Fungus-mediated plant-pollinator mutualism in Monoon laui (Annonaceae): functional differentiation of stigmatic exudate and petal nectar.

Background: Floral nectar typically functions as a pollinator reward in mutualistic flower-pollinator interactions, with this mutualism sometimes strengthened when plants provide the pollinators with brood sites and larval food as rewards. The functional and molecular mechanisms underpinning such rewards remain unclear.

Results: We present strong circumstantial evidence supporting a fungus-mediated plant-pollinator mutualism in a beetle-pollinated early-divergent angiosperm, Monoon laui (Annonaceae), which has flowers that produce exudates on both the stigmas and inner petals, with fungi that develop on the inner petals subsequently consumed by insect larvae. The identities of the pollinators and larvae, as well as the fungal communities borne on the pollinators and petals, indicate that the pollinators disperse fungi while ovipositing on the petals. The nutritional value of the two exudates reveals that the stigmatic exudate is sugar-rich, whereas the inner petal exudate has a greater amino acid content. Transcriptomic and proteomic comparisons between the two organs and their exudates corroborate the nutritional profiles, with a stronger immune response on stigmas.

Conclusions: Both stigmatic exudate and petal nectar of Monoon laui function as a pollinator reward, while petals with their nectar are moreover critical in the fungus-mediated plant-pollinator mutualism, as they are likely to be closely adapted to the requirements of the pollinators by providing them with brood sites and larval food, thereby increasing their population size during the flowering season and promoting pollination success.