Boyce Thompson Institute for Plant Research, Ithaca, NY, 14853, USA.
Keygene Inc, Rockville, MD, 20850, USA.
Planta. 2017 Oct;246(4):737-747. doi: 10.1007/s00425-017-2727-3. Epub 2017 Jul 1.
Whereas proline accumulates through de novo biosynthesis in plants subjected to osmotic stress, leucine, isoleucine, and valine accumulation in drought-stressed Arabidopsis thaliana is caused by abscisic acid-regulated protein degradation. In response to several kinds of abiotic stress, plants greatly increase their accumulation of free amino acids. Although stress-induced proline increases have been studied the most extensively, the fold-increase of other amino acids, in particular branched-chain amino acids (BCAAs; leucine, isoleucine, and valine), is often higher than that of proline. In Arabidopsis thaliana (Arabidopsis), BCAAs accumulate in response to drought, salt, mannitol, polyethylene glycol, herbicide treatment, and nitrogen starvation. Plants that are deficient in abscisic acid signaling accumulate lower amounts of BCAAs, but not proline and most other amino acids. Previous bioinformatic studies had suggested that amino acid synthesis, rather than protein degradation, is responsible for the observed BCAA increase in osmotically stressed Arabidopsis. However, whereas treatment with the protease inhibitor MG132 decreased drought-induced BCAA accumulation, inhibition of BCAA biosynthesis with the acetolactate synthase inhibitors chlorsulfuron and imazapyr did not. Additionally, overexpression of BRANCHED-CHAIN AMINO ACID TRANSFERASE2 (BCAT2), which is upregulated in response to osmotic stress and functions in BCAA degradation, decreased drought-induced BCAA accumulation. Together, these results demonstrate that BCAA accumulation in osmotically stressed Arabidopsis is primarily the result of protein degradation. After relief of the osmotic stress, BCAA homeostasis is restored over time by amino acid degradation involving BCAT2. Thus, drought-induced BCAA accumulation is different from that of proline, which is accumulated due to de novo synthesis in an abscisic acid-independent manner and remains elevated for a more prolonged period of time after removal of the osmotic stress.
虽然脯氨酸在受到渗透胁迫的植物中通过从头合成积累，但在干旱胁迫的拟南芥中，亮氨酸、异亮氨酸和缬氨酸的积累是由脱落酸调节的蛋白降解引起的。植物在应对多种非生物胁迫时，会大大增加游离氨基酸的积累。尽管对胁迫诱导脯氨酸增加的研究最为广泛，但其他氨基酸，特别是支链氨基酸（BCAA；亮氨酸、异亮氨酸和缬氨酸）的增加倍数通常高于脯氨酸。在拟南芥中，BCAA 会在干旱、盐胁迫、甘露醇、聚乙二醇、除草剂处理和氮饥饿时积累。缺乏脱落酸信号的植物积累的 BCAA 较少，但脯氨酸和大多数其他氨基酸则不会。之前的生物信息学研究表明，氨基酸合成而不是蛋白降解负责观察到的渗透胁迫下拟南芥中 BCAA 的增加。然而，用蛋白酶抑制剂 MG132 处理会降低干旱诱导的 BCAA 积累，而用乙酰乳酸合酶抑制剂chlorsulfuron 和 imazapyr 抑制 BCAA 合成则不会。此外，过表达响应渗透胁迫而上调的分支链氨基酸转移酶 2（BCAT2），会降低干旱诱导的 BCAA 积累。这些结果共同表明，渗透胁迫下拟南芥中 BCAA 的积累主要是蛋白降解的结果。在渗透胁迫解除后，BCAA 稳态通过涉及 BCAT2 的氨基酸降解逐渐恢复。因此，干旱诱导的 BCAA 积累与脯氨酸不同，脯氨酸是通过非依赖脱落酸的从头合成积累的，并且在去除渗透胁迫后更长时间内保持升高。
