POPULATION HETEROGENEITY OF HIGHER-PLANT MITOCHONDRIA IN STRUCTURE AND FUNCTION

Mitochondria of rapidly developing mungbean seedlings were fractionate d into four populations: two density classes, each from a 1500S and a 150S pellet. Each of the four populations exhibited cytochrome c oxida se (COX) activity and contained mitochondrial DNA and cardiolipin; pla stid and glyoxysome content were found to be relatively low Five mitoc hondrial membrane proteins, COXII/III, ATPase alpha/beta and porin, an d a matrix enzyme, manganese superoxide dismutase (MnSOD), were detect ed by immunoblots in all four populations. Another matrix enzyme, pyru vate dehydrogenase was detected only in the two respiratory-competent 1500S populations. The two 150S populations contained a previously uni dentified organelle that lacked demonstrable respiratory capability. T his organelle, which we have tentatively referred to as ''slow-sedimen ting (ss-) mitochondrion'', was small in size (below light-optics reso lution, 70-300 nm, majority less than or equal to 200 nm) and possesse d a peculiar looking boundary membrane, ribosomes, and an occasional p rominent electron-dense spot. Characteristically, ss-mitochondria were almost always in contact with a filament aligned membrane like struct ure of varying length. Cristae structure, while undetected in small ss -mitochondria, appeared in larger individuals. Typical mitochondria we re found in the denser 1500S population, while the lighter 1500S popul ation consisted of 300-800 nm mitochondria exhibiting a varying degree of size-dependent inner membrane folding. Using electron microscopy ( EM) immunolocalization and serial sectioning, we have identified in si tu organelles resembling in size and in fine structure the ss-mitochon dria, which also exhibit a size-dependent folding of the inner membran e. These results suggest that small ss-mitochondria may undergo a prog ressive development in situ. Taken together, our findings demonstrate the existence of a pattern of structure-function-coordinated gross het erogeneity among mitochondria. This pattern of mitochondrial heterogen eity, characterized both in isolated mitochondria and in situ, implies that small ss-mitochondria may represent a type of ''nascent mitochon dria'' derived from a yet unidentified mitochondria-propagation mode o perating during rapid seedling growth. Mitochondrial division by binar y fission, characterized by the appearance of dumbbell-shaped intermed iates, was also detected.