Migge A, Bork C, Hell R, Becker T W
Lehrstuhl für Genetik, Fakultät für Biologie, Universität Bielefeld, Germany.
Planta. 2000 Sep;211(4):587-95. doi: 10.1007/s004250000322.
Tobacco (Nicotiana tabacum L.) plants were subjected to a prolonged period of sulfur-deprivation to characterize molecular and metabolic mechanisms that permit control of primary N-metabolism under these conditions. Prior to the appearance of chlorotic lesions, sulfur-deprived tobacco leaves showed a strong decrease in the sulfate content and changes in foliar enzyme activities, mRNA accumulation and amino-acid pools. The basic amino acids glutamine, asparagine and arginine accumulated in the leaves of sulfur-deprived plants, while the foliar concentrations of aspartate, glutamate, serine or alanine remained fairly unchanged. Maximal extractable nitrate reductase (NR; EC 1.6.6.1) activity decreased strongly in response to sulfur-deprivation. The decrease in maximal extractable NR activity was accompanied by a decline in NR transcripts while the mRNAs of the plastidic glutamine synthetase (EC 6.1.3.2) or the beta-subunit of the mitochondrial ATP synthase were much less affected. Nitrate first accumulated in leaves of tobacco during sulfur-deprivation but then declined. An appreciable amount of nitrate was, however, present in severely sulfur-depleted leaves. The repression of NR gene expression is, therefore, not related to the decrease in the leaf nitrate level. However, glutamine- and/or asparagine-mediated repression of NR gene transcription is a possible mechanism of control in situations when glutamine and asparagine accumulate in leaves and provides a feasible explanation for the reduction in NR activity during sulfur-deprivation. The removal of reduced nitrogen from primary metabolism by redirection and storage as arginine, asparagine or glutamine combined with the down-regulation of nitrate reduction via glutamine- and/or asparagine-mediated repression of NR gene transcription may contribute to maintaining a normal N/S balance during sulfur-deprivation and indicate that the co-ordination of N- and S-metabolism is retained under these conditions.
对烟草（Nicotiana tabacum L.）植株进行长期缺硫处理，以表征在这些条件下控制初级氮代谢的分子和代谢机制。在出现褪绿病斑之前，缺硫烟草叶片的硫酸盐含量大幅下降，叶片酶活性、mRNA积累和氨基酸库发生变化。碱性氨基酸谷氨酰胺、天冬酰胺和精氨酸在缺硫植株的叶片中积累，而天冬氨酸、谷氨酸、丝氨酸或丙氨酸的叶片浓度基本保持不变。最大可提取硝酸还原酶（NR；EC 1.6.6.1）活性因缺硫而大幅下降。最大可提取NR活性的下降伴随着NR转录本的减少，而质体谷氨酰胺合成酶（EC 6.1.3.2）或线粒体ATP合酶β亚基的mRNA受影响较小。在缺硫期间，硝酸盐首先在烟草叶片中积累，但随后下降。然而，在严重缺硫的叶片中存在相当数量的硝酸盐。因此，NR基因表达的抑制与叶片硝酸盐水平的降低无关。然而，谷氨酰胺和/或天冬酰胺介导的NR基因转录抑制是一种可能的调控机制，在谷氨酰胺和天冬酰胺在叶片中积累的情况下可以对此作出解释，并为缺硫期间NR活性的降低提供了一个合理的解释。通过将还原态氮重定向并储存为精氨酸、天冬酰胺或谷氨酰胺，从初级代谢中去除还原态氮，同时通过谷氨酰胺和/或天冬酰胺介导的NR基因转录抑制来下调硝酸盐还原，这可能有助于在缺硫期间维持正常的氮/硫平衡，并表明在这些条件下氮和硫代谢的协调得以保留。
