Correlation of cytokinin levels in the endosperms and roots with cell number and cell division activity during endosperm development in rice

Cell number and cell division activity in rice (Oryza sativa) endosperms are possibly regulated by cytokinin levels in the endosperm and its source in the roots. This study tried to find the possible correlations among them. Six rice genotypes were grown in nutrient solution. Two patterns of endosperm cell division, synchronous and asynchronous, were observed among the genotypes based on the cell division rate of superior and inferior spikelets. Contents of zeatin (Z) + zeatin riboside (ZR) were much higher than those of N⁶-isopentenyladenine (iP) and N⁶-isopentenyladenosine (iPR) in both endosperms and roots. Changes in Z + ZR levels in endosperms were significantly correlated with those in roots, and both were very significantly correlated with the cell division rate. Changes in iP + iPR contents in the roots were not significantly correlated with those in the endosperms and the cell division rate. When roots were treated with kinetin, endosperm cell number and grain weight were increased. Such enhancement was more significantly achieved by the root kinetin treatment than by spraying kinetin on leaves and panicles. The results suggest that the cell number and cell division activity in rice endosperms are regulated by cytokinin levels in the endosperm and that root-derived Z + ZR play a pivotal role.