COS 190-1 - CANCELLED - Interactions between functional leaf traits and foliar endophytes in defense of tropical trees against natural enemies

Abstract: Leaf traits such as leaf chemistry, lifespan, toughness, and leaf mass per area (LMA) are a plant’s first line of defense against natural enemies. Plants that invest more in leaf tissue with longer lifespans and higher LMA consequently must invest in constitutive defenses (e.g., leaf toughness, thickness, and dense cell walls) as well. In contrast, plants that invest little in leaf nutrients and have low LMA may have greater levels of induced defenses. Together, leaf traits, whether constitutive or induced, represent an environmental filter for leaf microbial communities, especially foliar fungal endophytes (FEF), which may in turn play a role in plant defense.

Our overarching hypothesis is that FEF alter leaf defenses via tripartite interactions among FEF, leaf traits and plant enemies. To evaluate this hypothesis, we inoculated seedlings from seven tropical tree species with natural and diverse endophyte communities. We confirmed that endophyte inoculation was successful and described endophyte community structure via culturing and culture-free amplicon sequencing. We then measured leaf removal by leafcutter ants (Atta colombica) and leaf necrosis due to a generalist fungal pathogen (Calonectria sp.). We predicted that (1) A. colombica would remove less plant material from leaves with higher endophyte loads (i.e., higher endophyte abundance and richness) compared to leaves with lower endophyte loads, once variation in leaf traits was taken into account; (2) pathogen damage would decrease in leaves with high (relative to low) endophyte loads; and (3) FEF-mediated defense against both herbivores and pathogens will be most important in short lived leaves, as long-lived leaves are expected to rely more on constitutive defenses.

In contrast to our predictions, leaves of all tree species with high FEF abundance had no significant difference in leaf area removed by leaf-cutting ants relative to leaves with low FEF abundance. Likewise, pathogen damage did not differ meaningfully as a function of FEF abundance. Dimensionality reduction for leaf traits revealed correlations among LMA, toughness, thickness, and anthocyanin levels. Previous research showed that FEF abundance, diversity, and composition vary as a function of these leaf traits, suggesting that FEF community traits should be integrated into such models to understand their relative importance in defining leaf defense. 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.