Annals of Botany

Bred dwarf

Dwarf varieties of cereals are extensively used all round the world and have been one of the factors contributing to significant improvements in yield. In wheat, the dwarfing genes used in plant breeding programmes include Rht1 and Rht2, whose primary effect is to confer gibberellin insensitivity on the plant, as discussed by Botwright et al. (Wembley and Canberra, Australia, pp. 631–639). This results in reductions in internode and coleoptile lengths, which have been attributed to smaller epidermal cells, although, as the authors point out, previous studies have been mainly confined to one type of cell. In addition to height reduction, leaf area of seedlings is also markedly reduced and this affects seedling vigour. The latter term includes, amongst other things, the ability of seedlings to become established, especially under non-ideal conditions. Thus, the reduction in vigour in these dwarf lines provides a good reason to investigate other dwarfing genes, such as Rht8. In lines carrying this gene, the dwarfing may be overcome by supplying gibberellin and thus Rht8 is known as a gibberellin-sensitive dwarfing gene. The authors have carried out an extensive analysis (a) of two parental lines (a semi-dwarf line carrying Rht8 and a tall and very vigorous line carrying rht8) and (b) of doubled-haploid lines (DHLs) carrying either Rht8 (and therefore semi-dwarf) or rht8 (and therefore tall). The study included  measurement of the size and number of four epidermal cell types, the contribution of the cell types to leaf area, and estimates of seedling vigour. From this complex analysis, it is clear that in Rht8 dwarf lines, cell size and other cell characteristics (and hence leaf area) and seedling vigour segregate independently of the dwarfing trait, in contrast to the gibberellin-insensitive Rht1 and Rht2 lines. So, as the authors conclude, there is now the opportunity to select dwarf wheat lines without sacrificing leaf area or seedling vigour.