Secondary metabolites in a neotropical shrub: spatiotemporal allocation and role in fruit defense and dispersal

Abstract

Deciphering the ecological roles of plant secondary metabolites requires integrative studies that assess both the allocation patterns of compounds and their bioactivity in ecological interactions. Secondary metabolites have been primarily studied in leaves, but many are unique to fruits and can have numerous potential roles in interactions with both mutualists (seed dispersers) and antagonists (pathogens and predators). We described 10 alkenylphenol compounds from the plant species Piper sancti-felicis (Piperaceae), quantified their patterns of intraplant allocation across tissues and fruit development, and examined their ecological role in fruit interactions. We found that unripe and ripe fruit pulp had the highest concentrations and diversity of alkenylphenols, followed by flowers; leaves and seeds had only a few compounds at detectable concentrations. We observed a nonlinear pattern of alkenylphenol allocation across fruit development, increasing as flowers developed into unripe pulp then decreasing as pulp ripened. This pattern is consistent with the hypothesis that alkenylphenols function to defend fruits from pre-dispersal antagonists and are allocated based on the contribution of the tissue to the plant's fitness, but could also be explained by non-adaptive constraints. To assess the impacts of alkenylphenols in interactions with antagonists and mutualists, we performed fungal bioassays, field observations, and vertebrate feeding experiments. In fungal bioassays, we found that alkenylphenols had a negative effect on the growth of most fungal taxa. In field observations, nocturnal dispersers (bats) removed the majority of infructescences, and diurnal dispersers (birds) removed a larger proportion of unripe infructescences. In feeding experiments, bats exhibited an aversion to alkenylphenols, but birds did not. This observed behavior in bats, combined with our results showing a decrease in alkenylphenols during ripening, suggests that alkenylphenols in fruits represent a trade-off (defending against pathogens but reducing disperser preference). These results provide insight into the ecological significance of a little studied class of secondary metabolites in seed dispersal and fruit defense. More generally, documenting intraplant spatiotemporal allocation patterns in angiosperms and examining mechanisms behind these patterns with ecological experiments is likely to further our understanding of the evolutionary ecology of plant chemical traits.

Comparative Metabolomics of Fruits and Leaves in a Hyperdiverse Lineage Suggests Fruits Are a Key Incubator of Phytochemical Diversification

“…While sharing this general infructescence morphology, Piper species vary in growth form, from herbs and vines to shrubs and small trees (Gentry, 1990;Dyer and Palmer, 2004), as well as in shade tolerance, fruit size, seed number, and reproductive phenology (Greig, 1993a,b). Fruit antagonists of Piper include insect seed predators, which have been found to consume up to 87% of seeds (Greig, 1993a), and a largely uncharacterized suite of pathogens, which rapidly attack fruit upon ripening (Thies and Kalko, 2004;Whitehead and Bowers, 2014;Maynard et al, 2020). Leaves of Piper are subject to herbivory from a broad array of arthropods, including a genus of specialist geometrid moths, Eois, estimated to include over 1,000 species in the Neotropics (Brehm et al, 2016), as well as other geometrid moths, coleopterans, and orthopterans (Dyer and Palmer, 2004).…”

“…Secondary metabolites endemic to fruits, and with demonstrated functional significance in seed dispersal and/or fruit defense, have been shown in several systems, including iridoid glycosides in honeysuckles (Whitehead and Bowers, 2013a,b), capsaicinoids in Capsicum (Suzuki and Iwai, 1984;Tewksbury and Nabhan, 2001;Tewksbury et al, 2008), and amides and alkenylphenols in Piper (Whitehead et al, 2013(Whitehead et al, , 2016Whitehead and Bowers, 2014;Maynard et al, 2020). For capsaicinoids in Capsicum and alkenylphenols in Piper, the entire class of compounds is synthesized only in the fruits (Suzuki and Iwai, 1984;Maynard et al, 2020). Overall, these studies suggest that unique and potentially contrasting selective pressures on fruits may be an important factor shaping phytochemical diversification in plants.…”

Comparative metabolomics of fruits and leaves in a hyperdiverse lineage suggests fruits are a key incubator of phytochemical diversification

“…Fruits of Neotropical Piper are borne on distinct spike-shaped infructescences that are dispersed primarily by bats of the genus Carollia (Phyllostomidae). Fruit antagonists of Piper include insect seed predators, which have been found to consume up to 87% of seeds (Greig, 1993), and a largely uncharacterized suite of pathogens, which rapidly attack fruit upon ripening (Thies & Kalko, 2004;Whitehead & Bowers, 2014;Maynard et al, 2020). Leaves of Piper are subject to herbivory from a broad array of arthropods, including a genus of specialist geometrid moths, Eois, estimated to include over 1,000 species in the Neotropics (Brehm et al, 2016), as well as other geometrid moths, coleopterans, and orthopterans (Dyer & Palmer, 2004).…”

“…Ripe fruits were distinguished by a distinct softening and swelling of the fruit along an infructescence combined with a partial senescence of the infructescence from the branch (presumably to allow bats to easily remove the entire infructescence in flight). In most Piper species included in this study, one or a few infructescences ripen per day per plant during the fruiting period, and the vast majority of these are removed on the same night of ripening by bats (Thies & Kalko, 2004;Maynard et al, 2020). Those that are not removed rapidly decompose; therefore, we always took care to collect freshly-ripened infructescences.…”

“…Secondary metabolites endemic to fruits, and with demonstrated functional significance in seed dispersal and/or fruit defense, have been shown in several systems, including iridoid glycosides in honeysuckles (Whitehead & Bowers, 2013a, 2013b, capsaicinoids in Capsicum (Suzuki & Iwai, 1984;Tewksbury & Nabhan, 2001;Tewksbury et al, 2008), and amides and alkenylphenols in Piper (Whitehead et al, 2013(Whitehead et al, , 2016Whitehead & Bowers, 2014;Maynard et al, 2020). Further, capsaicinoids in Capsicum and alkenylphenols in Piper are synthesized only in the fruits of these taxa (Suzuki & Iwai, 1984;Maynard et al, 2020). Overall, these studies suggest that unique and potentially contrasting selective pressures on fruits may be an important factor shaping phytochemical diversification in plants.…”