Efficiently joins single-strand breaks in a double-stranded polydeoxynucleotide in an ATP-dependent reaction. Involved in DNA non-homologous end joining (NHEJ) required for double-strand break repair. May be involved for T-DNA integration even if not absolutely required. Seems to be dispensable under normal growth conditions. ATP + (deoxyribonucleotide)n-3'-hydroxyl + 5'-phospho-(deoxyribonucleotide)m = (deoxyribonucleotide)n+m + AMP + diphosphate. Interacts with XRCC4 via its tandem BRCT domains (PubMed:11029705). Interacts with POLL (PubMed:23660835). Widely expressed, with higher levels in young flowers and roots. Induced by gamma radiation and by white light, but not by UV-B. Regulated by ATM in response to DNA double strand breaks (DSBs). Belongs to the ATP-dependent DNA ligase family.