Date of Award
8-2017
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Forestry and Environmental Conservation
Committee Member
Dr. Nishanth Tharayil, Committee Chair
Committee Member
Dr. Paula Agudelo
Committee Member
Prof. Patrick Gerard
Committee Member
Prof. Hong Luo
Abstract
Current management practices overemphasizes on herbicides to manage weeds in crop production systems. However, indiscriminate use of herbicides to manage weeds has resulted in the development of resistance in several weed biotypes. Over-application on glyphosate to manage weeds in cropping system that uses RoundUp® Ready™ trait has resulted in the dominance of glyphosate resistant weeds across cropping systems. Glyphosate resistance is an important, economically unviable and rapidly escalating problem across agricultural production systems. To combat herbicide resistance, current recommendations advocate for changes in chemical and cultural practices of weed control, including rotation of herbicide regimen with herbicides with alternate modes of action, and formulation of cultural practices that would penalize the expression of resistance. Some of the bottlenecks in practicing these approaches are the current lack of knowledge about the weed cellular physiology that ensues resistance expression, the potential metabolic cost associated with this resistance expression, and the occurrence of compensatory pathways that could defray the cost of resistance expression. Adopting an alternate herbicide regimen without an understanding of the cellular physiology of resistance expression would result in the development of herbicide cross resistance in weeds, which would further aggravate the problem. To bridge this knowledge-gap, in this studies, metabolomics approach and complementary biochemical analyses were used to track the changes in cellular metabolism in weed species and biotypes that are resistant and naturally tolerant to glyphosate. In Ipomoea lacunosa, non-targeted metabolic profiling captured the differences in metabolic pool levels in two biotypes (WAS and QUI) with contrasting glyphosate tolerance (GR50 = 151 g ae ha-1 and 59 g ae ha-1). Metabolic profiling followed by pathway topological analysis captured innate metabolic differences (22 significantly different metabolites) between WAS and QUI biotypes. Despite the glyphosate dose being half the GR50 rate, shikimic acid accumulation was observed in both the biotypes. However, regardless of EPSPS inhibition, no changes in aromatic amino abundance was observed in the QUI biotype and WAS biotype, indicating their tolerance to the glyphosate. The results from this study implies that though I. lacunosa is tolerant to glyphosate, glyphosate exposure induces cellular metabolic perturbations. The varying tolerance to glyphosate could thus be due to physiological and metabolic adaptations between the different biotypes. Following through, metabolite and biochemical profiling of a susceptible (S) and resistant (R) biotype of Amaranthus palmeri identified physiological perturbations induced by glyphosate in both the biotypes at 8 and 80 hours after treatment (HAT). Compared to the S-biotype, the R-biotype had a 17 fold resistance to the normal field recommended rate of glyphosate. At 8HAT, shikimic acid accumulation in both S- and R-biotypes in response to glyphosate application indicated that the R-biotype was equally susceptible to glyphosate toxicity. The metabolite pool of glyphosate-treated R-biotype was similar to that of the water-treated (control) S and R-biotype, indicating physiological recovery at 80 HAT. A key finding from this study is that despite being resistant to glyphosate, Palmer amaranth biotypes initially sustained metabolic perturbation from glyphosate. However, what differentiates them from the susceptible biotypes is their ability to recover from the glyphosate induced metabolic disruptions. In response to glyphosate, glyphosate-treated R-biotype had lower reactive oxygen species (ROS) damage, higher ROS scavenging activity, and higher levels of secondary compounds of the shikimate pathway, leading to the finding that elevated anti-oxidant mechanisms in A. palmeri complements the resistance conferred due to increased EPSPS copy number. Furthermore, metabolite dynamics in response to glyphosate application studied using stable isotope resolved metabolomics revealed that despite glyphosate toxicity induced decrease in soluble proteins, a proportional increase in both 14N and 15N amino acids was observed in the susceptible plants. This indicates that following glyphosate treatment, a potential increase in de novo amino acid synthesis, coupled with a lower protein synthesis, and higher protein catabolism is observed in the S-biotype. In contrast, the R-biotype, though affected by glyphosate initially, had higher de novo amino acid synthesis without significant disruptions. Moreover, it is to be noted that although the initial assimilation of inorganic nitrogen to organic forms is less affected in the S-biotype than the R-biotype by glyphosate, amino acid biosynthesis downstream of glutamine is disproportionately disrupted. It is thus concluded that the herbicide-induced amino acid abundance in the S-biotype is contributed to by both protein catabolism, and de novo synthesis of amino acids such as glutamine and asparagine. Due to variability in the genetic makeup of populations, each biotype would exhibit different physiological manifestations when exposed to the same rate of glyphosate. Biochemical and metabolic profiling of five different Palmer amaranth biotypes indicated that both the S- and R-biotypes had comparable innate phytochemical profile and similar abundance in flavonoids and phenolic. However, compared to the S-biotypes, the R-biotypes had innately higher anti-oxidant capacity, and the antioxidant capacity was observed to correlate with the GR50 such that antioxidant capacity increased with increasing GR50. Upon treatment with glyphosate, there were significant alterations in the metabolic pool levels across all biotypes. After glyphosate treatment, the content of total phenolic and flavonoids decrease in S-biotypes, whereas the abundance of these metabolites either remained the same, or increased in the R-biotypes. These results indicate that antioxidant capacity is a complementary function aiding in conferring glyphosate resistance and the phytochemistry and the antioxidant capacity is partly induced after glyphosate application, rather than being constitutively expressed. Overall, these study demonstrates that, across biotypes and species, irrespective of their degree of resistance/tolerance, glyphosate not only perturbs shikimate pathway, but also a multitude of other metabolic pathways that are independent of shikimate pathway (secondary toxic effects) as early as eight hours after treatment. While in the susceptible biotypes these metabolic perturbations result in rapid cellular damage, these metabolic perturbations fail to translate to cellular damage in the resistant biotypes. The results indicate that the resistance of A. palmeri biotypes that were used in these studies partially stems from their ability to rapidly induce the production of phenylpropanoids soon after the glyphosate application. This induction of phytochemicals could quench the reactive molecules that are initially produced during the secondary metabolic perturbations, and would thus complement the glyphosate resistance in Amaranthus biotypes conferred by EPSPS gene amplification.
Recommended Citation
Maroli, Amith Sadananda, "Elucidating the Cellular Physiology Associated with the Herbicide (Glyphosate) Resistance and Tolerance in Agricultural Weeds Using Metabolomics Approach" (2017). All Dissertations. 2016.
https://open.clemson.edu/all_dissertations/2016