Guardians within: entomopathogenic fungi-driven antibiosis and compensatory growth combines to reduce the damage of herbivores to melon plants

Endophytic and rhizosphere-competent entomopathogenic fungi (EF) are important plant bodyguards, although the mechanisms underlying this phenomenon are poorly understood. Therefore, we aimed to elucidate the roles of antibiosis (lethal and sublethal effects), and potential growth compensation (in response to leaf injury) in melon plants exposed to cotton leafworm. Plants were inoculated with one of three EF strains (EAMa 0158-Su Metarhizium brunneum strain or EABb 04/01-Tip and EABb 01/33-Su Beauveria bassiana strains) by either foliar spray, seed dressing or soil drenching and then challenged with either multiple short-term, or single long-term Spodoptera littoralis larval infestation. Endophytic colonization and relative expression of plant defense genes were tracked using molecular techniques alongside evaluation of antibiosis effects on S. littoralis and plant tolerance to larval-induced leaf injuries. Inoculated plants exhibited antibiosis and potential growth compensation in responses to various S. littoralis challenges, which resulted in increased fresh and dry weight, chlorophyll content, number of secondary branches and stem diameter. Furthermore, up-regulation in the relative expression of ethylene (ACO1, ACO3, EIN2, EIN3) and jasmonic acid (LOX2)-related genes were observed, with the endophytic B. bassiana- induction of ethylene and jasmonic acid production being higher in S. littoralis infested plants. Our findings strongly confirm the EF multifunctionality and the involvement of the Endophytic EF triggered melon defensive system induction in the antibiosis and compensatory growth to protect melon plants from pest damage.