MYCORRHIZA-INDUCED ALLEVIATION OF PLANT DISEASE CAUSED BY CLAVIBACTER MICHIGANENSIS
SUBSP MICHIGANENSIS AND ROLE OF ETHYLENE IN MYCORRHIZA-INDUCED RESISTANCE IN TOMATO
The protective role of arbuscular mycorrhizal fungi (AMF) against the phytopathogen
Clavibacter michiganensis subsp. michiganensis (Cmm) was examined in tomato plants.
Seven different AMF isolates were used to determine which ones were able to induce
effectively resistance against Cmm. Stems of seven-week tomato plants were infected
with Cmm, then a disease severity index (DSI) was determined during the next three
weeks. In addition to different responses to mycorrhizal inoculation, three levels
of responses to the bacterial disease were recognized in treatments. Plants inoculated
with Rhizophagus irregularis (Ri) showed both the highest colonization and the highest
induced resistance to Cmm while the effect of Funneliformis mosseae, Gigaspora margarita
and Claroideoglomus claroideum on mycorrhizal colonization and on the induced resistance
were intermediate and high, respectively. Subsequently, Ri was chosen to inoculate
ethylene-insensitive tomato mutant line Never ripe (Nr) and its background (Pearson)
to investigate the possible role of ethylene (ET) in the myconbiza-induced resistance
(MIR). The results showed that Ri could induce systemic resistance against Cmm in
the Pearson background, whereas ET-insensitivity in Nr plants impaired MIR. These
results suggest that ET is required for Ri-induced resistance against Cmm. To our
knowledge, this is the first study to examine the effect of different AMF isolates
on the response of tomato plants to Cmm and involvement of ET in MIR against Cmm.