Chromatin structure variation in successful and unsuccessful arbuscular mycorrhizas of pea

Lincoln and Frisson varieties of endomycorrhiza-forming pea plants and isogenic mycorrhiza-resistant Frisson mutant (P2) plants were inoculated with Glomus mosseae. Nuclei released from inoculated and non-inoculated (control) roots were analysed for chromatin structure and activity using flow cytometric techniques. Chromatin accessibility to the specific DNA fluorochrome DAPI at saturating and non-saturating concentrations was measured. DNA fluorescence of nuclei of mycorrhizal Lincoln and wild genotype Frisson plants was significantly increased, compared to the controls, at saturating and, more strongly, at non-saturating DAPI concentrations. In contrast, the nuclei of inoculated P2 mutant roots showed a much lower increase in fluorescence, compared to uninoculated controls. Nuclei released from mycorrhiza-infected Lincoln roots were more sensitive to DNase I than those of uninfected ones. These results indicate a dramatic increase in that portion of the genome which can be transcribed in response to AM infection.