Proteoid root development of phosphorus deficient lupin is mimicked by auxin and phosphonate

White lupin (Lupinus albus L.) develops proteoid (cluster) roots in respons e to phosphorus deficiency. Proteoid roots are composed of tight clusters o f rootlets that initiate from the pericycle opposite protoxylem poles and e merge from every protoxylem pole within the proteoid root axis. Auxins are required for lateral root development, but little is known of their role in proteoid root formation. Proteoid root numbers were dramatically increased in P-sufficient (+P) plants by application of the synthetic auxin, naphtha lene acetic acid (NAA), to leaves, and were reduced in P-deficient (-P) pla nts by the presence of auxin transport inhibitors [2,3,5-triiodobenzoic aci d (TIBA) and naphthylphthalamic acid (NPA)]. While ethylene concentrations in the root zone were 1.5-fold higher in -P plants, there was no effect on proteoid root numbers of the ethylene inhibitors aminoethoxyvinylglycine (A VG) and silver thiosulphate. Phosphonate, which interferes with plant perce ption of internal P concentration, dramatically increased the number of pro teoid root segments in +P plants. Activities of phosphoenolpyruvate carboxy lase (PEPC), malate dehydrogenase (MDH) and exuded acid phosphatase in prot eoid root segments were not different from +P controls when NAA was applied to +P lupin plants, bur increased to levels comparable to -P plants in the phosphonate treatment. Addition of TIBA or NPA to -P plants reduced PEPC a nd MDH activity of -P proteoid roots to levels found in SP or -P normal roo t tissues, but did not affect acid phosphatase in root exudates. These resu lts suggest that auxin transport from the shoot plays a role in the formati on of proteoid roots during P deficiency. Auxin-stimulated proteoid root fo rmation is necessary, but not sufficient, to signal the up-regulation of PE PC and MDH in proteoid root segments. In contrast, phosphonate applied to P -sufficient white lupin elicits the full suite of coordinated responses to P deficiency (C) 2000 Annals of Botany Company.