Abstract
Helianthus annuus, the common sunflower, produces a complex array of secondary compounds that are secreted into glandular trichomes, specialized structures found on leaf surfaces and anther appendages of flowers. The primary components of these trichome secretions are sesquiterpene lactones (STL), a diverse class of compounds produced abundantly by the plant family Compositae and believed to contribute to plant defense against herbivory. We treated wild and cultivated H. annuus accessions with exogenous methyl jasmonate, a plant hormone that mediates plant defense against insect herbivores and certain classes of fungal pathogens. The wild sunflower produced a higher density of glandular trichomes on its leaves than the cultivar. Comparison of the profiles of glandular trichome extracts obtained by liquid chromatography–mass spectroscopy (LC-MS) showed that wild and cultivated H. annuus were qualitatively similar in surface chemistry, although differing in the relative size and proportion of various compounds detected. Despite observing consistent transcriptional responses to methyl jasmonate treatment, we detected no significant effect on glandular trichome density or LC-MS profile in cultivated or wild sunflower, with wild sunflower exhibiting a declining trend in overall STL production and foliar glandular trichome density of jasmonate-treated plants. These results suggest that glandular trichomes and associated compounds may act as constitutive defenses or require greater levels of stimulus for induction than the observed transcriptional responses to exogenous jasmonate. Reduced defense investment in domesticated lines is consistent with predicted tradeoffs caused by selection for increased yield; future research will focus on the development of genetic resources to explicitly test the ecological roles of glandular trichomes and associated effects on plant growth and fitness.
Highlights
Plants possess a variety of constitutive and induced defenses against herbivores and pathogens [1]
For ANN1238, mid-leaf glandular trichome density estimates were similar to those from the lower leaf edge, while estimates from the mid-leaf of HA89 were lower than glandular trichome density estimates from the leaf tip
Weedy H. annuus occupying agricultural habitats have shown transcriptional downregulation of genes associated with defense or biotic stress tolerance when compared to wild populations [51]
Summary
Plants possess a variety of constitutive and induced defenses against herbivores and pathogens [1]. Induced defenses are chemical or physical alterations in the plant, following an initial attack or stimulus, that affect subsequent herbivore or pathogen activity. Constitutive defenses are expected to be most important when pest pressure is high and fairly constant, while induced defenses are predicted to play a larger role when pest populations are variable and the costs of resistance are high [2,3,4]. Domesticated species have originated rapidly and recently and often exhibit a reduction in physical and chemical defenses [5]. Modern improvement programs attempt to re-introduce resistance to specific pests, which may mask growth-defense trade-offs initially associated with domestication
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