Phosphate transport by proteoid roots of Hakea sericea Manuel Fernando Sousa a , Arnoldo Rocha Fac ¸anha b , Rui Manuel Tavares a , Teresa Lino-Neto a , Herna ˆni Gero ´s a, * a Centro de Biologia, Departamento de Biologia, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal b Centro de Biocie ˆncias e Biotecnologia, Universidade Estadual do Norte Fluminense Darcy Ribeiro, Campos dos Goytacazes-RJ, CEP28013-600, Brazil Received 7 May 2007; received in revised form 4 August 2007; accepted 17 August 2007 Available online 31 August 2007 Abstract Up to now the higher capacity of proteoid roots to absorb inorganic phosphate from P i -poor soils has been related mainly to their increased root surface area and higher exudation of organic acids and phosphatases, while much less attention has been directed to their mechanisms of P i uptake. Here we report a characterization of the P i uptake kinetics of the proteoid root-forming species Hakea sericea Schrad. This Proteaceae is an Australian native, which is disseminating very fast through forests of the European south. Dense mats of proteoid roots were observed in the upper soil layers of the invaded area in Portugal where availability of P and N was shown to be very low. Plants grown hydroponically under low-P i also developed proteoid roots, and the proteoid clusters presented a major role in P i absorption in comparison to the non-proteoid portions of the root system as revealed by their higher 32 P i labeling. The 32 P i uptake by proteoid roots was dependent on H + gradient and yielded a biphasic kinetics, suggesting the involvement of H + /P i co-transport systems with K m values of 0.225 and 40.8 mMP i . The analogs phosphite (Phi) and arsenate, but not vanadate, inhibited competitively the P i absorption. Such biphasic P i uptake pattern with the highest affinity at submicromolar range is likely to be of critical importance for the capacity of this plant species to invade and proliferate throughout vast areas of nutrient-deprived soils. # 2007 Elsevier Ireland Ltd. All rights reserved. Keywords: Hakea sericea; Invasive species; Phosphate uptake; Phosphorus-poor soils; Proteaceae; Proteoid roots 1. Introduction Phosphorus (P) is an important plant macronutrient, making up about 0.2% of plant dry weight. P deficiency limits plant growth more frequently than any other nutrients except nitrogen [1–3]. Although total soil P content typically varies from 500 to 2000 mg kg 1 , total bioavailable P may be only a few mg kg 1 [2]. The form of P most readily accessed by plants is P i , the concentration of which rarely exceeds 10 mM in soil solutions [4]. The P i level in soil solution is regulated mainly by its interaction with organic or inorganic surfaces in the soil. Aluminium and iron ions in acid soils, and calcium ions in alkaline soils, interact strongly with P i and render it unavailable to plants. Consequently, plants have developed numerous morphological, physiological, biochemical and molecular adaptations to acquire P i [2,3]. Proteoid roots (or cluster roots) are considered, along with mycorrhizas and nitrogen-fixing nodules, to be one of the major adaptations to enhance nutrient acquisition [5]. Along a proteoid root, discrete clusters of closely spaced rootlets develop. The rootlets emerge in continuous rows from the cortex and are covered with root hairs, increasing the absorption surface area [6]. Species with proteoid roots can grow in soils with poorly available nutrients, and their induction has been reported mainly in response to low availability of P and Fe [7]. Until now the ability of proteoid roots in improving P i mobilization from soil has been related mainly to the increase of root surface area and exudation of carboxylic acids, acid phosphatases, phenolics, mucilages, and water [6,8,9]. Hakea sericea Schrad. is a Proteaceae native in Southeastern Australia and its ability to produce proteoid roots, together with the production of large number of seeds protected by woody follicles, efficient dispersal of seeds and rapid germination, have enabled the species to become well-adapted to cope with www.elsevier.com/locate/plantsci Plant Science 173 (2007) 550–558 Abbreviations: CCCP, carbonyl cyanide m-chlorophenylhydrazone; Phi, phosphite; TPP + , tetraphenylphosphonium. * Corresponding author. Tel.: +351 253 604048; fax: +351 253 678980. E-mail address: geros@bio.uminho.pt (H. Gero ´s). 0168-9452/$ – see front matter # 2007 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.plantsci.2007.08.006