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From root to fruit with cytokinin
Cereal endosperm is arguably the most economically and socially important plant tissue in the world; amongst the cereals rice plays an especially significant role. It is therefore surprising that we know so little about the molecular and physiological controls of the development of this remarkable tissue although there are encouraging signs that this deficiency is now being addressed. The work of Yang et al. (Universities of Yangzhou and Hong Kong, pp. 369-377) is an interesting example of this growing focus. The authors studied six rice genotypes differing in their pattern of endosperm cell division. In three genotypes, early endosperm divisions were synchronous and rapid in both superior and inferior spikelets which developed at the same rate. In the other three, divisions were asynchronous and proceeded more slowly; endosperm development in superior spikelets preceded that in inferior spikelets. Rigorous assay of cytokinins in the endosperm revealed the presence mainly of zeatin and zeatin riboside. With cell division rates and cytokinin contents known throughout the period of endosperm development in all the genotypes, it was possible to use this large data set to plot cell division rate against zeatin plus zeatin riboside content. The correlation was remarkable (r = 0.95) suggesting that in this real tissue cytokinins may be involved in the regulation of cell division, just as they are in cultured cells. But what is the origin of these endosperm-located cytokinins? The authors do not answer this question directly. However, they show first that the endosperm concentrations of zeatin plus zeatin riboside are strongly correlated with their concentrations in roots, and secondly that application of exogenous cytokinins to roots stimulates endosperm cell division (application to leaves or panicles was much less effective). These results are thus consistent with a role for cytokinins made in roots in the regulation of endosperm cell division.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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