A dominant mutation in tomato DNA POLYMERASE DELTA 1 causes geminivirus DNA replication catastrophe.

Abstract

Geminiviruses pose a severe threat to grain and vegetable crops worldwide, often resulting in significant economic losses. In cultivated tomato (<i>Solanum lycopersicum</i>), <i>Ty</i> resistance alleles have been introduced from wild tomato relatives, providing partial to strong resistance to geminivirus infections. The <i>Ty-6</i> resistance locus from <i>Solanum chilense</i> was previously mapped to chromosome 10. It was recently shown to contain a mutant allele of the <i>DNA POLYMERASE DELTA 1</i> (<i>POLD1</i>) gene that provides resistance to Tomato yellow leaf curl virus (TYLCV) infections. However, the resistance mechanism remained unknown. Here, we report another <i>POLD1</i> allele at the <i>Ty-6</i> locus of <i>S. chilense</i> with an E622D mutation in the catalytic site of the POLD1 protein. <i>POLD1^E622D</i> is maintained as a heterozygous dominant allele in <i>S. chilense</i> and the AVTO2225 breeding line. It provides full resistance to the severe TYLCV Thailand (TYLCTHV) strain. The E622D amino acid change does not alter the predicted structure of POLD1. Replication of the TYLCTHV genome in plants carrying the <i>POLD1^E622D</i> allele is severely compromised by a high frequency of mutations that accumulate in viral DNA, which results in nonfunctional proteins that are essential for continuous viral replication. Ectopically expressing the <i>POLD1^E622D</i> allele cDNA alone causes mutations in TYLCTHV genes in inoculated leaves. <i>S. chilense</i> and AVTO2225 plants carrying the <i>POLD1^E622D</i> allele mount a hypersensitive response after TYLCTHV infection, indicating that the defective virus genome cannot suppress the plant defense. The dominant <i>POLD1^E622D</i> allele is therefore an effective resistance gene that geminiviruses cannot overcome.