The Chl1 Arabidopsis mutant impaired in the nitrate-inducible NO3 transporter has acidic pH in the absence of nitrate.

Seedlings of wt and chl1, an Arabidopsis thaliana mutant impaired in nitrate transport, were grown in a nitrate-free medium, and a comparison was made between the values of cell sap pH, cytoplasmic pH and malate content detected in the two strains. In this condition, in which the presence of the mutation should not have any influence, cytoplasmic and cell sap pH of wt were higher than those of chl1 either when measured at the end of the growth period or after treatments in different nitrate-free media. The low intracellular pH detected in chl1 was not apparently due to a reduced activity of the plasmalemma H+-ATPase, since the cytoplasmic alkalinization following the activation of the H+ pump by K+ was larger in chl1 than in wt, the amplitude of the resulting pH change substantially depending on the actual value of cytosolic pH at the beginning of the treatment with K+ for both strains. The intracellular pH differences detected between wt and chl1 show that this mutant, defined as impaired in the nitrate-inducible NO3- transporter, exhibits phenotypic expression in the absence of nitrate too, thus suggesting that some other system, possibly involved in intracellular pH regulation, might be indirectly influenced by the mutation.