Figure 2.

Roles of NADK proteins in maintaining reactive oxygen species (ROS) homeostasis in different subcellular compartments of plant cells. (1) The generation of ROS in cells. NADPH oxidases (NOXs) are located on the plasma membrane which can generate superoxide radical (${\text{O}}_2^{-}$) by catalyzing NADPH. Superoxide dismutase (SOD) can transform ${\text{O}}_2^{-}$ to oxygen and hydrogen peroxide (H₂O₂), a main kind of stable ROS. The complex of Ca²⁺/CaM participate in the production of ROS by activating NOXs. Under stress conditions, intracellular Ca²⁺ elevated according to Ca²⁺ influx from apoplast spaces, which leads to the activation of Ca²⁺ sensor proteins like the calcium-dependent protein kinase (CDPK), calmodulin (CaM), calmodulin-like protein (CML), and calcineurin B-like protein (CBL). CaMs also contribute to the activation of NADKs. In chloroplasts, photosynthesis is the main source of generating ROS: photosynthetic electron transport chain and Photosystem (PS) I or II release ${\text{O}}_2^{-}$, which is then transformed to H₂O₂ by SOD. Excited chlorophylls can also generate singlet oxygen (¹O₂), a kind of ROS with high energy. In mitochondria, ${\text{O}}_2^{-}$ is formed by the respiratory electron transport chain, which can be transformed to H₂O₂. In peroxisomes, H₂O₂ is generated through three main pathways including fatty acid oxidation, xanthine oxidation and glycolate oxidation. (2) The scavenging systems of ROS in cells. As an intermediate, ${\text{O}}_2^{-}$ can be immediately transformed to H₂O₂ by SOD, therefore, H₂O₂ becomes the main object of the ROS scavenging systems. The ascorbate-glutathione cycle and related enzymes including glutathione reductase (GR), dehydroascorbate reductase (DHAR), glutathione dehydrogenase (GDH) and ascorbate peroxidases (APX) function in both cytosol and other subcellular compartments such as in chloroplasts, mitochondria and peroxisomes. Besides, glutathione peroxidase (GPX) and catalase (CAT) can also detoxify H₂O₂. In the ROS scavenging systems, reducing power from the photosynthetic apparatus and NADPH are needed to support the operation. (3) NADKs distribute in different subcellular compartments, where they play an important role in the NADPH supply system. Firstly, NADKs can phosphorylate NAD to NADP in the different subcellular compartments. Then, dehydrogenases including glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6GPDH), NADP-dependent isocitrate dehydrogenase (ICDH), NADP-dependent malic enzyme (ME), NADP-dependent aldehyde dehydrogenase (ALDH) and NADP-dependent glutamate dehydrogenase (NADP–GDH) can transform NADP to NADPH. Transhydrogenases in mitochondria can also generate NADPH by transferring the hydrogen ion from NADH to NADP. Red arrows and blue arrows indicate the ROS generation and scavenging pathways, respectively. ASC, ascorbate; APX, ascorbate peroxidases; CAT, catalase; DHA, dehydroascorbate; DHAR, dehydroascorbate reductase; GR, glutathione reductase; GSH, glutathione; GSSG, oxidized glutathione; GDH, glutathione dehydrogenase; ROS, reactive oxygen species.