A functional role for the colleters of coffee flowers

Colleters are protuberances or trichomes that produce and release an exudate that overlays vegetative or reproductive buds. Colleters have a functional definition, as they are thought to protect young tissues against dehydration and pest attack. Decaffeinated coffee plants, named Decaffito, have recently been obtained through chemical mutagenesis, and in addition to the absence of the alkaloid, the flowers of these plants open precociously. Decaffito mutants exhibit minimal production and secretion of the exudate by the colleters. We compared these mutants with normal coffee plants to infer the functional role of colleters and the secreted exudate covering flower buds. Decaffito mutants were obtained by sodium azide mutagenesis of Coffea arabica cv. Catuaı́ seeds. Wild-type plants were used as controls and are referred to as Catuaı́. The flower colleters were analysed by scanning and transmission electron microscopy in addition to histochemical analysis. Histochemical analysis indicated the presence of heterogeneous exudate in the secretory cells of the colleters of both variants of coffee trees. Alkaloids were detected in Catuaı́ but not in Decaffito. Transmission electron microscopy revealed that the secretory cells in the Catuaı́ colleters possessed the normal and common characteristics found in secretory structures. In the secretory cells of the Decaffito colleters, it was not possible to identify any organelles or even the nucleus, but the cells had a darkened central cytoplasm, indicating that the secretion is produced in low amounts but not released. Our results offer a proof of concept of colleters in coffee, strongly indicating that the exudate covering the flower parts works as an adhesive to keep the petals together and the flower closed, which in part helps to avoid dehydration. Additionally, the exudate itself helps to prevent water loss from the epidermal cells of the petals.