Physiological aspects of cluster root function and development in phosphorus-deficient white lupin (Lupinus albus L.)

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 ace tic acid is supplied to the growth medium of P sufficient plants, and inhib itory effects of kinetin application suggest the involvement of endogenous phytohormones (auxins and cytokinins), which may act in an antagonistic man ner in the P-starvation response. Phosphorus deficiency-induced adaptations of white lupin involved in P acquisition and mobilization of sparingly ava ilable P sources, are predominantly confined to the cluster roots, and more over to distinct stages during their development. Increased accumulation an d exudation of citrate and a concomitant release of protons were found to b e mainly restricted to mature root clusters after prolonged culture (3-4 we eks) under P-deficient conditions. Inhibition of citrate exudation by exoge nous application of anion channel antagonists such as ethacrynic- and anthr acene-9-carboxylic acids may indicate involvement of an anion channel. Phos phorus deficiency-induced accumulation and subsequent exudation of citric a cid seems to be a consequence of both enhanced biosynthesis and reduced tur nover of citric acid in the cluster root tissue, indicated by enhanced expr ession of sucrose synthase, fructokinase, phosphoglucomutase, phosphoenol-p yruvate carboxylase, but reduced activity of aconitase and slower root resp iration. The release of acid phosphatase and of phenolic compounds (isoflav onoids) as well as the induction of a putative high-affinity P uptake syste m was more highly expressed in juvenile, mature and even senescent cluster regions than in apical zones of non-proteoid roots. An AFLP-cDNA library fo r cluster root-specific gene expression was constructed to assist in the id entification of further genes involved in cluster root development. (C) 200 0 Annals of Botany Company.