According to the endosymbiotic theory, mitochondria are descended from specialised bacteria (probably purple nonsulfur bacteria) that somehow survived endocytosis by another species of prokaryote or some other cell type, and became incorporated into the cytoplasm [ref].
And plasmids naturally exist in bacterial cells, and they also occur in some eukaryotes [ref].
I was however taught that mitochondria have no plasmid and only have circular DNA. If the endosymbiotic theory is true, then how come mitochondria have no plasmid?