Studying the cell wall of Medicago sativa stems in response to long-term cadmium exposure

Abstract

Environmental pollution with heavy metals is a major issue of the recent decade. Especially cadmium (Cd) is of great concern due to its persistence and high toxicity even in trace amounts. Plants have developed several mechanisms to counteract exposure to abiotic stress such as Cd and existing data suggest that the cell wall plays an important role in the defence strategy of plants during heavy metal exposure. The experimental set-up of the present thesis particularly focused on long-term Cd exposure of Medicago sativa L. using realistic soil concentrations (10 mg Cd kg-1 soil dry weight (DW)), which makes the study relevant from an agricultural and societal point of view. Thereby, the influence on the cell wall structure, cell wall- and soluble proteins as well as plant metabolism in stems of M. sativa was investigated and results were complemented with targeted gene expression analyses. Cadmium exposure altered the abundance of cell wall proteins involved in multiple physiological processes such as defence response and oxidationreduction processes, but also carbohydrate metabolism and cell wall remodelling. Especially the abundance and enzymatic activity of pectin methylesterase increased in response to Cd exposure, which influenced the pectin pattern and promotes the creation of Cd binding sites as well as its sequestration in the cell wall. Observed changes in the polysaccharide composition support the hypothesis that Cd exposure initiates alterations in the cell wall structure, which establish the cell wall as a physical barrier against the external threat. Additionally, Cd exposure stimulated the phenylpropanoid pathway towards the biosynthesis of isoflavonoids rather than lignin and led to an accumulation of isoflavone conjugates. Although a growth impairment of juvenile plants was observed, Cd exposure to 10 mg Cd kg-1 soil DW did neither affect the biomass production of mature M. sativa plants nor its physiological performance. During long-term exposure to realistic Cd concentrations M. sativa seemed to readjust its primary and secondary metabolite profile and established a new metabolic steady-state, which conveyed acclimation to the applied stress. This said, M. sativa would be a considerable candidate for phytoremediation attempts to clean-up contaminated sites.