Abstract
Monodehydroascorbate reductase (MDAR) is an enzyme involved in ascorbate recycling. Arabidopsis thaliana has five MDAR genes that encode two cytosolic, one cytosolic/peroxisomal, one peroxisomal membrane-attached, and one chloroplastic/mitochondrial isoform. In contrast, tomato plants possess only three enzymes, lacking the cytosol-specific enzymes. Thus, the number and distribution of MDAR isoforms differ according to plant species. Moreover, the physiological significance of MDARs remains poorly understood. In this study, we classify plant MDARs into three classes: class I, chloroplastic/mitochondrial enzymes; class II, peroxisomal membrane-attached enzymes; and class III, cytosolic/peroxisomal enzymes. The cytosol-specific isoforms form a subclass of class III and are conserved only in Brassicaceae plants. With some exceptions, all land plants and a charophyte algae, Klebsormidium flaccidum, contain all three classes. Using reverse genetic analysis of Arabidopsis thaliana mutants lacking one or more isoforms, we provide new insight into the roles of MDARs; for example, (1) the lack of two isoforms in a specific combination results in lethality, and (2) the role of MDARs in ascorbate redox regulation in leaves can be largely compensated by other systems. Based on these findings, we discuss the distribution and function of MDAR isoforms in land plants and their cooperation with other recycling systems.
Highlights
Ascorbate is a multifunctional soluble compound that acts as a redox buffer and serves as an electron donor for many enzymatic reactions [1]
Genome searches were performed using BLASTP with the A. thaliana AMDAR2 protein as the query, which was chosen because AthMDAR2 has no extra sequences, such as transmembrane domain and targeting signal
We explore the physiological roles of A. thaliana AthMDARs
Summary
Ascorbate is a multifunctional soluble compound that acts as a redox buffer and serves as an electron donor for many enzymatic reactions [1]. Its functions are not fully understood, many of them are linked to plant responses to light environments, as plants accumulate high levels of this compound in their leaves under illumination [1]. Leaf ascorbate content is further enhanced by high light (HL) stress through the activation of the D-mannose/L-galactose pathway [2,3], which predominates ascorbate biosynthesis in plants [2,4] This environmental stimulus promotes the production of reactive oxygen species (ROS), such as hydrogen peroxide (H2 O2 ), superoxide anion radical, singlet oxygen, and hydroxyl radical, from photosynthesis and photorespiration [5,6,7].
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