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Nectar secretion in a dry habitat: structure of the nectary in two endangered Mexican species of Barkeria (Orchidaceae).

ABSTRACT: Barkeria scandens and B. whartoniana are endangered, endemic taxa from Mexico. They are epiphytes adapted to dry habitats. Since these plants are xerophytic, their flowers were investigated for structural adaptations to nectar secretion. The flowers of both species are structurally similar, and contrary to most claims for the genus, have functional floral nectaries comprising a nectary chamber and a narrow tubular cuniculus. Nectar is present in both these structures, and contains sugars and lipid-like compounds. The nectary tissue is composed of a single-layered epidermis overlying 1-2 layers of subepidermal secretory parenchyma. The outer tangential wall of the epidermal cells is thick and multi-layered, whereas the cuticle, which often shows blistering, is lamellate and possesses micro-channels. Lipid-like material occurs both between the microfibrils of the cell wall and in the micro-channels. Robust secretory tissue, thick cell walls, and lipid-like nectar components limit nectar evaporation. Moreover, the rigidity of the nectary potentially makes it possible for red-flowered B. scandens to switch from entomophily to ornithophily.

SUBMITTER: Stpiczynska M

PROVIDER: S-EPMC8340907 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:BACKGROUND AND AIMS: It had previously been assumed that Maxillaria spp. produce no nectar. However, nectar has recently been observed in Maxillaria coccinea (Jacq.) L.O. Williams ex Hodge amongst other species. Furthermore, it is speculated that M. coccinea may be pollinated by hummingbirds. The aim of this paper is to investigate these claims further. METHODS: Light microscopy, histochemistry, scanning and transmission electron microscopy. KEY RESULTS: This is the first detailed account of nectar secretion in Maxillaria Ruiz & Pav. A 'faucet and sink' arrangement occurs in M. coccinea. Here, the nectary is represented by a small protuberance upon the ventral surface of the column and nectar collects in a semi-saccate reservoir formed by the fusion of the labellum and the base of the column-foot. The nectary comprises a single-layered epidermis and three or four layers of small subepidermal cells. Beneath these occur several layers of larger parenchyma cells. Epidermal cells lack ectodesmata and have a thin, permeable, reticulate cuticle with associated swellings that coincide with the middle lamella between adjoining epidermal cells. Nectar is thought to pass both along the apoplast and symplast and eventually through the stretched and distended cuticle. The secretory cells are collenchymatous, nucleated and have numerous pits with plasmodesmata, mitochondria, rough ER and plastids with many plastoglobuli but few lamellae. Subsecretory cells have fewer plastids than secretory cells. Nectary cells also contain large intravacuolar protein bodies. The floral morphology of M. coccinea is considered in relation to ornithophily and its nectary compared with a similar protuberance found in the entomophilous species M. parviflora (Poepp. & Endl.) Garay. CONCLUSIONS: Flowers of M. coccinea produce copious amounts of nectar and, despite the absence of field data, their morphology and the exact configuration of their parts argue strongly in favour of ornithophily.

Project description:BackgroundNectar is a major floral attractant and reward for insects that ensures pollination. Liriodendron, a genus of the Magnoliaceae family, includes only two relict species, L. chinense and L. tulipifera, which are considered "basal angiosperms" according to plant evolutionary history. The flowers of Liriodendron plants are insect pollinated and secrete nectar to attract pollinators. To date, the morphology and anatomy of nectaries, the mechanism of nectar secretion and the molecular mechanism of nectary development in Liriodendron remain poorly understood.MethodsIn this study, we examined the nectary surface cells and change in starch in L. tulipifera by using scanning electron microscopy and periodic acid-Schiff techniques to select appropriate samples for subsequent research. Transcriptome sequencing was of the top and middle parts of immature nectaries and the middle part of mature and postsecretory nectaries in L. tulipifera was performed. We evaluated the expression profiles of 21 DEGs that are closely related to nectary development and nectar secretion for real-time quantitative PCR analysis.ResultsL. tulipifera nectaries are starch-storing nectaries and are located in the top and middle parts of L. tulipifera petals. After analyzing the RNA-seq data, we obtained 115.26 Gb of clean data in 12 libraries and mapped the results to the L. chinense reference genome with 71.02-79.77% efficiency. In total, 26,955 DEGs were identified by performing six pairwise comparisons. The flavonoid biosynthesis, phenylpropanoid biosynthesis, anthocyanin biosynthesis and starch and sucrose metabolism pathways were enriched and related to nectar secretion and pigment change. We identified 56 transcription factor families, and members of the TCP, Trihelix, C2H2, ERF, and MADS families changed dynamically during nectary development. Moreover, to further verify the accuracy of the RNA-seq results, we validated the expression profiles of 21 candidate genes.ConclusionsWe evaluated the nectary development and secretion processes comprehensively and identified many related candidate genes in L. tulipifera. These findings suggest that nectaries play important roles in flavonoid synthesis and petal color presentation.

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Nectar secretion in a dry habitat: structure of the nectary in two endangered Mexican species of Barkeria (Orchidaceae).

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