Mitochondria in plant cells, just like in animal cells, originate from pre-existing mitochondria through a process called binary fission. This is similar to how bacteria reproduce. Here's how it works:

1. Replication of Mitochondrial DNA: The circular DNA molecule within the mitochondrion replicates itself.

2. Growth and Division: The mitochondrion grows in size and eventually divides into two daughter mitochondria, each containing a copy of the mitochondrial DNA.

Important Note: Mitochondria have their own DNA, separate from the nuclear DNA in the plant cell. This is because mitochondria were once free-living bacteria that were engulfed by early eukaryotic cells and formed a symbiotic relationship. This theory is known as the endosymbiotic theory.