Ascorbic acid (vitamin C), one of the most vital antioxidant molecules in plants, plays a dual – role in combating oxidative stress. It not only directly neutralizes reactive oxygen species (ROS) but also supplies the essential reducing power for ascorbate peroxidase (APX). APX catalyzes the decomposition of hydrogen peroxide (H₂O₂), playing a pivotal role in maintaining the cellular redox balance. However, the continuous operation of this protective system depends on the ascorbic acid regeneration cycle. Monodehydroascorbate (MDHA), the primary product of ascorbic acid oxidation, can be regenerated through the reduction reaction catalyzed by MDHA reductase (MDHAR) or spontaneous disproportionation. Nevertheless, there are still gaps in the functional studies of MDHAR subtypes, and a systematic comparison of each subtype’s response to oxidative stress remains to be conducted.
Recently, a research paper titled “Cytosolic Monodehydroascorbate Reductase 2 Promotes Oxidative Stress Signaling in Arabidopsis” was published in the Plant, Cell & Environment journal. This study elucidates the functional differences among MDHAR subtypes, which is of great significance for understanding the precise regulation of the plant antioxidant network.
The researchers first explored the functions of different MDHAR subtypes in Arabidopsis thaliana. qRT – PCR and enzyme activity assays showed that under standard growth conditions with a low turnover rate of the ascorbate – glutathione pathway, none of the MDHAR subtypes were essential for plant growth and development. Building on this, the researchers utilized the catalase – deficient mutant cat2 (this mutant is continuously under oxidative stress due to the increased demand for the hydrogen peroxide metabolism reduction pathway) to further investigate the functional importance of different MDHAR subtypes under oxidative stress conditions. Further qRT – PCR analysis revealed that among the four MDAR genes, the cytosolic subtypes (MDHAR2 and MDHAR3) responded most significantly to intracellular oxidative stress. This induced expression was accompanied by an enhancement of MDHAR activity in the cat2 mutant, with the increase in NADPH – dependent activity being more pronounced than that of NADH – dependent activity. Subsequently, the researchers further crossed each mutant with cat2 to construct double – mutant systems (lacking both the main leaf catalase activity and specific MDHAR subtypes). Gene expression analysis showed that the effects of each mutation on MDAR genes in the cat2 background were consistent with their action patterns in the Col – 0 background.
The researchers also observed two phenotypic effects caused by the cat2 mutation: one was a significant inhibition of rosette leaf growth compared to Col – 0; the other was the spontaneous formation of necrotic spots on the leaves, similar to the hypersensitive response to pathogens. Through phenotypic analysis of the double mutants, it was found that in the cat2 background, the loss of peroxisomal MDHAR function inhibited plant growth, while the loss of cytosolic MDHAR2 did not affect growth but eliminated most of the transcriptomic and phenotypic responses to oxidative stress. The effects of the mdar2 mutation included reduced salicylic acid accumulation and increased glutathione oxidation, and these phenotypes could be restored by complementing the MDAR2 gene (Figure 2). Finally, to deeply analyze the impact of the mdar2 mutation on the cat2 response, the researchers conducted transcriptomic analysis. The analysis confirmed that functional MDHAR2 is an essential component for mediating the downstream signaling response triggered by oxidative stress in cat2. The comprehensive results indicate that cytosolic MDHAR2 is non – essential under optimal growth conditions but plays a crucial role in promoting the biotic defense response triggered by oxidative stress.
This study aimed to explore the functional differences of specific MDHAR subtypes under normal growth and oxidative stress conditions. The results of single – mutant analysis are consistent with previous reports, confirming that these enzymes are not essential under standard growth conditions. However, in the context of oxidative stress, the importance of certain MDHARs becomes evident. This work provides a new perspective for understanding the non – classical functions of the antioxidant system and also offers new targets for plant defense responses against oxidative stress and crop stress – resistant breeding.
Xu, D., Trémulot, L., Yang, Z., Mhamdi, A., Châtel – Innocenti, G., Mathieu, L., Espinasse, C., Van Breusegem, F., Vanacker, H., Issakidis – Bourguet, E., & Noctor, G. (Publication year). Cytosolic Monodehydroascorbate Reductase 2 Promotes Oxidative Stress Signaling in Arabidopsis.