Global Metabolome of Palmer Amaranth (Amaranthus palmeri) Populations Highlights the Specificity and Inducibility of Phytochemical Responses to Abiotic Stress

Commonalities in adaptive responses to abiotic stressors
could
contribute to the development of cross-resistance in weeds. The degree
to which herbicide-induced changes in weeds parallel those induced
by other abiotic stress remains unknown. We investigated the specificity
of metabolic perturbations induced by glyphosate and drought across
three glyphosate-resistant (GR) and two glyphosate-susceptible (GS)
biotypes of Palmer amaranth (Amaranthus palmeri) using global metabolomics approaches. Compared to GS-biotypes,
in the absence of stress, the GR-biotypes had a higher abundance of
primary metabolites, including sugars, nonaromatic amino acids, and
organic acids. However, despite having a higher 5-enolpyruvylshikimate-3-phosphate
synthase (EPSPS) gene copy number that could upregulate the phenylpropanoid
metabolism, the nonstressed GR-biotypes were less abundant in specialized
(secondary) metabolites. Under glyphosate stress, 80% of metabolites,
including shikimate, that accumulated in GS-biotypes also increased
in the GR-biotypes. However, glyphosate triggered the preferential
accumulation of glycosides of dihydroxylated and methoxylated flavanols
with higher antioxidant potential, and ferulic acid derivatives, specifically
in GR-biotypes. The disruption of the shikimate pathway and the accumulation
of phenylpropanoids upon glyphosate exposure suggest that the stress
response of GR-biotypes could be partly induced. This differential
response was less evident in other phytochemical classes and under
drought, highlighting that the phytochemical responses are stress-specific
rather than biotype-specific.