Polyploidy in tomato roots as affected by arbuscular mycorrhizal colonization

Nuclear changes in roots of tomato (Lycopersicon esculentum), a plant with a small genome, during the establishment of arbuscular mycorrhizal (AM) col onization were studied using light and electron microscopy, as well as flow and static cytometry. Nuclei of mycorrhizal root cortex cells were larger and had more decondensed chromatin than those of controls. Significant ploi dy distribution differences were observed between nuclei of AM colonized an d control roots, and a strong correlation between nuclear polyploidization and AM colonization was found. Polyploidization and decondensation are usua lly associated with high metabolic activity. The metabolic activity of myco rrhizal root cells, evaluated in this work as respiratory activity by using a cytochemical assay for succinate dehydrogenase combined with image analy sis, increased in comparison to controls. The meaning of polyploidization i s discussed in relation to the structural and metabolic modifications induc ed by mycorrhization.