Physiological Aspects of Cluster Root Function and Development in Phosphorus-deficient White Lupin (Lupinus albus L.)

Abstract

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 sufficient plants, and inhibitory effects 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 deficiency-induced adaptations of white lupin, involved in P acquisition and mobilization of sparingly available P sources, are predominantly confined 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-deficient 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 deficiency-induced 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 (isoflavonoids) as well as the induction of a putative high-affinity 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-specific gene expression was constructed to assist in the identification of further genes involved in cluster root development.