Mitochondria are pleomorphic organelles capable of constant fusion and fission. Live fluorescent microscopy was employed to investigate mitochondrial pleomorphy during fluctuations in light, sugar and O2. Light and sugar are shown to induce fission whereas a green-to-red photo-convertible mEos fluorescent protein targeted to mitochondria (mitoEos) reveals that hypoxia induces fusion, leading to giant mitochondria. The endoplasmic reticulum (ER) is shown to be a mediator of mitochondrial fission and acts as a mould for morphological transitions displayed by mitochondria. Simultaneous live-imaging also reveals sustained mitochondria-peroxisome interactions, which are apparent in anisotropy1, a cell wall mutant shown here to be light sensitive. Triple transgenic lines were created to differentially label mitochondria, peroxisomes and the ER and show that the ER cages the other organelles including chloroplasts during these sustained interactions. Together, simultaneous live-imaging of mitochondria, peroxisomes and the ER using double and triple Arabidopsis thaliana transgenics further illuminate organelle pleomorphy and interactivity.