Abstract
Western flower thrips (WFT), Frankliniella occidentalis, is a serious insect pest of Chrysanthemum [Chrysanthemum × morifolium Ramat. (Asteraceae)]. Here we have investigated whether genotypic variation in constitutive and inducible resistance to WFT correlates with phenotypic differences in leaf trichome density and the activity of the defense-related enzyme polyphenol oxidase (PPO) in chrysanthemum. Non-glandular and glandular leaf trichome densities significantly varied among ninety-five chrysanthemum cultivars. Additional analyses in a subset of these cultivars, differing in leaf trichome density, revealed significant variation in PPO activities and resistance to WFT as well. Constitutive levels of trichome densities and PPO activity, however, did not correlate with chrysanthemum resistance to WFT. Further tests showed that exogenous application of the phytohormone jasmonic acid (JA) increased non-glandular trichome densities, PPO activity and chrysanthemum resistance to WFT, and that these effects were cultivar dependent. In addition, no tradeoff between constitutive and inducible resistance to WFT was observed. JA-mediated induction of WFT resistance, however, did not correlate with changes in leaf trichome densities nor PPO activity levels. Taken together, our results suggest that chrysanthemum can display both high levels of constitutive and inducible resistance to WFT, and that leaf trichome density and PPO activity may not play a relevant role in chrysanthemum defenses against WFT.
Highlights
Plants have evolved sophisticated constitutive and inducible defenses to defend themselves against arthropod herbivores
Our study has demonstrated that there is great variation in both constitutive and jasmonic acid (JA)-inducible resistance to Western flower thrips (WFT) among chrysanthemum cultivars, opening new avenues for pest-resistance breeding in this horticultural species
We found that this variation could not be explained by differences in leaf trichome densities or polyphenol oxidase activity, which are relevant plant defenses in other plant-herbivores interactions systems
Summary
Plants have evolved sophisticated constitutive and inducible defenses to defend themselves against arthropod herbivores These defenses can be physical structures (e.g., trichomes, thorns, spines) and/or chemical components (e.g., secondary metabolites and defense-related enzymes) present in the plant prior to herbivory (Rosner and Hannrup 2004; Franceschi et al 2005; Mithöfer and Boland 2012) or induced upon herbivore attack (Erb et al 2012). Both the expression of constitutive and inducible defenses can vary within and among plant species (Underwood et al 2000; Koornneef et al 2004; Zhang et al 2020). Identification and characterization of novel sources of pest resistance is a fundamental step to transfer these defenses into susceptible plant varieties through plant breeding programs (Macel et al 2019; Visschers et al 2019).
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