Evaluating the Role of Endophyte-Rich Leaves in Protecting Tropical Trees Against a Generalist Herbivore and a Pathogen

Abstract

1.Plants use chemical compounds and physical barriers to defend themselves against natural enemies. For instance, tough leaves are considered to be better defended than soft leaves and we can measure this spectrum of defenses via leaf functional traits. Leaf traits such as leaf chemistry, lifespan, toughness, and leaf mass per area often are a plants first line of defense. Plants with longer lifespans that invest more in leaf tissue and higher leaf mass per area (LMA) typically feature robust constitutive defenses (e.g., leaf toughness, thickness, and dense cell walls) as well. In contrast, plants that invest more in leaf nutrients and have low LMA often invest more in induced defenses. Whether constitutive or induced, leaf traits represent an environmental filter for foliar endophytic fungi (FEF), which may play an additional role in plant defense. Our overarching assumption is that FEF alter leaf fate by interacting directly or indirectly with leaf traits, thus reshaping successive FEF colonization, development of leaf traits and response to plant enemies. To evaluate this hypothesis, we inoculated seedlings of seven tropical tree species that varied in leaf traits with natural and diverse endophyte communities. We confirmed the success of our inoculations of low FEF load (E-) and high FEF load (E+) treatment groups via culturing and culture-free amplicon sequencing. We then measured leaf removal by leaf-cutter ants and leaf necrosis due to a generalist fungal pathogen. Across the experiment, we observed greater herbivory in the E- treatment compared to the E+ treatment, but no difference in pathogen damage. However, within E- and E+ treatment groups, leaves exposed to pathogen had greater leaf damage than non-exposed leaves. Dimensionality reduction of leaf functional traits (i.e., LMA, toughness, thickness, and anthocyanin levels) revealed relationships among traits and distinct host species characteristics. All leaf functional traits had significant correlations with FEF community composition. In turn, indicator species analyses reveal functional traits and taxonomic identities of FEF associated with high- and low leaf damage by natural enemies, suggesting new insights into cryptic roles of foliar symbionts in extending and modifying plant defenses in tropical forests.