Counting the costs: Nitrogen partitioning in Sorghum mutants

Long-standing growth/defence theories state that the production of defence compounds come at a direct cost to primary metabolism when resources are limited. However, such trade-offs are inherently difficult to quantify.We compared the growth and nitrogen partitioning in wild type Sorghum bicolor (L.) Moench, which contains the cyanogenic glucoside dhurrin, with unique mutants that vary in dhurrin production. The totally cyanide deficient 1 (tcd1) mutants do not synthesise dhurrin at all whereas mutants from the adult cyanide deficient class 1 (acdc1) have decreasing concentrations as plants age. Sorghum lines were grown at three different concentrations of nitrogen. Growth, chemical analysis, physiological measurements and expression of key genes in biosynthesis and turnover were determined for leaves, stems and roots at four developmental stages. Nitrogen supply, ontogeny, tissue type and genotype were all important determinants of tissue nitrate and dhurrin concentration and turnover. The higher growth of acdc1 plants strongly supports a growth/defence tradeoff. By contrast, tcd1 plants had slower growth early in development, suggesting that dhurrin synthesis and turnover may be beneficial for early seedling growth rather than being a cost. The relatively small trade-off between nitrate and dhurrin suggests these may be independently regulated.