Do cyanogenic glucosides help sorghum manage a fluctuating nitrogen supply?

Abstract

Sorghum (Sorghum bicolor [L.] Moench) is an important forage crop that contains the cyanogenic glucoside dhurrin that releases hydrogen cyanide when tissue is damaged. The acyanogenic (dhurrin-free) sorghum line tcd1 was developed to eliminate the risk of cyanide poisoning from sorghum forage. However, dhurrin may also play a role in nitrogen accumulation and storage. We tested whether dhurrin offers the cyanogenic sorghum line BTx623 a growth advantage relative to tcd1 , when nitrogen is limiting and variable. BTx623 and tcd1 were grown under two 42-day nitrogen treatments: high dose, low frequency ('surge') and low dose, high frequency ('pulse'). BTx623 exhibited no growth advantage or disadvantage compared to tcd1 under either treatment. Young BTx623 plants had high concentrations of dhurrin for defence but rapidly recycled this during nitrogen deficiency under the surge treatment, demonstrating dhurrin's role in both defence and nitrogen storage. At later stages, surge plants appeared to accumulate influxes of nitrogen in nitrate and amino acids but not dhurrin. There was evidence of gene expression promoting increased biosynthesis and reduced recycling of dhurrin following surge nitrogen applications but not pulse applications. These results deepen our understanding of dhurrin's role in nitrogen metabolism and demonstrate tcd1 's potential as a safe forage.