The impact of acetohydroxyacid synthase inhibiting herbicides on symbiotic nitrogen fixation of grain and pasture legumes.

Abstract

Group B herbicides inhibit the acetohydroxyacid synthase (AHAS - also known as acetolactate synthase) enzyme in the pathway of branched chain amino acid synthesis. These herbicides have gained widespread use in Australia, however potential impacts on nitrogen fixation by legumes have not been comprehensively assessed. Group B herbicides recommended for in-crop application to grain and pasture legume species were assessed for impacts on growth, nodulation and nitrogen fixation. Although it was demonstrated that nitrogen fixation can be affected by these herbicides, the range of responses indicated that multiple mechanisms could be responsible. These could include a reduction nitrogen fixation directly coupled to reduced plant growth; more specific and direct disruption of nitrogen fixation related to the inhibition of nodulation; or other mechanisms yet to be defined that could include affects on the rhizobia. To begin to understand these mechanisms, a herbicide tolerant Medicago littoralis cultivar ‘FEH-1’ was compared to Herald. Decreased nodulation, nitrogen fixation and acetylene reduction activity due to herbicide application were primarily related to the susceptibility of the plant to the herbicide. Thus herbicide tolerant legumes have the potential to alleviate suboptimal nitrogen fixation due to group B herbicides. A proteomics study of the response of root tips of model legume Medicago truncatula A17 to flumetsulam and metsulfuron methyl was conducted to identify more specifically the herbicide impacts on plant physiology. An increased abundance of stress response proteins and a decline in the abundance of some metabolic proteins was found, including a reduction in the abundance of glutamine synthetase which is expected to have direct consequences for the regulation of nitrogen fixation. Observations of root morphology revealed changes to root hairs and the development of lateral roots related to the disruption of meristems, with likely consequences for infection and nodule development. The results from this thesis confirm the potential for acetohydroxyacid synthase inhibiting herbicides to reduce nitrogen fixation of legumes. In addition to a general effect on nitrogen fixation via coupling to reduced plant growth, more specific biochemical and morphological mechanisms that disrupt nodulation are plausible.