Oxidative regulation of chloroplast enzymes by thioredoxin and thioredoxin-like proteins in Arabidopsis thaliana

Yuichi Yokochi, Yuka Fukushi, Ken-ichi Wakabayashi, Keisuke Yoshida, and Toru Hisabori

PNAS December 21, 2021 118 (51) e2114952118

Significance

Plants modulate photosynthesis activity in response to the surrounding environment. It is well known that the redox-responsive protein thioredoxin (Trx) activates photosynthesis-related enzymes in the light. However, the factors involved in deactivating them are not well understood. Recent in vitro experiments suggest that several Trx and Trx-like proteins serve as oxidation factors for Trx-targeted proteins; thus, we examined their functions in vivo. Consequently, we found that f-type Trx and two types of Trx-like proteins, Trx-like 2 and atypical Cys His-rich Trx, were involved in oxidative deactivation of photosynthesis-related enzymes (e.g., fructose-1,6-bisphosphatase, Rubisco activase, and the ATP synthase γ-subunit). Thus, this study reveals the functions of oxidation factors in vivo and elucidates the regulation system for photosynthesis in the dark.

Abstract

Thioredoxin (Trx) is a protein that mediates the reducing power transfer from the photosynthetic electron transport system to target enzymes in chloroplasts and regulates their activities. Redox regulation governed by Trx is a system that is central to the adaptation of various chloroplast functions to the ever-changing light environment. However, the factors involved in the opposite reaction (i.e., the oxidation of various enzymes) have yet to be revealed. Recently, it has been suggested that Trx and Trx-like proteins could oxidize Trx-targeted proteins in vitro. To elucidate the in vivo function of these proteins as oxidation factors, we generated mutant plant lines deficient in Trx or Trx-like proteins and studied how the proteins are involved in oxidative regulation in chloroplasts. We found that f-type Trx and two types of Trx-like proteins, Trx-like 2 and atypical Cys His-rich Trx (ACHT), seemed to serve as oxidation factors for Trx-targeted proteins, such as fructose-1,6-bisphosphatase, Rubisco activase, and the γ-subunit of ATP synthase. In addition, ACHT was found to be involved in regulating nonphotochemical quenching, which is the mechanism underlying the thermal dissipation of excess light energy. Overall, these results indicate that Trx and Trx-like proteins regulate chloroplast functions in concert by controlling the redox state of various photosynthesis-related proteins in vivo.

See: https://www.pnas.org/content/118/51/e2114952118

Fig. 1.

Phenotypes of mutant plants deficient in Trx-like proteins. (A) Visible phenotypes of plants deficient in Trx-like proteins. Plants were grown for 4 wk. (B) Confirmation of the knockout of Trx-like proteins by Western blotting. Arrows indicate the proteins of interest. Bands without arrows are nonspecific proteins. Leaf extract from the ACHT1 overexpressor was used as a positive control. Coomassie brilliant blue (CBB)-stained Rubisco large subunit (RbcL) is shown as a loading control. (C) Physiological parameters of plants deficient in Trx-like proteins. Each value is presented as the mean ± SD (n = 5). Different letters indicate significant differences among plant lines (P < 0.05; one-way ANOVA and Tukey honestly significant difference [HSD]).