Abscisic acid-regulated protein degradation causes osmotic stress-induced accumulation of branched-chain amino acids in Arabidopsis thaliana

Jul 3, 2017Planta

Abscisic acid-controlled protein breakdown leads to branched-chain amino acid buildup during water stress in Arabidopsis plants

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Abstract

Drought-stressed Arabidopsis thaliana accumulates branched-chain amino acids (BCAAs) primarily through protein degradation.

BCAAs, including leucine, isoleucine, and valine, accumulate more significantly than proline during drought stress.
Accumulation of BCAAs is regulated by abscisic acid and is linked to protein degradation rather than biosynthesis.
Plants deficient in abscisic acid signaling show reduced BCAA levels but not reduced proline levels.
Treatment with protease inhibitors decreases BCAA accumulation, supporting the role of protein degradation.
Overexpression of an enzyme involved in BCAA degradation leads to decreased BCAA accumulation under drought conditions.
Unlike proline, which is synthesized anew, BCAA levels return to normal after osmotic stress relief primarily through degradation.

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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.

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Abscisic acid-regulated protein degradation causes osmotic stress-induced accumulation of branched-chain amino acids in Arabidopsis thaliana

Abstract

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