NCS2 mutants: ultrastructural observations

NCS2 mutant plants are characterized by pale green stripes on leaves and defective kernel sectors on ears. Specific mitochondrial DNA changes were found to correlate with the phenotypic alterations (Newton and Coe, PNAS 83:7363). Furthermore, specific differences in mitochondrial gene expression are found in mutant plant material (Feiler and Newton, MNL 1987). We were interested in determining whether ultrastructural alterations could be observed in the mutant tissues and organelles. For this initial study, a pale green sector on an NCS2 leaf was compared with a section of a leaf from a normal plant. Both plants were field-grown and leaf samples were chosen at comparable stages of development.

Light and electron microscopic examinations revealed an altered general morphology of affected NCS2 tissue. At the lower magnifications of the light microscope, one can observe abrupt shifts from normal to affected cells. A conspicuous asymmetry in the vascular bundles and bundle sheaths is seen, especially under low magnification of the resin prepared material. This distortion could reflect the actual situation in the plant or it may result from the cells being more fragile and subject to damage by the rather harsh preparation procedures. At higher magnifications (30,000-fold) the internal organization of the organelles can be compared. In mutant cells, the mitochondria have lost most of their internal membrane (cristae) structure. The inner and outer mitochondrial membranes appear to be more closely appressed. Ribosomes appear to be present and many DNA-like fibrils are also observed in the mutant mitochondrial matrix.

Chloroplasts are also affected in pale green sectors of NCS2 plants. Thylakoid membranes are present but there is some loss of stacking and a reduction in stromal constituents. In the bundle sheath cells, normal starch grains are absent and appear to be replaced by osmiophilic bodies.

The maternally inherited NCS2 mutants have altered mitochondria and chloroplasts in affected tissues. The correlation between phenotype and the mitochondrial genome is well established for this mutant. The effect on the chloroplast is presumed to be a physiological effect. It is not yet known whether the mitochondrial ultrastructural alterations are a result of a direct effect on mitochondrial biogenesis or of an indirect physiological effect.

Deborah Thompson and Kathleen Newton

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