Abstract
Correlative data from monocots suggest that cytokinin (CK) regulates seed development. The involvement of CKs in seed growth was investigated using pea, a eudicot with an unknown CK profile, as a model system. CK profiles were measured by liquid chromatography – tandem mass spectrometry against major stages of embryogenesis, which were documented histologically. Like other grain legumes, CK levels of developing pea seeds fluctuated through development and had mainly nucleotide and riboside forms. Among the 11 CKs detected, cis-isomers (cis-[9R]Z (zeatin riboside), and cis-[9RMP]Z (zeatin riboside 5′ monophosphate)), along with their isopentenyl precursors, were the major forms during pea embryogenesis, whereas corresponding trans-isomers appeared as minor constituents. Highest CK concentrations occurred at the heart-shape stage, when there are high rates of cell division and sugar metabolism. To assess the significance of high CK concentrations observed at the heart-shape stage, a bioassay was developed wherein heart-shaped embryos were excised and cultured on medium containing either cis-[9R]Z, trans-[9R]Z, or kinetin. Growth of cultured heart-shaped embryo explants was significantly augmented by all exogenous CKs relative to controls that were not supplemented with CK. Moreover, at concentrations equivalent to those experienced by an embryo in vivo, cis-[9R]Z was active in enhancing the growth of cultured pea embryos to an extent equal to that of trans-[9R]Z. Overall, the results endorse a growth-promoting role for cis-CKs during seed development.
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