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

Thymidine kinase 1 (TK1) phosphorylates thymidine nucleosides to generate thymidine monophosphate. This reaction belongs to the pyrimidine salvage route that is phylogenetically conserved. In the model plantArabidopsis thaliana, TK activity contributes to maintain nuclear and organellar genome integrity by providing deoxythymidine-triphosphate (dTTP) for DNA synthesis. Arabidopsis has two TK1 genes (TK1aandTK1b) and double mutants show an albino phenotype and develop poorly. In contrast, maize (Zea mays L.) has a single TK1 (ZmTK1) gene and mutant plants are albino and display reduced genome copy number in chloroplasts. We studied the role ofZmTK1during development and genotoxic stress response by assessing its activity at different developmental stages and by complementing Arabidopsistk1mutants. We found thatZmTK1transcripts and activity are present during germination and throughout maize development. We show that ZmTK1 translocation to chloroplasts depends on a 72-amino-acid N-signal and its plastid localization is consistent with its ability to complement Arabidopsistk1bmutants which are hypersensitive to ciprofloxacin (CIP), a genotoxic agent to organellar DNA. Also,ZmTK1partly complemented the Arabidopsis double mutant plants during development. Our results contribute to the understanding of TK1 function in monocot species as an organellar enzyme for genome replication and repair.