Citrate exudation by maize roots; A possible mechanism of resistance to aluminium

Low-molecular-weight organic anions have been intensively studied as possibly involved in a mechanism of Al resistance in plants, due to their involvement in many metabolic processes and to their negative charge, conferring them the capacity to complex metals. The objective of the thesis was to study the root exudation of organic anions as a potential mechanism of Al resistance operating in maize ( Zea mays L.). The effect of Al exposure on root organic anion exudation was studied with roots grown under sterile conditions, with maize genotypes that differ in sensitivity to Al. Citrate accounted for the majority of the organic anions exuded, followed by malate, trans -aconitate, fumarate, and cis -aconitate. Along the longitudinal axis of fully developed seminal roots citrate was exuded mainly in the regions of root apices, either belonging to the main root or to the lateral roots in the most basal part of the main root. Rates of citrate exudation from root apices of eight genotypes exposed to Al correlated significantly well with their relative Al resistance; a less inhibited root elongation accompanied higher exudation rates.The effects of Al on nutrient uptake were also studied in the thesis. The spatial localisation of nutrient uptake on the root axis of maize seedlings was assessed and revealed that Al is affecting nutrient uptake widely along the longitudinal axis of the root. Compared with the pattern of citrate exudation along similar axes it seems that citrate is probably primarily involved in making plants resistant to Al by detoxifying Al around the root meristems, the most sensitive part for root growth. Local citrate exudation does not seem to be directly involved in nutrient uptake, because the segment with the highest citrate exudation (the apex) shows almost no nutrient uptake, while the root zone with the highest nutrient uptake shows almost zero citrate exudation.The question whether quantitatively the amounts of these small ligands released in the root environment are adequate to explain resistance to Al was tackled by combining experimental and modelling work. The results of the simulations strongly support the notion that citrate indeed can underlie Al resistance in maize. For the conditions considered in this study, detoxification of apoplastic Al and protection of this compartment seem more realistic and more important than those in the interface root-outer solution.