Plant Physiology 93:642-647 (1990)
© 1990 American Society of Plant Biologists
Environmental and Stress Physiology
Induction of Nitrate Transport in Maize Roots, and Kinetics of Influx, Measured with Nitrogen-13 1
David J. Hole,
Ali M. Emran,
Youhanna Fares and
Malcolm C. Drew
Department of Horticultural Sciences, Texas A&M University, College Station, Texas 77843-2133,
Positron Diagnostic and Research Center, University of Texas Health Science Center at Houston, Texas 77030,
Biosystec Inc., Neal Pickett Dr., College Station, Texas 77840
Unlike phosphate or potassium transport, uptake of nitrate by roots is induced, in part, by contact with the substrate ion. Plasmalemma influx of 13N-labeled nitrate in maize roots was studied in relation to induction of the uptake system, and the influence of short-term N starvation. Maize (Zea mays) roots not previously exposed to nitrate had a constitutive transport system (state 1), but influx increased 250% during six hours of contact with 100 micromolar nitrate, by which time the transport mechanism appeared to be fully synthesized (state 2). A three-day period of N starvation prior to induction and measurement of nitrate influx resulted in a greater capacity to transport nitrate than in unstarved controls, but this was fully expressed only if roots were kept in contact with nitrate for the six hours needed for full induction (state 2E). A kinetic analysis indicated a 160% increase in maximum influx in N-starved, induced roots with a small decrease in Km. The inducible component to nitrate influx was induced only by contact with nitrate. Full expression of the nitrate inducible transport system was dependent upon mRNA synthesis. An inhibitor of cytoplasmic protein synthesis (cycloheximide) eliminated the formation of the transport system while inhibition by chloramphenicol of mitochondrial- or plastid-coded protein synthesis had no effect. Poisoning of membrane-bound proteins effectively disabled both the constitutive and induced transport systems.
1 Research supported by Texas Agricultural Experimental Station Project H-6850, and by a grant from Texas A&M University to M.C.D. Texas Agricultural Experiment Station Technical Article No. 25044.
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