Stable isotope resolved metabolomics reveals the role of anabolic and catabolic processes in glyphosate-induced amino acid accumulation in Amaranthus palmeri biotypes

Authors: MAROLI, AMITH - Clemson University; Nandula, Vijay; Duke, Stephen; THARAYIL, NISHANTH - Clemson University

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/29/2016
Publication Date: 7/29/2016
Citation: Maroli, A.S., Nandula, V.K., Duke, S.O., Tharayil, N. 2016. Stable isotope resolved metabolomics reveals the role of anabolic and catabolic processes in glyphosate-induced amino acid accumulation in Amaranthus palmeri biotypes. Journal of Agricultural and Food Chemistry. 64:7040-7048.

Interpretive Summary: Since its evolution in 2005, a result of the rapid and widespread adoption of glyphosate-resistant crops associated with the intense use of glyphosate and lack of rotation with non-glyphosate-resistant crops, glyphosate-resistant Palmer amaranth has wreaked havoc across row crop growing regions of the southern US. Extensive energies have been expended towards gaining a better understanding of the biology and prolific nature of this weed. In such efforts, scientists from the Crop Production Systems Research Unit of USDA-ARS, Stoneville, MS, Natural Products Utilization Research Unit of USDA-ARS, Oxford, MS, and Clemson University conducted studies to characterize the role of anabolic (de novo synthesis) vs catabolic (protein catalysis) processes contributing to free amino acid pools in glyphosate susceptible (S) and resistant (R) Palmer amaranth biotypes. Following exposure to glyphosate (0.4 kg ae ha-1), the proportion of 15N amino acids in the total amino acid pool increased in the S-biotype, compared to R-biotype, which indicates an increase in the de novo amino acid synthesis, coupled with a lower protein synthesis rate. However, the efficiency of the enzymes glutamine synthetase/glutamate synthase evaluated as a function of Gln/Glu ratio in the glyphosate treated S and R biotypes were 14 and 1.4 respectively, which indicated that though the initial nitrogen assimilation in glyphosate treated S-biotype is less affected, amino acids biosynthesis downstream of glutamine is disproportionately disrupted. It is thus concluded that the increase in total amino acid pool in the S biotype following herbicide stress, though partly contributed by protein catabolism, is also contributed through the de novo synthesis of N-donating amino acids such as glutamine and asparagine. The above results add extremely valuable insights that could potentially be exploited in controlling troublesome weeds such as Palmer amaranth.

Technical Abstract: Using stable isotope resolved metabolomics (SIRM), we characterized the role of anabolic (de novo synthesis) vs catabolic (protein catalysis) processes contributing to free amino acid pools in glyphosate susceptible (S) and resistant (R) Amaranthus palmeri biotypes. Following exposure to glyphosate (0.4 kg ae ha-1), the proportion of 15N amino acids in the total amino acid pool increased in the S-biotype, compared to R-biotype, which indicates an increase in the de novo amino acid synthesis, coupled with a lower protein synthesis rate. However, the efficiency of the GS/GOGAT cycle evaluated as a function of Gln/Glu ratio in the glyphosate treated S and R biotypes were 14 and 1.4 respectively, which indicated that though the initial nitrogen assimilation in glyphosate treated S-biotype is less affected, amino acids biosynthesis downstream of glutamine is disproportionately disrupted. It is thus concluded that the increase in total amino acid pool in the S biotype following herbicide stress, though partly contributed by protein catabolism, is also contributed through the de novo synthesis of N-donating amino acids such as glutamine and asparagine.