Nuclear changes in pathogen-infected tomato roots

We have investigated nuclear changes induced in tomato (Lycopersicon esculentum) root plants by two different pathogens, a phytoplasma of the stolbur group and the soil-borne fungus Phytophthora nicotianae var. parasitica, using light and electron microscopy as well as flow cytometry. Our results show that both pathogens strongly influenced nuclear structure, causing pyknosis and chromatolysis, and induced variations in proportion of nuclear populations with different DNA content, increasing the 2C and decreasing the 4C nuclear populations. These latter results suggest a block of the endoreduplication process in the presynthetic phase of the host cell cycle. Moreover, in pathogen-infected roots, nuclei of the same DNA content were less fluorescent, and therefore had a lower DNA content, compared to those of controls, as confirmed by morphological analyses. Our findings suggest that different stimuli may evoke similar nuclear changes, and confirm the usefulness of flow cytometry as a tool to recognize the stress conditions related to the presence of a pathogen.