FIGURE 1.

Roles of auxins and cytokinins in different biotic interactions affecting the root system. The schematized root represents different interactions occurring on root systems from different plant species: ectomycorrhizae, formed mainly on tree roots; arbuscular endomycorrhyzae, occurring in most land plants; symbiotic nodulation in legumes (≪rhizobia≫) or actinorhizal plants (≪Frankia≫); galls formed in roots of some Brassicaceae plants. The effect of symbionts and oomycete pathogens on lateral root development is also depicted. Auxin metabolism and/or response is positively associated to ECM, AM, rhizobia and Frankia symbiotic interactions (either in relation to infections and/or to nodule organogenesis), to the interaction with several root pathogens (acting either positively or negatively on pathogenic susceptibility), and positively associated to root gall formation in response to P. brassicae. Auxins produced by different rhizospheric microorganisms stimulate lateral root formation. Cytokinin metabolism and/or response is associated to rhizobia nodulation, either positively in the cortex or negatively in the epidermis, to the interaction with several root pathogens (acting mostly positively on pathogenic susceptibility), and positively associated to root gall formation in response to P. brassicae. +, indicates a positive effect; -, a negative effect. Dark green: root hairs and epidermis; pale green, cortex; middle green, nodule primordium; blue, endodermis; yellow, pericycle; pink, stele tissues including vascular bundles.