Colonization by arbuscular mycorrhizal and endophytic fungi enhanced terpene production in tomato plants and their defense against a herbivorous insect

Terpenoids serve as an important form of chemical defense for plants. A greenhouse study was conducted to investigate the effects of two types of beneficial fungi on the accumulation of terpenoids in tomato plants and on defense against herbivorous insects. Control tomato plants without any fungal inoculation constitutively made monoterpenes and sesquiterpenes. Inoculation by Rhizophagus intraradices (N.C. Schenck & G.S. Sm.) C. Walker & A. Schussler, an arbuscular mycorrhizal fungus, and Beauveria bassiana (Bals.-Criv.) Vuill., an endophytic entomopathogenic fungus, individually or in combination, led to enhanced levels of monoterpenes and sesquiterpenes, which included new monoterpenes not found in the control plants. Herbivore feeding assays using beet armyworm (Spodoptera exigua Hubner) were performed to compare the levels of defense in tomato plants with or without fungal inoculation. Beet armyworm larvae fed on tomato plants inoculated by either or both types of fungi were found to gain significantly less weight than those fed on control non-inoculated plants. This suggests that fungus-inoculated tomato plants had a stronger defense response against beet armyworm than control plants, which may be partly attributed to the difference in the levels of terpenoids.