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Thymidine kinases ( TKs) are important components in the nucleotide salvage pathway. However, knowledge about plant TKs is quite limited. In this study, the molecular function of TKs in Arabidopsis thaliana was investigated., Two TKs were identified and named At TK1 and At TK2. Expression of both genes was ubiquitous, but AtTK1 was strongly expressed in high-proliferation tissues. AtTK1 was localized to the cytosol, whereas AtTK2 was localized to the mitochondria. Mutant analysis indicated that the two genes function coordinately to sustain normal plant development., Enzymatic assays showed that the two TK proteins shared similar catalytic specificity for pyrimidine nucleosides. They were able to complement an Escherichia coli strain lacking TK activity. 5′-Fluorodeoxyuridine (FdU) resistance and 5-ethynyl 2′-deoxyuridine (EdU) incorporation assays confirmed their activity in vivo. Furthermore, the tk mutant phenotype could be alleviated by nucleotide feeding, establishing that the biosynthesis of pyrimidine nucleotides was disrupted by the TK deficiency. Finally, both human and rice ( Oryza sativa) TKs were able to rescue the tk mutants, demonstrating the functional conservation of TKs across organisms., Taken together, our findings clarify the specialized function of two TKs in A. thaliana and establish that the salvage pathway mediated by the kinases is essential for plant growth and development. [ABSTRACT FROM AUTHOR]

Genetic imprinting refers to the unequal expression of paternal and maternal alleles of a gene in sexually reproducing organisms, including mammals and flowering plants. Although many imprinted genes have been identified in plants, the functions of these imprinted genes have remained largely uninvestigated., We report genome-wide analysis of gene expression, DNA methylation and small RNAs in the rice endosperm and functional tests of five imprinted genes during seed development using Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated gene9 (CRISPR/Cas9) gene editing technology., In the rice endosperm, we identified 162 maternally expressed genes (MEGs) and 95 paternally expressed genes (PEGs), which were associated with miniature inverted-repeat transposable elements, imprinted differentially methylated loci and some 21-22 small interfering RNAs (siRNAs) and long noncoding RNAs (lncRNAs). Remarkably, one-third of MEGs and nearly one-half of PEGs were associated with grain yield quantitative trait loci. Most MEGs and some PEGs were expressed specifically in the endosperm. Disruption of two MEGs increased the amount of small starch granules and reduced grain and embryo size, whereas mutation of three PEGs reduced starch content and seed fertility., Our data indicate that both MEGs and PEGs in rice regulate nutrient metabolism and endosperm development, which optimize seed development and offspring fitness to facilitate parental-offspring coadaptation. These imprinted genes and mechanisms could be used to improve the grain yield of rice and other cereal crops. [ABSTRACT FROM AUTHOR]