The regulation of PEP carboxylase gene expression by nitrogen in maize leaf tissue

通过对氮素胁迫恢复过程中玉米叶片基底部(即光合作用成熟细胞)的代谢中间产物、酶活性、蛋白质和mRNAs的分析，得到以下结果：(A)玉米叶片中PEP羧基酶(PEPC)的选择性积累主要在转录水平上调节，以响应N的有效性。研究了N-诱导PEPC N依赖基因表达的可行性。这些结果表明，谷氨酰胺可能是调控PEPC N依赖基因表达的一个积极信号。结果还表明，谷氨酰胺的积累主要是由于N胁迫恢复过程中谷氨酰胺合成酶和铁氧还蛋白依赖的谷氨酸合成酶的差异表达所致。(B)除了N源外，细胞分裂素是诱导N胁迫植物离体叶中PEPC基因的必需成分，具有激素特异性。(C)分析了光合作用C4途径的核心酶碳酸酐酶(CA)的基因表达，并与N胁迫恢复过程中PEPC的基因表达进行了比较。结果表明，CA和PEPC在对N有效性的响应中受到相同或非常相似的机制的协调调控。(D)检测了不同N条件下叶肉细胞和束鞘细胞胞浆中谷氨酰胺合成酶的基因表达。然而，无论氮素条件如何，该酶的编码水平都没有变化。这表明，至少对PEPC和CA基因而言，N依赖的基因表达调控是特异的。

The following results were obtained by analyzing metabolic intermediates, enzyme activities, proteins, and mRNAs in the basal region of leaves, namely, photosynthetically maturing cells, of maize plants during recovery from N-stress.(A)The selective accumulation of PEP carboxylase(PEPC)is regulated primarily at transcriptional level in responding to N-availability. The N-avaialbility for induction of the N-dependent gene expression of PEPC was studied. The results obtained indicate that glutamine can be a positive signal for the control of the N-dependent gene expression of PEPC. The results also suggested that the accumulation of glutamine is mainly due to the differential expression of glutamine synthetase and ferredoxin-dependent-glutamate synthase during recovery from N-stress.(B)In addition to N-source cytokinin is an essential component for the induction of PEPC gene in the detached leaves of N-stressed plants, showing a hormonal specificity.(C)The gene expression of carbonic anhydrase(CA), a core enzyme in the C4pathway of photosynthesis, was analyzed in comparison to that of PEPC during recovery from N-stress. The results obtained show that CA and PEPC are coordinately regulated by the same or very similar mechanism in responding to N-availability.(D)The gene expression of glutamine synthetase located in the cytosols of mesophyil and bundle sheath cells was examined under various N-conditions. However, the levels of MRNA encoding for this enzyme unchanged regardless of N condition. This indicates that the N-dependent regulation of gene expression is specific at least for PEPC and CA genes.