Fruit Rot, Antifungal Defense, and Palatability of Fleshy Fruits for Frugivorous Birds

Abstract

We tested several predictions concerning fungal rot and antifungal defense of vertebrate-dispersed fruit. ( 1) Fungal fruit rot should be generally deterrent to frugivores. But because fungi may vary in their negative effects, antifungal defenses should be allocated in relationship to net effects of particular species on dispersal. (2) Defenses of fruits under selection for high palatability should be directed primarily toward microbes ("microbespecific" defenses). Lower quality fruits, for which high palatability is not crucial to dispersal, may be defended by more general defenses.(3) Preferred fruits should be defended to a lower degree than less preferred fruits, as a result of consistently high dispersal rates (Removal Rate model).We tested our hypotheses using real fruits and artificial-pulp media of six temperate ericaceous plants in feeding trials with captive birds and in fungal growth trials. Plants had been assigned to two temperate fruiting classes (Summer Small-seeded, SS, or Fall Lowquality, FL), and differed in ripening phenology, persistence times, and fungal resistance in the field. We compared feeding rates on fruits rotted by members of a suite of fruit rot fungi. We also determined feeding rates on artificial-pulp media that either contained ripe fruit extracts or did not, and we determined fungal growth rates on identical media. We found that: ( 1) fungal species depressed feeding rates in general accordance with their reported vertebrate toxicity. Relative to nontoxic fungi, toxic fungi were inhibited more strongly by secondary fruit pulp extracts. (2) Artificial-pulp media ofSS fruits were preferred in feeding trials, due both to basic nutritional composition and nondeterrent (microbespecific) secondary defense. (3) In accordance with the Removal-Rate model, artificial-pulp media of the preferred SS fruits showed significant antifungal activity, but at a much lower level than that of FL fruits. Our results suggest that a primary function of secondary chemicals in ripe fruits may be to defend against destructive fungi that compete with seed dispersers for the pulp. Our results generally support the hypothesis that selective pressures may result in ripe fruit defenses that are directed primarily against organisms having strong negative effects upon dispersal. As such, factors influencing risk of pulp consumption by nondispersers may be useful predictors of fruit defense patterns.

Why fruit rots: theoretical support for Janzen's theory of microbe–macrobe competition

“…Janzen [5,6] was perhaps the first to realize their active role through chemical defence of the resource they are exploiting. He developed a much-cited theory that is based on the fact that many vertebrates, including humans, are very sensitive to microbial spoilage of potential foods and find such spoilage highly aversive [7]. For example, cedar waxwings (Bombycilla cedrorum) display a strong preference for uninfected fruits; dropping more microbially infected fruits during their foraging from trees [8].…”

The persistence of ripe fleshy fruits in the presence and absence of frugivores

“…These distinctions are important because of the very different consequences for the plant, the frugivores and the microbes. Rewards for vertebrate dispersal agents are equally attractive to microbes, and many antimicrobial compounds are likely to be equally unattractive to vertebrates, so a trade-off between attracting dispersal agents and avoiding microbial decay may place limits on fruit persistence (Cipollini and Stiles 1993;Cipollini and Levey 1997). The ability to retain ripe fruits in an attractive condition for a prolonged period will be particularly advantageous for plant species that depend on rare or unreliable seed-dispersal agents.…”

The MYB182 Protein Down-Regulates Proanthocyanidin and Anthocyanin Biosynthesis in Poplar by Repressing Both Structural and Regulatory Flavonoid Genes

“…PAs are present in many plants but are especially abundant in trees, where they are found in vegetative tissues such as leaves, bark, and roots (Barbehenn and Constabel, 2011). They are also present in seeds and many types of fruit, where they are thought to discourage premature consumption by frugivores or prevent spoilage by fungi (Cipollini and Stiles, 1993).…”