Cercospora beticola Toxins (X. Inhibition of Plasma Membrane H+-ATPase by Beticolin-1) |
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Authors: | Simon-Plas F Gomes E Milat M L Pugin A Blein J P |
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Institution: | Laboratoire de Phytopharmacie, Institut National de la Recherche Agronomique/Universite de Bourgogne UA692, bv 1540, 21034 Dijon Cedex, France. |
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Abstract: | Influxes of 13NH4+ across the root plasmalemma were measured in intact seedlings of Picea glauca (Moench) Voss. Two kinetically distinct uptake systems for NH4+ were identified. In N-deprived plants, a Michaelis-Menten-type high-affinity transport system (HATS) operated in a 2.5 to 350 mu]M range of external NH4+ concentration (NH4 +]o). The Vmax of this HATS was 1.9 to 2.4 mu]mol g-1 h-1, and the Km was 20 to40 mu]M. At NH4+]o from 500 mu]M to 50 mM, a linear low-affinity system (LATS) was apparent. Both HATS and LATS were constitutive. A time-dependence study of NH4+ influx in previously N-deprived seedlings revealed a small transient increase of NH4+ influx after 24 h of exposure to 100 mu]M NH4+]o. This was followed by a decline of influx to a steady-state value after 4 d. In seedlings exposed to 100 mu]M external NO3- concentration for 3 d, the Vmax for NH4+ uptake by HATS was increased approximately 30% compared to that found in N-deprived seedlings, whereas LATS was down-regulated. The present study defines the much higher uptake capacity for NH4+ than for N03- in seedlings of this species. |
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