Formation of appressoria by the arbuscular mycorrhizal fungus Gigaspora margarita on roots of Allium cepa is linked with root age

Spores of the arbuscular mycorrhizal fungus, Gigaspora margarita, were placed near the root tip, the middle of the root (equal distance from root base and root tip), or the root base (close to the shoot) of the first primary root of 9-day-old onion. Two weeks later, the number and position of appressoria and the appressoria with penetrating hyphae were determined in the first and the newly formed second primary roots. The total number of appressoria was not significantly different among the treatments. Inoculation near the root tip of the first primary root resulted in the formation of a large number of appressoria on the first primary root and the formation of about three times fewer appressoria on the second primary root. Inoculation near the base of the first primary root resulted in the formation of no appressoria on the first primary root, whereas many appressoria were formed on the second primary root. Our results suggest that the root age is a determinant of the appressorium formation.