Cluster roots--an underground adaptation for survival in extreme environments.

Cluster roots are a characteristic of members of the Proteaceae and of several other plant species that are adapted to habitats of extremely low soil fertility, usually without formation of mycorrhizal associations. Functionally linked with intense mobilization of nutrients (P, Fe, Zn, Mn) by root-induced chemical changes (pH, root exudates, redox potential) in the rhizosphere, cluster-rooted plant species can serve as model plants to study rhizosphere processes and regulatory aspects of plant adaptations for chemical mobilization of nutrients in the rhizosphere.