Fluctuations of endogenous hormones in isolated rice embryos during embryogenesis and early stages of germination

Fluctuations of levels of several endogenous plant hormones in isolated rice ( Oryza sativa ssp. japonica) embryos during early and mid-embryogenesis and early stages of germination were studied by enzyme-linked immunosorbent assay (ELISA). Embryos were collected at different days after pollination (DAP) and different days after imbibition (DAI) of mature seeds. The contents of gibberellin(1) (GA(1)), abscisic acid (ABA), isopentenyladenine and isopentenyladenosine ( iPAs), zeatin and zeatin riboside ( ZRs) were immunochemically assayed. The GA(1) level was the highest among all hormones tested. The variations of GA(1) levels were opposite with the ABA levels, with some exceptions. During early and mid-embryogenesis, the levels of GA(1) and ABA were the highest at 4 DAP. From 8 to 18 DAP, GA(1) level declined, whereas the ABA level increased. During germination, GA(1) level increased at 2 DAI whereas simultaneously the ABA content decreased. The highest ratio of GA(1)/ABA was observed at 2 DAI The levels of iPAs and ZRs were maxima in the embryos at 4 DAP, decreased to a very low level and maintained constant thereafter. Our results provide further evidence that GA(1) plays an important role in the early stages of embryo development and germination. The balance between GA(1) and ABA, rather than their absolute contents, controls these processes throughout the development, whereas iPAs and ZRs may play important roles in early embryogenesis. The use of isolated embryos as starting material avoids the usual interferences with other tissues such as the endosperm. In addition, this is the first report dealing with the hormonal balance of early-embryos in rice.