soybean. Significant correlations between yield components and the most active CK form, tZ (trans-Zeatin), were detected at both R4 and R5 stages. A similar trend was observed for cZ (cis-Zeatin), indicating a possible role of both zeatin isomers in pod and early seed development. Positive, significant relationships between yield and cytokinins were maintained also at R6 stage; however, a switch in hormone profiles and increased levels of isopentenyl adenine (iP) types of CK in high yielding cultivars suggested that the presence of iP derivatives allowed developing seeds to maintain their active role in sink organs and attract assimilates during the seed filling phases, when the metabolism of the maturing plant was generally slowing down. Results suggested that cytokinin metabolites or their associated genes, may serve as a valuable, early indicators of yield performance in marker-assisted breeding programs for soybean or be manipulated through gene editing techniques.