Nucleus size in the host cells of an Arbuscular Mycorrhizal system: A mathematical approach to estimate the role of ploidy and chromatin condensation

In order to better understand which are the factors involved in the control of nuclear size, and more precisely, to clarify why nuclear hypertrophy doesn’t always follow genome increase, biological techniques combined with a strict mathematical approach have been applied to study the nuclei of root cortical cells of Lycopersicon escu-lentum Mill cv Early Mech. Tomato is a multiploid plant, with three different levels of DNA content, therefore it is especially suitable to study nucleus size according to genome size. In addition, as arbuscular mycorrhizal colonization strongly affects nucleus organization, part of the plants were inoculated with the arbuscular mycorrhizal fungus Glomus mosseae BEG 12. Quantitative and qualitative nuclear changes have been analyzed in mycorrhizal and non-mycorrhizal roots of tomato, by means of microscopy, immuno-labelling and flow cytometry. The results, supported by mathematical analysis, clearly show that increased ploidy is necessary, but not sufficient, to explain nuclear hypertrophy, in which chromatin decondensation is also involved and related to cell metabolic activity.