Endonuclease that can use RNA, single-stranded and double-stranded DNA as substrates (PubMed:23620482). Hydrolyzes single-stranded DNA and RNA without apparent specificity for bases during senescence. Endonuclease that recognizes and cleaves all types of mismatches with high efficiency, including heteroduplex double-stranded DNA. Maybe involved in programmed cell death (PCD) and senescence. Endonucleolytic cleavage to 5'-phosphomononucleotide and 5'-phosphooligonucleotide end-products. Binds 3 divalent metal cations (PubMed:23620482). Uses Ca(2+) ions with ssDNA as substrate (PubMed:23620482). Can also use Mn(2+) with lower efficiency with ssDNA and dsDNA as substrates (PubMed:23620482). Optimum pH is 8 with RNA, ssDNA and dsDNA as substrates. Monomer. Mostly expressed in flowers and during leaf and stem senescence, and, to a lower extent, detectable at low levels in roots, leaves, and stems. Particularly expressed in senescing tissues in a NAC92/ORE1-dependent manner. Present in the margins and tips of the oldest leaves, senescent leaves, differentiating xylem and at the abscission zone of flowers. In flowers, expressed in developing anthers and seeds. Accumulates in stigma, mature anthers, sepals, and petals of older/fully opened flowers. Also present in floral organs after fertilization. In mature siliques, observed in abscission zones and in the distal portion of the valve margins. Directly induced by NAC92 during senescence onset. ENDO1 is used to detect mutations generated by ethyl methan sulfonate (EMS) to produce a TILLING (targeting-induced local lesions in genomes) platform (PubMed:17651368, PubMed:18433472). Belongs to the nuclease type I family.