Glutathione transferases catalyze recycling of auto‐toxic cyanogenic glucosides in sorghum

Abstract

Cyanogenic glucosides are nitrogen-containing specialized metabolites that provide chemical defense against herbivores and pathogens via the release of toxic hydrogen cyanide. It has been suggested that cyanogenic glucosides are also a store of nitrogen that can be remobilized for general metabolism via a previously unknown pathway. Here we reveal a recycling pathway for the cyanogenic glucoside dhurrin in sorghum (Sorghum bicolor) that avoids hydrogen cyanide formation. As demonstrated in vitro, the pathway proceeds via spontaneous formation of a dhurrin-derived glutathione conjugate, which undergoes reductive cleavage by glutathione transferases of the plant-specific lambda class (GSTLs) to produce p-hydroxyphenyl acetonitrile. This is further metabolized to p-hydroxyphenylacetic acid and free ammonia by nitrilases, and then glucosylated to form p-glucosyloxyphenylacetic acid. Two of the four GSTLs in sorghum exhibited high stereospecific catalytic activity towards the glutathione conjugate, and form a subclade in a phylogenetic tree of GSTLs in higher plants. The expression of the corresponding two GSTLs co-localized with expression of the genes encoding the p-hydroxyphenyl acetonitrile-metabolizing nitrilases at the cellular level. The elucidation of this pathway places GSTs as key players in a remarkable scheme for metabolic plasticity allowing plants to reverse the resource flow between general and specialized metabolism in actively growing tissue.

The Structure and Function of Major Plant Metabolite Modifications

“…However, phenylacylation of flavonoids has been reported in species of other families, albeit at a different structural position, for example in Scots pine (Jungblut et al, 1995), and indeed, periodically although relatively uncommonly occurs across the various families of the green lineage (Tohge et al, 2018). Similar advances recently have been made with regard to nitrogen containing compounds such as glucosinolates (Leong and Last, 2017), terpenes (Aharoni et al, 2003;Umehara et al, 2008;Chen et al, 2011), and fatty acid derivatives (Kachroo et al, 2003) in Arabidopsis, as well as compounds such as triterpene saponins (Achnine et al, 2005;Thimmappa et al, 2014), cyanogenic glucosides (Bjarnholt et al, 2018;Sun et al, 2018), acyl sugars (Leong and Last, 2017), and SGAs (Itkin et al, 2013;Schwahn et al, 2014) in other species.…”

Elucidation of the Amygdalin Pathway Reveals the Metabolic Basis of Bitter and Sweet Almonds (Prunus dulcis)

“…This could possibly supplement the activity of PdUGT94AF1 and PdUGT94AF2 with respect to amygdalin formation in specific cell types at some stage of cotyledon ontogeny. Alternatively, it could serve to trigger the initiation of endogenous recycling processes of prunasin (Pičmanová et al, 2015;Bjarnholt et al, 2018) or prunasin hydrolysis.…”

“…The genes encoding the enzymes required for the biosynthesis of prunasin and amygdalin are present in the genomes of both sweet and bitter almonds (Sánchez-Pérez et al, 2008). The key to unraveling the molecular basis for the sweet kernel trait, therefore, may be linked to a lack of expression of the biosynthetic genes or the complete endogenous remobilization of the amygdalin as a source of sugar and reduced nitrogen during seed development (Pičmanová et al, 2015;Bjarnholt et al, 2018). The expression analyses of the biosynthetic genes presented here documents that it is the lack of expression of PdCYP79D16 in the sweet genotype, echoed by the strongly reduced expression of PdCYP71AN24, that results in the sweet kernel trait (Fig.…”

Prediction of Dhurrin Metabolism by Transcriptome and Metabolome Analyses in Sorghum

“…Afterward, nitrilase complexes ( Sb NIT4B2/A or Sb NIT4B1/A) produce ammonia (NH 3 ) and L-aspartic acid, and/or L-asparagine from β -cyanoalanine [ 15 ]. The second is a non-toxic catabolic recycling pathway in which unknown enzyme which could be glutathione S-transferase (GST) or BGD catalyze the conversion of dhurrin to p -hydroxyphenylacetonitrile (pHPAN) and glutathione (GSH) is needed to detach a glucose from dhurrin [ 15 , 16 , 17 ]. The glutathionylated-pHPAN (GS-pHPAN) conjugate is converted to pHPAN by GST-like enzymes (GSTL1 or GSTL2) and GSHs [ 17 ].…”