利用銨鹽突變株探討植物氮利用與光合作用之交互調控

Abstract

氮是構成核酸與胺基酸的重要元素，也是限制植物生長的要素之一，土壤中的硝酸鹽是大多數植物主要的氮素來源，根部吸收週遭環境中的硝酸鹽後，需要經過一連串的同化作用，將硝酸鹽還原成銨鹽，銨鹽經固氮形成麩醯胺後，再轉換成其他胺基酸供植物生長利用。過量的銨鹽對植物是種傷害，必須盡快的在葉綠體內進行固氮解毒，為了避免銨鹽的過度累積，植物必須嚴格的調控硝酸鹽以及銨鹽同化作用的效率，來維持體內銨鹽的含量。光合作用提供植物體內各種代謝反應所需的能量、碳源及還原劑，其中約有10％的化學能，被用於硝酸鹽以及銨鹽的同化作用，這是僅次於二氧化碳同化作用外，需要消耗最多能量與碳水化合物的代謝反應。碳水化合物與胺基酸都是植物組成的必要成分，然而光合作用產生的化學能有限，如何依據植物的生理需求來調控能量及相關物質的分配便顯得十分重要，現今認為氮與碳同化作用間具有高度的協調性，不過詳細的調控機制以及兩者間的交互作用在植物生理上扮演的角色仍有待進一步的研究。 本篇研究利用順向遺傳學的方式，篩選出具有銨鹽毒害過度敏感性狀的阿拉伯芥突變株，透過多種生化與分生技術發現，此突變株第四號染色體上的At4g14605基因產生缺失。雖然At4g14605基因被預期是粒線體轉錄終止因子蛋白家族中的一員（mitochondrial transcription termination factor-related protein, mTERF-related），但我們的研究發現，At4g14605蛋白會進入葉綠體內並參予葉綠體基因體轉錄機制的調控，當At4g14605基因發生突變時，會影響葉綠體基因體中光系統II相關基因在RNA與蛋白質層次上的表現，這樣的結果可能會干擾光系統II複合體的形成，導致光合作用無法正常的進行，只能仰賴光系統I製造ATP，而降低了還原劑的產生，可能連帶影響了硝酸鹽及銨鹽同化作用的過程，利用這些研究，我們闡述了一個新的氮碳交互調控之可能機制。

For most of plants, nitrate and ammonium are the two major nitrogen sources. Nitrate taken into plants can be stored, or assimilated first to ammonium and then incorporated into carbon skeleton to generate amino acids. Excess ammonium is toxic to most plant. To avoid the toxicity, ammonium has to be incorporated into amino acids in chloroplast efficiently. In addition, nitrate and ammonium transport and assimilation system should be precisely regulated to keep balanced with energy, reducing power and carbon fixed from photosynthesis. About 10 % of the photosynthesis outcome is used in nitrogen metabolism. Nitrogen metabolism and carbon fixation are the most energy consuming pathway for plant. To avoid competition, these two pathways have to be coordinately regulated. However, the interaction and regulation mechanism between nitrogen metabolism and carbon fixation remain to be elucidated. In this study, using genetic mapping and microarray analysis, an Arabidopsis ammonium sensitive and chlorate resistant mutant was found to be mutated at At4g14605 gene. Although At4g14605 is predicted to be a mitochondrial transcription termination factor-related protein, we found that it is targeted to chloroplast, and affect the expression of several photosystem II (PSII) related gene. Therefore, blocking the function of this gene will disrupt the normal assembling of PSII complex and reduce the cyclic electron transfer chain in photosynthesis, a key process to provide reducing power for carbon fixation and nitrogen metabolism.