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Key role for polyploidy in promoting leaf length in Lolium species
It is obvious that both cell division and cell enlargement are essential processes in plant growth. However, that simple statement raises some very interesting questions concerning the control and integration of the two processes in the production of organs of particular shapes and sizes. Thus, the recent discovery of genes that regulate the cell cycle in relation to leaf size is very exciting. Equally interesting is the evidence that in many, but certainly not all species, the phase of cell enlargement is preceded or accompanied by endo-reduplication of DNA (replication of DNA in the absence of mitosis). Taken with observations that across the plant kingdom cell size is positively correlated with genome size, this suggests a role for genome size in regulating cell enlargement. The data presented here by Sugiyama (Hirosaki, Japan, pp. 931–938) certainly point in this direction. The author compared diploid and tetraploid populations in two Lolium species with respect to leaf size, leaf cell division and leaf cell enlargement. In both species, but especially in L. perenne, the tetraploids had longer leaves than the diploids. This was not due in any way to differences in cell division: cell cycle times and cell production rates showed no significant differences between diploids and tetraploids. Instead, as shown by detailed analysis of cell lengths from the leaf bases up through the zones of division and elongation, the greater leaf lengths were clearly correlated with greater cell lengths at maturity. Statistical analysis showed that although the tetraploids exhibited both a greater rate of cell elongation and a longer phase of cell elongation than the diploids, it was the increased rate of elongation that made by far the major contribution to increased cell length. The challenge now is to discover the mechanism(s) by which genome size affects so specifically this particular facet of cell growth.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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