Cellular homeostasis is maintained by the proteasomal degradation of regulatory and misfolded proteins, which sustains the amino acid pool. Although proteasomes alleviate stress by removing damaged proteins, mounting evidence indicates that severe stress caused by salt, metal(oids), and some pathogens can impair the proteasome. However, the consequences of proteasome inhibition in plants are not well understood and even less is known about how its malfunctioning alters metabolic activities. Lethality causes by proteasome inhibition in non-photosynthetic organisms stem from amino acid depletion, and we hypothesized that plants respond to proteasome inhibition by increasing amino acid biosynthesis. To address these questions, the short-term effects of proteasome inhibition were monitored for 3, 8 and 48 h in the roots of Brassica napus treated with the proteasome inhibitor MG132. Proteasome inhibition did not affect the pool of free amino acids after 48 h, which was attributed to elevated de novo amino acid synthesis; these observations coincided with increased levels of sulfite reductase and nitrate reductase activities at earlier time points. However, elevated amino acid synthesis failed to fully restore protein synthesis. In addition, transcriptome analysis points to perturbed abscisic acid signaling and decreased sugar metabolism after 8 h of proteasome inhibition. Proteasome inhibition increased the levels of alternative oxidase but decreased aconitase activity, most sugars and tricarboxylic acid metabolites in root tissue after 48 h. These metabolic responses occurred before we observed an accumulation of reactive oxygen species. We discuss how the metabolic response to proteasome inhibition and abiotic stress partially overlap in plants.

中文翻译：

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甘蓝型油菜根中的蛋白酶体抑制作用增加了氨基酸合成，从而抵消了减少的蛋白水解作用。

通过调节和错误折叠的蛋白质的蛋白酶体降解来维持细胞稳态，这维持了氨基酸库。尽管蛋白酶体通过去除受损的蛋白质减轻了压力，但越来越多的证据表明，由盐，金属（类固醇）和某些病原体引起的严重压力会损害蛋白酶体。然而，蛋白酶体抑制在植物中的后果尚不为人所知，对其功能失常如何改变代谢活性的了解甚少。非光合生物中蛋白酶体抑制的致死性源自氨基酸的消耗，我们假设植物通过增加氨基酸的生物合成来响应蛋白酶体的抑制。为了解决这些问题，对蛋白酶体抑制作用的短期影响在根部的3、8和48小时内进行了监测。甘蓝型油菜用蛋白酶体抑制剂MG132处理。蛋白酶体抑制作用在48小时后不影响游离氨基酸池，这归因于从头氨基酸合成的增加。这些观察结果与在较早时间点亚硫酸还原酶和硝酸还原酶活性的升高水平相吻合。但是，提高的氨基酸合成不能完全恢复蛋白质合成。此外，转录组分析表明，蛋白酶体抑制8小时后，脱落酸信号受到干扰，糖代谢降低。蛋白酶体抑制作用增加了48小时后根组织中替代氧化酶的水平，但降低了乌头酸酶的活性，大部分糖和三羧酸代谢产物。这些代谢反应发生在我们观察到活性氧积累之前。