Mitochondria are essential and dynamic organelles in eukaryotes. Cardiolipin 
(CL) is a key phospholipid in mitochondrial membranes, playing important roles 
in maintaining the functional integrity and dynamics of mitochondria in animals 
and yeasts. However, CL's role in plants is just beginning to be elucidated. In 
this study, we used Arabidopsis thaliana to examine the subcellular distribution 
of CL and CARDIOLIPIN SYNTHASE (CLS) and analyzed loss-of-function cls mutants 
for defects in mitochondrial morphogenesis and stress response. We show that CL 
localizes to mitochondria and is enriched at specific domains, and CLS targets 
to the inner membrane of mitochondria with its C terminus in the intermembrane 
space. Furthermore, cls mutants exhibit significantly impaired growth as well as 
altered structural integrity and morphogenesis of mitochondria. In contrast to 
animals and yeasts, in which CL's effect on mitochondrial fusion is more 
profound, Arabidopsis CL plays a dominant role in mitochondrial fission and 
exerts this function, at least in part, through stabilizing the protein complex 
of the major mitochondrial fission factor, DYNAMIN-RELATED PROTEIN3. CL also 
plays a role in plant responses to heat and extended darkness, stresses that 
induce programmed cell death. Our study has uncovered conserved and 
plant-specific aspects of CL biology in mitochondrial dynamics and the organism 
response to environmental stresses.