| Abstract: | The polypeptide composition of the membranes from corn (Zeamays L.) seedling roots upon nitrate induction was determinedby two-dimensional gel electrophoresis and silver-staining.The synthesis of five polypeptides (49, 48, 35, 33, and 32 kDa)in the tono-plast fraction and four polypeptides (50, 49, 38,and 33 kDa) in the plasma membrane fraction was induced by both2.5 mM Ca(NO3)2 and 5 mM KNO3. Extensive washing of the membraneswith salt and NaOH demonstrated that three induced polypeptides(49, 48, and 35 kDa) in the tonoplast fraction and two inducedpolypeptides (49 and 33 kDa) in the plasma membrane fractionwere integral proteins. After incubation of seedlings in N-freemedium for 4 d, the 49 and 32 kDa polypeptides in the tonoplastfraction had disappeared. By the sixth day in N-free medium,the 35 kDa polypeptide had disappeared from the tonoplast fraction.The 50 kDa polypeptide of the plasma membrane fraction was nolonger detectable in seedlings incubated for 6 d in N-free medium.The size of the spots corresponding to the 33 kDa polypeptidesof both membrane fractions and to the 49 kDa polypeptide ofthe plasma membrane fraction was reduced following incubationof seedlings in N-free medium. The changes in nitrate-inducedpolypeptides in both membrane fractions following transfer toN-free medium correlated with a reduced capacity to take upnitrate in the treated seedlings. The results support the conclusionthat the nitrate-induced polypeptides may be involved in nitratetransport across the tonoplast and plasma membrane. Key words: Nitrate transport, induction, membrane peptides |
