PLANT PHYSIOLOGY , Vol 110, Issue 1 105-110, Copyright © 1996 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
The Fungicide Phosphonate Disrupts the Phosphate-Starvation Response in Brassica nigra Seedlings
C. Carswell, B. R. Grant, M. E. Theodorou, J. Harris, J. O. Niere and W. C. Plaxton
Departments of Biology and Biochemisty, Queens University, Kingston, Ontario, Canada K7L 3N6 (C.C., M.E.T., W.C.P.)
The development of Brassica nigra seedlings over 20 d of growth was
disrupted by the fungicide phosphonate (Phi) in a manner inversely
correlated 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 weights and
root:shoot ratios of Pi-limited (0.15 mM) seedlings were significantly
reduced at 1.5 mM Phi, and they progressively declined to about 40% of
control values as medium Phi concentration was increased to 10 mM.
Intracellular Pi levels generally decreased in Phi-treated seedlings, 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, respectively. Extractable
activities of the Pi-starvation-inducible enzymes phosphoenolpyruvate
phosphatase and inorganic pyrophosphate-dependent phosphofructokinase 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 inorganic pyrophosphate-dependent
phosphofructokinase activities by Pi limitation was reduced by 40 to 90%,
whereas (b) soluble protein concentrations and the activities of the
ATP-dependent phosphofructokinase and pyruvate kinase were unaffacted. It
is concluded that Phi specifically interrupts processes involved in
regulation of the Pi-starvation response in B. nigra.
