Proteoid Root Development of Phosphorus Deficient Lupin is Mimicked by Auxin and Phosphonate

Abstract

White lupin ( Lupinus albus L.) develops proteoid (cluster) roots in response to phosphorus deficiency. Proteoid roots are composed of tight clusters of rootlets that initiate from the pericycle opposite protoxylem poles and emerge 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, naphthalene acetic acid (NAA), to leaves, and were reduced in P-deficient (−P) plants by the presence of auxin transport inhibitors [2,3,5-triiodobenzoic acid (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 aminoethoxyvinvylglycine (AVG) and silver thiosulphate. Phosphonate, which interferes with plant perception of internal P concentration, dramatically increased the number of proteoid root segments in +P plants. Activities of phosphoenolpyruvate carboxylase (PEPC), malate dehydrogenase (MDH) and exuded acid phosphatase in proteoid root segments were not different from +P controls when NAA was applied to +P lupin plants, but increased to levels comparable to −P plants in the phosphonate treatment. Addition of TIBA or NPA to −P plants reduced PEPC and MDH activity of −P proteoid roots to levels found in +P or −P normal root tissues, but did not affect acid phosphatase in root exudates. These results suggest that auxin transport from the shoot plays a role in the formation of proteoid roots during P deficiency. Auxin-stimulated proteoid root formation is necessary, but not sufficient, to signal the up-regulation of PEPC 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 Copyright 2000 Annals of Botany Company