Editorial: NAD Metabolism and Signaling in Plants

摘要

Since its discovery more than a century ago and the conferral of several Nobel laureates for ground-breaking works concerning the molecule, nicotinamide adenine dinucleotide (NAD) is recognized as a fascinating metabolic cornerstone for all living organisms ( Berger et?al., 2004 ). This ubiquitous pyridine nucleotide (and its phosphorylated relative, NADP) is an energy cofactor and signal-carrying molecule exerting vital functions in plant metabolic pathways and regulatory processes ( Hashida et?al., 2009 ; Gakière et?al., 2018b ; Gakière et?al., 2018a ). Molecular studies on NAD involved transcript, protein, and metabolite profiling, and revealed a particularly complex signaling network that can impact not only the core biochemistry of the plant (e.g. photosynthesis, respiration, energy), but also responses to various stress, more importantly immune responses ( Zhang and Mou, 2009 ; Pétriacq et?al., 2012 ; Pétriacq et?al., 2016 ; Alferez et?al., 2018 ; Hashida et?al., 2018 ). Remarkably, compelling evidence indicate that NAD can be released into the extracellular space, where it is sensed by cell surface receptors ( Wang et?al., 2017 ; Wang et?al., 2019 ). However, the detailed mechanisms by which NAD acts as a regulator of plant physiology are still poorly understood. This Research Topic gathers some cardinal knowledge on the functions of NAD in plants. Two papers in the issue focus on the key enzyme NAD kinase which phosphorylates NAD(H) forming NADP(H) ( Gakière et?al., 2018a ) _(.) The first review by Ishikawa and Kawai-Yamada examined the roles of the enzyme in cyanobacteria—the evolutionary ancestor of land plant chloroplasts which has a far simpler level of compartmentation, in the oxidative pentose phosphate pathway, the glycolysis, and the pyridine nucleotide transhydrogenase proton translocation system. Survey of 72 cyanobacterial genomes revealed that 69 express a typical NAD kinase, raising the intriguing questions how typical NAD kinases contribute to NAD(P)(H) homeostasis in cyanobacteria and how this is regulated in the species containing the atypical NAD kinases. Also intriguing is the genetic identification of one of the latter as a factor linked to fast growth ( Ungerer et?al., 2018 ). However, a number of open questions remain, mainly why cyanobacteria need multiple NAD kinases and what the specific roles of the paralogs are. Further biochemical work alongside that on the membrane permeability of NAD(P)H will go a long way to addressing these questions. Secondly, Tai et al. reviewed the interplay of NAD kinases with calmodulin-mediated calcium signaling. They described the importance of the balance between NADP(H) and NAD(H) providing evidence suggesting that in response to allosteric regulation NAD kinase is a key regulator of the NADP(H)/NAD(H) ratio ( Li et?al., 2018 ). They then listed the evidence that calmodulin activates NAD kinase before performing a detailed analysis of wheat calmodulins, including predictions of gene and protein structures alongside subcellular localization and structure function studies. This provided a comprehensive overview of the putative regulatory control these proteins exhibit on NAD kinase ( Tai et al. ). Next, the review described the feedback regulation by NAD ~(+) derivatives, which is necessary for maintaining the dynamic balance of the cellular metabolites that ensure the normal operation of life activities. Under sunlight, photosynthetic electron transfer chain is the primary source of NADPH as reducing power for assimilation of CO _(2) by the Calvin cycle. However, under stress conditions that weaken enzyme activity involved in the Calvin cycle, decline of NADPH usage and NADP ~(+) recycling occurs and photosynthetic electron flow can be overloaded resulting in generation of reactive oxygen species. Thus, the NADP(H) pool size and redox state are crucial for the chloroplastic redox balance. An important fact is that, at the beginning of photosynthesis, NADP ~(+) supply control is dominated by de novo NADP ~(+) synthesis rather than recycling from the Calvin cycle. In addition, the activity of NADP ~(+) synthesis and the NADP pool size varies depending on the light conditions and the ferredoxin-thioredoxin system, by which NADP ~(+) is reduced to NADPH. NAD ~(+) is exclusively produced in the cytosol, although NADP ~(+) is produced at on demand sites (cytosol, chloroplasts, and peroxisomes) by various isoforms of NAD kinase. Therefore, the regulatory mechanism of cytosolic NAD ~(+) supply is also involved in the chloroplastic NADP ~(+) supply control. Hashida and Kawai-Yamada summarized the regulatory mechanisms of NADP ~(+) production, focusing on the interactions, crosstalk, and co-regulation between chloroplasts and the cytoplasm at the level of NAD ~(+) metabolism and molecular transport. NAD metabolism underlies a number of NAD-processing reactions. To start recycling NAD from nicotinate (NA), plants developed various strategies to overcome