Modulation of NO3- uptake by water-extractable humic substances: involvement of root plasma membrane H(+)ATPase
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The effect of a water extractable humic substances fraction (WEHS) on nitrate uptake and plasma membrane (pm) H+-ATPase activity of maize roots was investigated. Four days old maize root seedlings were exposed for 4 to 24 h to a nutrient solution containing 200 μM nitrate in the absence or presence of 5 mg org. C · L-1 WEHS. Plants exposed to nitrate developed a higher capacity to absorb the anion (induction): the net uptake rate progressively increased up to 12 h of contact with the solution; thereafter, a decline was observed. When WEHS was present together with nitrate in the nutrient solution, the induction of nitrate uptake was evident and maximal already 4 h after starting the treatment. The rate of net nitrate uptake decreased only slightly during the remaining period (4-24 h). Stimulation of net nitrate uptake rate was also observed when WEHS was added to a nitrogen- or nitrate-free nutrient solution or to a 5 mM CaSO4 solution. The activity of pmH+-ATPase raised upon exposure of the roots to nitrate with the same pattern observed for nitrate uptake. The contemporary presence of nitrate and WEHS caused a further stimulation of the pmH+-ATPase activity after 4 h treatment. An increase in the enzyme activity was also observed when plants were treated for 4 h in the presence of WEHS in CaSO4, nitrogen- or nitrate-free solutions. However, when nitrate was present the enhancement was even greater. Results support the idea that the plasma membrane proton pump might be one of the primary targets of the action of humic substances on plant nutrient acquisition. A role of WEHS in the modulation of nitrate uptake via an interaction with the pm H+-ATPase is also discussed.