THE FUNGICIDE PHOSPHONATE DISRUPTS THE PHOSPHATE-STARVATION RESPONSE IN BRASSICA-NIGRA SEEDLINGS

The development of Brassica nigra seedlings over 20 d of growth was di srupted by the fungicide phosphonate (Phi) in a manner inversely corre lated with nutritional inorganic phosphate (Pi) levels. The growth of Pi-sufficient (1.25 mM Pi) seedlings was suppressed when 10, but not 5 , mM Phi was added to the nutrient medium. In contrast, the fresh weig hts and root:shoot ratios of Pi-limited (0.15 mM) seedlings were signi ficantly reduced;at 1.5 mM Phi, and they progressively declined to abo ut 40% of control values as medium Phi concentration was increased to 10 mM. Intracellular Pi levels generally decreased in Phi-treated seed lings, and Phi accumulated in leaves and roots to levels up to 6- and 16-fold that of Pi in Pi-sufficient and Pi-limited plants, respectivel y. Extractable activities of the Pi-starvation-inducible enzymes phosp hoenolpyruvate phosphatase and inorganic pyrophosphate-dependent phosp hofructokinase were unaltered in Pi-sufficient seedlings grown on 5 or 10 mM Phi. However, when Pi-limited seedlings were grown on 1.5 to 10 mM PHi (a) the induction of phosphoenolpyruvate phosphatase and inorg anic pyrophosphate-dependent phosphofructokinase activities by Pi limi tation was reduced by 40 to 90%, whereas (b) soluble protein concentra tions and the activities of the ATP-dependent phosphofructokinase and pyruvate kinase were unaffected. It is concluded that Phi specifically interrupts processes involved in regulation of the Pi-starvation resp onse in B. nigra.