Roles of hormone signaling on defense responses of invasive Sphagneticola trilobata to pathogen and insect herbivore

Abstract

Invasive species often dominate new environments and alter ecosystem diversity and function. Two key mechanisms proposed to explain this success are the Enemy Release Hypothesis (ERH), which suggests that invasive species escape their natural enemies, and the Novel Weapons Hypothesis (NWH), which posits that they possess unique biochemical traits that give them a competitive advantage over native species. Here, we tested the induced defense responses in a highly invasive plant, Sphagneticola trilobata, against the pathogenic fungi (Rhizoctonia solani) and a lace bug herbivore (Corythucha marmorata). The endogenous contents of five hormones: Jasmonic acid (JA), Ethylene (ET), Salicylic acid (SA), Abscisic acid (ABA), and Gibberellic acid (GA) after treatment with either pathogen or herbivore showed higher hormone levels against pathogen and lower levels against herbivore. Further, expression analyses with 17 (nine JA and eight ET pathway) genes were tested against each pathogen and herbivore treatment. In particular, ETHYLENE RESPONSE FACTOR (ERF) branch of the JA pathway is induced for necrotrophic pathogen. Whereas, the basic helix-loop-helix (bHLH) protein (MYC2) branch is induced for herbivore. The differential hormone levels and expression between treatments could be due to different mode of infection mechanisms used by the fungus (necrotrophy) and an insect (piercing and sucking). Specifically, S. trilobata enhance defenses in the invaded area, and these defenses differ between responses to a pathogen and herbivore. Furthermore, our study further confirms that S. trilobata do not reduce their investment in defenses due to a lack of enemies. It may be an adaptation mechanism to fine-tune their defense mechanism while encountering multiple stresses during invasion process of S. trilobata, particularly against local natural enemies.