Reduced arbuscular mycorrhizal colonization in tomato ethylene mutants

Plant hormones are likely key regulators of arbuscular mycorrhizae (AM) development. However, their roles in AM are not well known. Here Mutants in five hormone classes introgressed in a single tomato (Lycopersicon esculentum Mill. Syn Solanum lycopersicum L.) background (cv. Micro-Torn) were used to determine their effects on AM development and the expression of defense-related genes (chitinases and beta-1,3-glucanases) in roots. Under low P conditions, mutant epinastic (epi) and Never ripe (Nr), ethylene overproducer and low sensitivity, respectively, had the intraradical colonization by Glomus clarum highly inhibited, as compared to the control Micro-Tom (MT). No significant alterations in fungal colonization were observed in mutants affecting other hormone classes. Under low P conditions, the steady state levels of transcripts encoding a class I basic chitinase (chi9) were higher in mycorrhizal epi and Nr mutant roots as compared to MT controls. In contrast the steady state levels of a class III acidic beta-1,3-glucanase (TomPR-Q'a) transcripts in mycorrhizal epi Mutant roots were significantly lower than in mycorrhizal NIT roots. Root colonization in epi mutants was accompanied by several alterations in fungal morphology, as compared to root colonization in NIT controls. The data suggest that ethylene may play an important role in controlling intraradical arbuscular mycorrhizal fungal growth.