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

Abstract

Sorghum bicolor (L.) Moench produces the nitrogen-containing natural product dhurrin that provides chemical defense against herbivores and pathogens via the release of toxic hydrogen cyanide gas. Drought can increase dhurrin in shoot tissues to concentrations toxic to livestock. As dhurrin is also a remobilizable store of reduced nitrogen and plays a role in stress mitigation, reductions in dhurrin may come at a cost to plant growth and stress tolerance. Here, we investigated the response to an extended period of water limitation in a unique EMS-mutant adult cyanide deficient class 1 (acdc1) that has a low dhurrin content in the leaves of mature plants. A mutant sibling line was included to assess the impact of unknown background mutations. Plants were grown under three watering regimes using a gravimetric platform, with growth parameters and dhurrin and nitrate concentrations assessed over four successive harvests. Tissue type was an important determinant of dhurrin and nitrate concentrations, with the response to water limitation differing between above and below ground tissues. Water limitation increased dhurrin concentration in the acdc1 shoots to the same extent as in wild-type plants and no growth advantage or disadvantage between the lines was observed. Lower dhurrin concentrations in the acdc1 leaf tissue when fully watered correlated with an increase in nitrate content in the shoot and roots of the mutant. In targeted breeding efforts to down-regulate dhurrin concentration, parallel effects on the level of stored nitrates should be considered in all vegetative tissues of this important forage crop to avoid potential toxic effects.

Regulation of dhurrin pathway gene expression during Sorghum bicolor development

“…In sorghum, such investments are plastic, with increased allocation of resources to dhurrin in plants experiencing stress (e.g. Rosati et al 2019 ; O'Donnell et al 2013 ; Gleadow et al 2016 )), or in the case presented here, transcript level variation guided by the diel cycle (Fig. 5 ).…”

“…While the biosynthetic pathway is well characterised (Halkier and Møller 1989 ; Kahn et al 1997 ; Møller and Conn 1979 ), little is known about the transcriptional regulation of genes in the pathway (Busk and Møller 2002 ). The concentration of dhurrin in sorghum is dependent on the genotype (Emendack et al 2018 ; Hayes et al 2015 ; Burke et al 2013 ), tissue type (O’Donnell et al 2013 ), plant age (Blomstedt et al 2018 ), stage of development (Miller et al 2014 ) and growing conditions (O’Donnell et al 2013 ; Gleadow et al 2016 ; Rosati et al 2019 ). Information on pathway gene expression in organs, tissues and cells during development and under different environmental conditions is needed to provide information for gene regulatory network analysis and to elucidate the signalling pathways that regulate expression of the genes involved in dhurrin synthesis, transport, hydrolysis and re-mobilization.…”

Light‐driven catalysis with engineered enzymes and biomimetic systems

“…5) [83]. Plants synthesize dhurrin as a cyanide-containing defense against pests, and its production pathway has been the focus of research aiming to increase insect resistance in plants that do not naturally produce dhurrin [85,86]. The biosynthesis of dhurrin in Sorghum bicolor is therefore well understood and the genes for encoding the pathway are cloned, making it a viable model system [87].…”

Transcript profiles of wild and domesticated sorghum under water-stressed conditions and the differential impact on dhurrin metabolism

“…The dhurrin content in forage sorghum may be high and upon bio-activation generate a HCN content exceeding the 600-ppm maximum content for safe grazing by cattle (Gleadow et al 2016 ). Humans may inadvertently have selected for cyanogenic plants over non-cyanogenic plants during evolution (Cowan et al 2020 ; Jones 1998 ) either because their improved resistance to herbivores (McKey et al 2010 ; Jones 1998 ) or better nitorgen use efficiency (Myrans et al 2020 ; Rosati et al 2019b ). Dhurrin levels decrease with tissue age in sorghum and the highest biosynthesis and accumulation rate is observed in young sorghum seedlings (Busk and Møller 2002 ; Gleadow and Woodrow 2002b ).…”