Nuclear changes in pathogen-infected tomato roots

We have investigated nuclear changes induced in tomato (Lycopersicon escule ntum) plant roots by two different pathogens, a phytoplasma of the stolbur group and the soil-borne fungus Phytophthora nicotianae var. parasitica, us ing light and electron microscopy as well as flow cytometry, Our results sh ow that both pathogens strongly influenced nuclear structure, causing pykno sis and chromatolysis, and induced variations in proportion of nuclear popu lations with different DNA content, increasing the 2C and decreasing the 4C nuclear populations. These latter results suggest a block of the endoredup lication process in the presynthetic phase of the host cell cycle. Moreover , in pathogen-infected roots, nuclei of the same DNA content were less fluo rescent, and therefore had a lower DNA content, compared to those of contro ls, as confirmed by morphological analyses. Our findings suggest that diffe rent stimuli may evoke similar nuclear changes, and confirm the usefulness of flow cytometry as a tool to recognize the stress conditions related to t he presence of a pathogen.