Deep in the Jelly: Histochemical and Functional Aspects of Mucilage-Secreting Floral Colleters in the Orchids Elleanthus brasiliensis and E. crinipes

Abstract

Colleters are trichomes or emergencies that produce a sticky exudate consisting of a mixture of mucilage, lipids, terpenes, and phenolic compounds. Colleters occur in at least 60 families of angiosperms; however, reports of them are scarce for the Orchidaceae. Elleanthus brasiliensis is distinguished by the presence of an abundant gelatinous secretion that covers almost all of its inflorescences. We aimed to describe the histology of colleters in inflorescences of E. brasiliensis and Elleanthus crinipes , and to analyze the chemical composition of their secretion to better understand the functions of these secretory structures. Due to the low frequency of colleters and lack of visible secretion in E. crinipes , histochemical tests and chemical analyses were not performed for this species. Colleters are of a brush type and their secretion has, at the same time, hydrophilic and lipophilic components. Histochemical tests further revealed the presence of pectin, mucilage, lipids, terpenes, phenolic compounds, and proteins. The GC-MS analysis confirmed the presence of γ-sitosterol and palmitic, linoleic, and stearic acids in the secretion of E. brasiliensis . Infrared analysis indicated the possible presence of polysaccharides in the secretion. The occurrence of colleters in both species studied and in other orchids described in the literature suggests that these structures are common in the inflorescences of tropical orchids. In these environments, the hydrated polysaccharides in the secretion form a dense matrix that can act as a physical barrier, and terpenes may help to protect against herbivores and pathogenic microorganisms. This information broadens our knowledge of the morphological and chemical diversity of the secretions produced by orchid colleters.

More than Scales: Evidence for the Production and Exudation of Mucilage by the Peltate Trichomes of Tillandsia cyanea (Bromeliaceae: Tillandsioideae)

“…Histochemical tests confirm mucilaginous secretion and indicate the presence of proteins in the exudate. Secretions of this nature were observed previously in other similar secretory systems, including colleters in species of Orchidaceae [ 31 , 39 ] and several eudicotyledons [ 36 , 51 , 52 ]. The positive reaction provided by ruthenium red in the central portion of the shield indicates the presence of structural acid polysaccharides, rather than accumulated secretion inside cell compartments.…”

“…Many of the features presented by the secretory trichomes of T. cyanea , particularly the restriction of secretory activity to young portions and the mucilaginous nature of the exudate, allow their classification as colleters. This functional concept of colleter has been attributed to numerous secretory systems of this type, especially in the face of the recent descriptions of glands associated with juvenile organs in monocots [ 16 , 31 , 34 , 35 , 38 , 39 ]. Such glands seem to perform the same functions as traditional colleters, but usually lack the typical and relatively more complex structure often found in eudicotyledons [ 36 ].…”

“…A role in protection against desiccation is a recognized function of some exudates in young reproductive axes of angiosperms [ 31 , 34 , 38 , 39 , 52 , 69 ] in which water supply via xylem may be inefficient, and secretions could avoid water loss due to excessive transpiration or increase water uptake through the cuticle [ 45 , 47 , 69 ]. In this context, an additional role in water absorption must be considered concerning the trichomes of T. cyanea due to their similarity with the absorbing scales of bromeliad leaves, with which they share many structural features related to water uptake ability.…”

“…These structures are currently characterized from a functional point of view as glands that produce and release mucilaginous or resinous secretion for protection against desiccation or herbivore/pathogen attack in young parts of the plant [ 16 , 31 , 32 , 33 , 34 , 35 ]. While colleters are widely distributed in eudicotyledons [ 36 ], their presence has been demonstrated in only a few monocot families [ 16 , 31 , 33 , 34 , 35 , 37 , 38 , 39 ].…”

The Renaissance of Plant Mucilage in Health Promotion and Industrial Applications: A Review

“…In flowering plants of the Orchidaceae, mucilage maintains hydration [66]. According to a study by Cassola et al on Elleanthus brasiliensis, mucilage can protect against pathogens and herbivores due to the presence of terpenes and other compounds secreted by the inflorescences [67]. In addition to taking up and storing water and nutrients, including macro-and micronutrients, the mucilages produced by plant roots protect against toxic compounds.…”