Hakea sericea Schrad. - a model to study phosphate uptake in proteoid roots

Abstract

P is an important macronutrient, making up about 0.2% of a plant dry weight. Pi is the form of P most readily accessed by plants, the concentration of which rarely exceeds 10 µM in soil. Here we studied the mechanisms involved on Pi uptake from soil in Hakea sericea Schrad. (Proteacea), an Australian invader of natural habitats, able to develop proteoid roots as a response to P deficiency. H. sericea proteoid roots have efficient transporters for extraction of Pi from soil. Uptake studies supported the involvement of two Pi transport systems with the following kinetic parameters: for the high-affinity system Km, 6 µM Pi and Vmax, 5 µmolh-1g-1 FW; for the low-affinity system Km, 100 µM Pi and Vmax, 24 µmolh-1g-1 FW. The protonophore CCCP (50 µM) inhibited up to 60% the initial uptake rates of 5-50 µM Pi (high-affinity range) and 100-500 µM (low-affinity range), suggesting H+-dependent transport. Both transporters were competitively inhibited by phosphite and 100 µM mersalyl reduced by 50% the initial uptake rates of 10 µM Pi. PCR using degenerate primers designed for the conserved regions of Pi transporter genes from higher plants allowed the amplification of two 437 bp fragments sharing 77,4% identity with each other. Both nucleotide sequences comprise an incomplete ORF coding for transmembrane domains and exhibit homology with other plant Pi transporters. These nucleotide sequences are being used as probes in the identification of complete sequences of Pi transporter genes in H. sericea genome.