Unique positioning of mitochondria in developing microspores and pollen grains in Pharbitis nil: mitochondria cover the nuclear surface at specific developmental stages

Changes in the number and distribution of mitochondria in microspores and p ollen grains during male gametogenesis in Pharbitis nil were examined with Technovit sections stained with 3,3'-dihexyloxacarbocyanine iodide. The num ber of mitochondria per microspore or pollen grain of P. nil increased cons tantly and dramatically during male gametogenesis. During this process. mit ochondria exhibited characteristic localizations: subpopulations of mitocho ndria covered the surface of the microspore and vegetative nuclei before an d again just after postmeiotic mitosis I (9 and 5 days before flowering, re spectively). The mitochondria also surrounded the generative nucleus 2 days after postmeiotic mitosis I (5 days before flowering), although the densit y of mitochondria on the nuclear surface was lower. Electron microscopy sho wed that the mitochondria were about 30 nm from the nuclear envelope and th at each mitochondrion was located near a nuclear pore. The characteristic l ocalization of mitochondria in P. nil pollen may serve as a model to analyz e the mechanisms that control mitochondrial positioning within a cell and i nteractions between mitochondria and nuclei.