Role of pyrimidine salvage pathway in the maintenance of organellar and nuclear genome integrity

Abstract

Summary Nucleotide biosynthesis proceeds through a de novo pathway and a salvage route. In the salvage route, free bases and/or nucleosides are recycled to generate the corresponding nucleotides. Thymidine kinase (TK) is the first enzyme in the salvage pathway to recycle thymidine nucleosides as it phosphorylates thymidine to yield thymidine monophosphate. The Arabidopsis genome contains two TK genes −TK1a and TK1b− that show similar expression patterns during development. In this work, we studied the respective roles of the two genes during early development and in response to genotoxic agents targeting the organellar or the nuclear genome. We found that the pyrimidine salvage pathway is crucial for chloroplast development and genome replication, as well as for the maintenance of its integrity, and is thus likely to play a crucial role during the transition from heterotrophy to autotrophy after germination. Interestingly, defects in TK activity could be partially compensated by supplementation of the medium with sugar, and this effect resulted from both the availability of a carbon source and the activation of the nucleotide de novo synthesis pathway, providing evidence for a compensation mechanism between two routes of nucleotide biosynthesis that depend on nutrient availability. Finally, we found differential roles of the TK1a and TK1b genes during the plant response to genotoxic stress, suggesting that different pools of nucleotides exist within the cells and are required to respond to different types of DNA damage. Altogether, our results highlight the importance of the pyrimidine salvage pathway, both during plant development and in response to genotoxic stress.

Nucleotide Metabolism in Plants

“…However, it is unclear, why thymidine salvage is of such importance. TK occurs in the cytosol, the mitochondria, and the plastids and is of particular importance for chloroplast maintenance when germinating seedlings turn autotrophic (Pedroza-García et al, 2019). The other deoxynucleosides are salvaged by an enzyme with broad deoxynucleoside specificity (dNK; Fig.…”

Maize Thymidine Kinase Activity Is Present throughout Plant Development and Its Heterologous Expression Confers Tolerance to an Organellar DNA-Damaging Agent

“…Also, a few TK genes have been described in plant species; in rice, TK mRNA levels are higher in mature as well as in meristematic tissues, thus the pyrimidine salvage pathway might function in other events requiring DNA synthesis, such as DNA repair. The Arabidopsis thaliana genome contains two TK genes (TK1a and TK1b), and single homozygous mutants of any of these genes show a lower growth rate at seedling developmental stages and TK activity is required for optimal growth as the double mutant displays an albino phenotype and delayed development [8]. TK1a and TK1b proteins have distinct subcellular localization; while the former is a cytosolic enzyme, the latter localizes in the mitochondria [9] and chloroplasts [8].…”

“…The Arabidopsis thaliana genome contains two TK genes (TK1a and TK1b), and single homozygous mutants of any of these genes show a lower growth rate at seedling developmental stages and TK activity is required for optimal growth as the double mutant displays an albino phenotype and delayed development [8]. TK1a and TK1b proteins have distinct subcellular localization; while the former is a cytosolic enzyme, the latter localizes in the mitochondria [9] and chloroplasts [8]. TK1a and TK1b show ubiquitous expression during plant growth and development [9,10].…”

“…TK1a and TK1b show ubiquitous expression during plant growth and development [9,10]. TK1 activity has a substantial role during early plant growth for plastid development and maintenance before germinating seedlings turn autotrophic [8]. TK1 genes in Arabidopsis display a differential response to ultraviolet-C (UV-C) radiation, while TK1a gene is strongly induced 1-3 h post irradiation, TK1b transcript levels remain constant.…”

“…Moreover, overexpression of the TK1a gene confers tolerance to genotoxic stress [10]. These differences are also reflected in their roles in genotoxic stress response: whereas TK1a contributes to DNA repair of the nuclear genome, TK1b is involved in maintaining the integrity of organellar genomes [8].…”