Microscopy of a cytoplasmic male-sterile soybean from an interspecific cross between Glycine max and G. soja (Leguminosae).

Cytoplasmic male sterility has been found independently in soybean three times since 1995, but no microscopic investigation has been published. The purpose of this microscopic study was to establish the developmental sequence leading to sterility in a cytoplasmic male-sterile soybean line that has been found to be stable under all environmental conditions tested and to demarcate the temporal and spatial parameters that result in degeneration of the microspores and pollen grains. Light microscopy showed an abnormal development and/or premature degeneration of the tapetum after meiosis II, but some pollen grains persisted until after microspore mitosis. The pollen grains never completely filled with reserves. Premature formation of the endothecium also was evident. Histochemical staining for water-insoluble carbohydrates revealed an abnormal pattern of starch deposition in anther walls that coincided with lack of pollen filling. Electron microscopy showed degeneration of the inner mitochondrial membrane in the tapetal cells as the first detectable change leading to cell degeneration. Subsequently, the tapetal endoplasmic reticulum exhibited atypical concentric rings. Pollen grains displayed mitochondria with unusually enlarged inner mitochondrial spaces, degraded plastids, a rudimentary intine, and no starch or lipid reserves. Results link mitochondrial degeneration, premature formation of the endothecium, and energy deprivation to male sterility.