Highly efficient Pi uptake enables Hakea sericea to invade nutrient-poor soils

Abstract

Hakea sericea Schrad., a Proteaceae native from South-eastern Australia can now be found naturalized in several countries, such as Portugal and South Africa. Well adapted to the Australian impoverished soils due to the formation of proteoid roots, and relying on several mechanisms to cope with forest fires, it rapidly spread throughout the mountain areas of North-western Portugal, where very low concentrations of available P and N were found (3.1 and 14 mg kg-1, respectively). Although several authors have pinpointed the ability of typical proteoid roots to exude organic acids in order to solubilize precipitated forms of Pi, here we show that plant survival relies also on the enhancement of Pi uptake capacity. Kinetic studies performed in non-excised proteoid roots and in proteoid root segments have shown a complex model of Pi absorption: two H+-dependent transport systems with high- (Km, 6 µM Pi) and low-affinity (Km, 100 µM Pi) plus a non-saturating "diffusion-like" component (kd, 0.02 L h-1 g-1 FW) were found. The high-affinity system appears to be of major physiological importance to plant survival in such P-poor soil. When Pi uptake was measured from pH 4.5 to 6.5, analysis of Km variation suggested that H2PO4- is the preferred transported form by both the high- and the low-affinity Pi transport systems. This anionic Pi form should be the most readily available, taking into account the acidic character of the soil (pH 5.4). Plants grown in a Pi-poor nutrient solution were able to produce proteoid roots that exhibited Pi absorption characteristics similar to those of wild-grown plants. The rates of Pi uptake in proteoid roots of H. sericea were compared with those reported for proteoid- and non-proteoid-root forming plants. It is discussed that the high Pi transport capacity in H. sericea proteoid roots can account for the occurrence of P toxicity, a characteristic observed in many Proteaceae, which may allow the design of alternative strategies to control the invader.