Counting the costs: nitrogen partitioning in Sorghum mutants

Abstract

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.

The Interplay Between Water Limitation, Dhurrin, and Nitrate in the Low-Cyanogenic Sorghum Mutant adult cyanide deficient class 1

“…In stressed plants, where photosynthetic rate is reduced, cyanogenic glucosides may also provide a ready source of nitrogen, remobilized when the stress is alleviated (Selmar et al, 1988; Kongsawadworakul et al, 2009; O’Donnell et al, 2013; Bjarnholt et al, 2018; Schmidt et al, 2018). The cross-over of cyanogenic glucosides for use in primary and secondary metabolism is demonstrated by the negative effects on plant growth at specific developmental stages when they are reduced or removed, as seen in cassava ( Manihot esculenta Crantz) (Jørgensen et al, 2005) and the acyanogenic sorghum line totally cyanide deficient 1 ( tcd1 ) (Blomstedt et al, 2012; Blomstedt et al, 2018).…”

“…Developmental regulation of dhurrin formation in sorghum is confounded by environmental factors such as drought and nitrogen application that can induce higher dhurrin concentrations (O’Donnell et al, 2013; Neilson et al, 2015; Gleadow et al, 2016a; Blomstedt et al, 2018; Emendack et al, 2018). This renders crop toxicity difficult to predict.…”

“…Sorghum also accumulates nitrate, particularly in the sheath tissue (O’Donnell et al, 2013; Blomstedt et al, 2018). Like dhurrin, nitrate concentrations are affected by the environment.…”

“…Drought stress adds to the accumulation of nitrate as the ability to assimilate nitrate into protein is reduced (Sivaramakrishnan et al, 1988). There is conflicting evidence as to whether lower dhurrin concentrations are associated with higher nitrate levels in sorghum (O’Donnell et al, 2013; Neilson et al, 2015; Gleadow et al, 2016a; Blomstedt et al, 2018). Furthermore, whether there is a stoichiometric trade-off in the allocation of nitrogen to dhurrin and nitrate remains unclear.…”

“…When reaching 3 to 4 weeks of age, the acdc1 mutant decreases the dhurrin concentration in its leaf tissue more rapidly than wild-type plants (Blomstedt et al, 2012). By 5 weeks of age, the acdc1 mutant has significantly lower dhurrin concentrations in the leaf tissue in comparison to wild-type plants, and by 8 weeks of age, only negligible concentrations of dhurrin remain in the acdc1 leaf tissue (Blomstedt et al, 2018). No sequence changes in the coding regions of the dhurrin biosynthetic genes ( CYP79A1 , CYP71E1 and UGT85B1 ) are present in acdc1 .…”