Arbuscular mycorrhizal fungi induce polyploidization in tomato roots.

BERTA, GRAZIELLA¹, FUSCONI, ANNA², SAMPò, SIMONETTA¹, PERTICONE, SONIA¹, LINGUA, GUIDO¹.

¹ Dipartimento di Scienze e Tecnologie Avanzate dell'Università di Torino, corso Borsalino 54, 15100 Alessandria, ITALY. ²Dipartimento di Biologia Vegetale dell'Università di Torino, viale Mattioli 25, 10125 Torino, ITALY.

Significant nuclear hypertrophy is one of the cytological modifications induced in the host by arbuscular mycorrhizal (AM) fungi. Hitherto, it was explained only by chromatin decondensation, which is particular characteristic of arbuscule-containing cells, suggesting that the presence of the fungus stimulates and/or increases transcription in the host nuclei. However, cytomolecular analyses have revealed that in tomato (Lycopersicon esculentum) roots, AM fungi also induce polyploidization as well as chromatin decondensation.
Tomato, like other plants with a small genome, is multiploid, with 2C, 4C and 8C populations of nuclei. Flow cytometry of nuclei extracted from the differentiated portion of the roots showed that the presence of AM fungi increases the number of 8C nuclei, altering the proportion of the three populations. Static microfluorimetry of cortical cells has confirmed these results and given measurements of the DNA content in the cells directly involved in the symbiosis. In the colonised cells of mycorrhizal plants, about 90 % of nuclei have a 8C DNA content, whilst the 2C population is almost lacking. These results show, for the first time, that an AM fungus can induce ploidy variations in the host cells, probably acting on the passage between the post-synthetic phase (G2) and mitosis. This is in agreement with the recent report of enhanced transcription in AM roots of a recently isolated gene, coding for a protein involved in the control of the transition to the synthetic phase of the cell cycle.
It is well known that DNA replication activates previously inactive genes and reprograms the genome. Decondensed chromatin and increased ploidy may thus be complementary means by which cells increase and modify their metabolic activity.