Effect of cadmium on growth, isoflavonoid and protein accumulation patterns in mycorrhizal roots of Medicago truncatula

Ecosystems are submitted to various abiotic stresses, among which heavy metals represent major industrial pollutants. Cadmium (Cd), that has damaging effects on plant metabolism, occurs in agricultural environments through industrial pollution and human activities, including phosphate fertiliser and sewage sludge applications. Metal availability to plants can be modulated by soil microorganisms, including arbuscular mycorrhizal (AM) fungi. In the present work, Cd effects on the model legume Medicago truncatula inoculated or not with the AM fungus Glomus intraradices have been studied at 3 levels: (1) plant biomass production together with green part chlorophyll quantification and root isoflavonoid accumulation, (2) G. intraradices development inside the roots and (3) root profiles of total proteins. A Cd concentration of 2 ppm caused a reduction of root growth, which was alleviated in colonized plants. Cd application led to an increased accumulation of some isoflavonoids [daidzein, ononin, malonylononin and medicarpin-3-O-(6’-malonylglucoside)] in nonmycorrhizal roots compared to mycorrhizal roots. Metabolic markers of mycorrhization (apocarotenoids) were partly reduced in Cd treated plants. Global changes in root protein accumulation of Cd-stressed M. truncatula during the symbiotic interaction with G. intraradices were monitored by differential protein display (2-dimensional electrophoresis). Cd provoked changes in protein accumulation in root tissues, some of them being reverted by the AM symbiosis. The proteins whose abundance was modified in Cd and/or G. intraradices-treated roots are being identified by mass spectrometry. Results will be discussed in relation to a possible role of AM symbiosis in detoxification and/or resistance mechanisms towards Cd in M. truncatula plants.