Phosphate transport by proteoid roots of Hakea sericea

Up to now the higher capacity of proteoid roots to absorb inorganic phosphate from Pi-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 Pi uptake. Here we report a characterization of the Pi 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-Pi also developed proteoid roots, and the proteoid clusters presented a major role in Pi absorption in comparison to the non-proteoid portions of the root system as revealed by their higher 32Pi labeling. The 32Pi uptake by proteoid roots was dependent on H+ gradient and yielded a biphasic kinetics, suggesting the involvement of H+/Pi co-transport systems with Km values of 0.225 and 40.8 μM Pi. The analogs phosphite (Phi) and arsenate, but not vanadate, inhibited competitively the Pi absorption. Such biphasic Pi 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.