Hole D J, Emran A M, Fares Y, Drew M C
Department of Horticultural Sciences, Texas A&M University, College Station, Texas 77843-2133.
Plant Physiol. 1990 Jun;93(2):642-7. doi: 10.1104/pp.93.2.642.
Unlike phosphate or potassium transport, uptake of nitrate by roots is induced, in part, by contact with the substrate ion. Plasmalemma influx of (13)N-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 K(m). 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.
与磷酸盐或钾的转运不同，根系对硝酸盐的吸收部分是由与底物离子的接触诱导的。研究了玉米根系中（13）N标记硝酸盐的质膜流入与吸收系统诱导以及短期氮饥饿影响之间的关系。先前未接触过硝酸盐的玉米（Zea mays）根系具有组成型转运系统（状态1），但在与100微摩尔硝酸盐接触的6小时内，流入量增加了250%，此时转运机制似乎已完全合成（状态2）。在诱导和测量硝酸盐流入之前进行三天的氮饥饿处理，导致根系转运硝酸盐的能力比未饥饿的对照更强，但只有在根系与硝酸盐保持6小时以实现完全诱导（状态2E）时，这种能力才能完全表达出来。动力学分析表明，氮饥饿且已诱导的根系最大流入量增加了160%，而米氏常数（K(m)）略有下降。硝酸盐流入的可诱导成分仅由与硝酸盐的接触诱导产生。硝酸盐诱导转运系统的完全表达依赖于mRNA合成。细胞质蛋白合成抑制剂（环己酰亚胺）消除了转运系统的形成，而氯霉素对线粒体或质体编码蛋白合成的抑制则没有影响。膜结合蛋白中毒有效地使组成型和诱导型转运系统均失效。
