Tomato ethylene mutants exhibit differences in arbuscular mycorrhiza development and levels of plant defense-related transcripts

The role of ethylene on arbuscular mycorrhiza (AM) development remains unclear. We used tomato ethylene mutants in near isogenic genotypes to evaluate AM development and growth differences and whether changes in levels of plant defense-related transcripts in roots were correlated with those differences. The cultivar Micro-Tom (MT) and mutants overproducing ethylene (epinastic, epi) and insensitive to ethylene (Never-ripe, Nr) were inoculated with Glomus clarum. The epi mutants exhibited reduced intraradical colonization at all time-points and reduced frequencies of hyphopodia and arbuscules at 30 and 37 days after inoculation (DAI) compared to MT. At these times and 23 DAI, epi roots contained higher levels of mRNAs encoding Chi3 and Chi9 chitinases, Cu-Zn superoxide dismutase, catalase and a peroxidase than MT roots. In contrast, Nr showed higher intraradical colonization and frequencies of arbuscules, external hypha and hyphopodia but only at 23 DAI. At the preceding 16 DAI time point, levels of antioxidant enzyme mRNAs were higher while Chi3, Chi9 and GlucB mRNAs were lower than in MT roots. Our study provides evidence that specific stages of AM development were altered negatively in epi and positively in Nr. These effects on fungal growth are most likely due to altered ethylene signaling modulating the plant defense system.