Plant Physiol, August 2002, Vol. 129, pp. 1763-1772

Inhibition of Phosphate Uptake in Corn Roots by Aluminum-Fluoride Complexes¹

Laboratório de Biologia Celular e Tecidual and Laboratório de Fisiologia e Bioquímica de Microrganismos, Centro de Biociências e Biotecnologia, Universidade Estadual do Norte Fluminense, Campos dos Goytacazes-RJ, CEP 28015-620, Brazil

F forms stable complexes with Al at conditions found in the soil. Fluoroaluminate complexes (AlF_x) have been widely described as effective analogs of inorganic phosphate (Pi) in Pi-binding sites of several proteins. In this work, we explored the possibility that the phytotoxicity of AlF_x reflects their activity as Pi analogs. For this purpose, ³²P-labeled phosphate uptake by excised roots and plasma membrane H⁺-ATPase activity were investigated in an Al-tolerant variety of maize (Zea mays L. var. dwarf hybrid), either treated or not with AlF_x. In vitro, AlF_x competitively inhibited the rate of root phosphate uptake as well as the H⁺-ATPase activity. Conversely, pretreatment of seedlings with AlF_x in vivo promoted no effect on the H⁺-ATPase activity, whereas a biphasic effect on Pi uptake by roots was observed. Although the initial rate of phosphate uptake by roots was inhibited by AlF_x pretreatment, this situation changed over the following minutes as the rate of uptake increased and a pronounced stimulation in subsequent ³²Pi uptake was observed. This kinetic behavior suggests a reversible and competitive inhibition of the phosphate transporter by fluoroaluminates. The stimulation of root ³²Pi uptake induced by AlF_x pretreatment was tentatively interpreted as a phosphate starvation response. This report places AlF₃ and AlF₄ among Al-phytotoxic species and suggests a mechanism of action where the accumulation of Pi-mimicking fluoroaluminates in the soil may affect the phosphate absorption by plants. The biochemical, physiological, and environmental significance of these findings is discussed.