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Proteoid roots are bottlebrush-like clus-;ers of rootlets which form along lateral roots. They are :haracteristic of most species of the Proteaceae, which ire mainly distributed in Australia and South Africa. Homologous root clusters are present in species of the Casuarinaceae, Mimosaceae, Fabaceae, Myricaceae and Moraceae. Many similarities exist between these species in relation to morphology and function of root clusters. Many are non-mycorrhizal and are highly efficient in phosphorus (P) acquisition. In these species, proteoid roots and proteoid-like root clusters are abundant when grown on infertile soils. Their formation is predominantly affected by the P status of the plants, being induced at low P levels and repressed at high P levels. Proteoid roots and proteoid-like root clusters play an important role in acquisition of P and other mineral nutrients. Although increase in root surface area may be a contributing factor, in many species these roots excrete large amounts of organic acids and phenolics. The excretion of these compounds in a small soil volume gives rise to extensive nutrient mobilization by acidification, reduction and chelation of sparingly soluble forms of P and micronutrients such as Fe and Mn.

Cluster root formation in white lupin (Lupinus albus L.) is induced mainly by phosphorus (P) starvation, and seems to be regulated by the endogenous P status of the plant. Increased formation of cluster roots, when indole acetic acid is supplied to the growth medium of P sucient plants, and inhibitory eects of kinetin application suggest the involvement of endogenous phytohormones (auxins and cytokinins), which may act in an antagonistic manner in the P-starvation response. Phosphorus de®ciency-induced adaptations of white lupin, involved in P acquisition and mobilization of sparingly available P sources, are predominantly con®ned to the cluster roots, and moreover to distinct stages during their development. Increased accumulation and exudation of citrate and a concomitant release of protons were found to be mainly restricted to mature root clusters after prolonged culture (3±4 weeks) under P-de®cient conditions. Inhibition of citrate exudation by exogenous application of anion channel antagonists such as ethacrynic-and anthracene-9-carboxylic acids may indicate involvement of an anion channel. Phosphorus de®ciencyinduced accumulation and subsequent exudation of citric acid seems to be a consequence of both enhanced biosynthesis and reduced turnover of citric acid in the cluster root tissue, indicated by enhanced expression of sucrose synthase, fructokinase, phosphoglucomutase, phosphoenol-pyruvate carboxylase, but reduced activity of aconitase and slower root respiration. The release of acid phosphatase and of phenolic compounds (iso¯avonoids) as well as the induction of a putative high-anity P uptake system was more highly expressed in juvenile, mature and even senescent cluster regions than in apical zones of non-proteoid roots. An AFLP-cDNA library for cluster root-speci®c gene expression was constructed to assist in the identi®cation of further genes involved in cluster root development.