The Influence of Arginine-specific Reagents on Nitrate Uptake by Corn Seedlings

Abstract

The effects of three arginine-specific reagents on NO3 uptake were studied using corn seedlings (Zea mays L., Golden Cross Bantam). In the presence of borate, 0-25 mM 2,3-butanedione (BD) and 10 mM 1,2-cyclohexanedione (CHD) inhibited NO3 uptake by 76% and 68%, respectively, compared to the controls. However, in the absence of borate, only 18% and 38% inhibition was observed for 0-25 mM BD and 1-0 mM CHD, respectively. Similarly, 0-5 mM phenylglyoxal (PGO) resulted in 75% inhibition. The degree of inhibition of nitrate uptake exhibited a concentration-dependence with respect to the reagents. Corn seedlings are 2- or 3-fold more sensitive to BD than to PGO and CHD, respectively, presumably due to the unfavourable steric effects of the benzal ring. Uptake of NO3 was partially restored after removal of BD, CHD, and PGO from the uptake medium. No significant differences were observed for the ATPase and plasma membrane-associated vanadate-sensitive H+-ATPase or K+-stimulated ATPase activity in homogenates and microsomal fractions prepared from corn seedlings which had been incubated for 2 h in the presence or absence of 0-5 mM BD or 10 mM PGO. This suggests that inhibition of nitrate uptake by the arginine specific reagents was not caused by the indirect effect of their binding and inhibiting H+-ATPase. The fact that the arginine specific reagents strongly inhibit NOi uptake indicates that the NO^ transport system has arginine residues at or near the active site.