True nectar or stigmatic secretion? Structural evidence elucidates an old controversy regarding nectaries in Anthurium

Anatomical, histochemical, and developmental approaches reveal the long-term functioning of the floral nectary in Tocoyena formosa (Rubiaceae)

Tocoyena formosa has a persistent floral nectary that continues producing nectar throughout flower and fruit development. This plant also presents an intriguing non-anthetic nectary derived from early-developing floral buds with premature abscised corolla. In this study, we characterize the structure, morphological changes, and functioning of T. formosa floral nectary at different developmental stages. We subdivided the nectary into four categories based on the floral and fruit development stage at which nectar production started: (i) non-anthetic nectary; (ii) anthetic nectary, which follows the regular floral development; (iii) pericarpial nectary, derived from pollinated flowers following fruit development; and (iv) post-anthetic nectary that results from non-pollinated flowers after anthesis. The nectary has a uniseriate epidermis with stomata, nectariferous parenchyma, and vascular bundles, with a predominating phloem at the periphery. The non-anthetic nectary presents immature tissues that release the exudate. The nectary progressively becomes more rigid as the flower and fruit develop. The main nectary changes during flower and fruit development comprised the thickening of the cuticle and epidermal cell walls, formation of cuticular epithelium, and an increase in the abundance of calcium oxalate crystals and phenolic cells near the vascular bundles. Projections of the outer periclinal walls toward the cuticle in the post-anthetic nectary suggest nectar reabsorption. The anatomical changes of the nectary allow it to function for an extended period throughout floral and fruit development. Hence, T. formosa nectary is a bivalent secretory structure that plays a crucial role in the reproductive and defensive interactions of this plant species.

First Report of Colleters in Araceae: A Case Study in Anthurium andraeanum Reveals Diverse Mucilage Glands Associated with the Developing Shoot

Araceae comprises a diverse group of plants that grow in various habitats, ranging from submerged aquatics to lithophytes. Thus, aroids are likely to show diverse glands acting in several plant-environment interactions, including colleters that protect young shoots. Based on this premise and the lack of studies regarding secretory structures in Araceae, we employed standard light and electron microscopy methods to test the hypothesis that colleters are present in Anthurium. Our main goals were to identify mucilage glands in A. andraeanum by conducting a detailed anatomical study of their structure, ultrastructure, and secretory activity. We found finger-like colleters in the apex of young leaves, spathes, and unexpanded cataphylls as well as secreting zones at the apex of expanded cataphylls, at the margins of non-fused cataphylls, and throughout the keels in two-keeled cataphylls. The colleters develop precociously and senesce shortly afterwards. Ultrastructural data and histochemistry confirmed the production of a polysaccharide-rich secretion that fills the spaces within the developing shoot. As far we know, this is the first time that colleters have been reported for Araceae. The functional roles of the secretion and the position of finger-like colleters concerning the 'precursor tip' of monocotyledons are discussed. Future research correlating secretory activity in colleters of species from different habitats might reveal a great diversity of mucilage glands with ecological and evolutionary significance to the family.

Transcriptomic analysis of deceptively pollinated Arum maculatum (Araceae) reveals association between terpene synthase expression in floral trap chamber and species-specific pollinator attraction

Deceptive pollination often involves volatile organic compound emissions that mislead insects into performing nonrewarding pollination. Among deceptively pollinated plants, Arum maculatum is particularly well-known for its potent dung-like volatile organic compound emissions and specialized floral chamber, which traps pollinators-mainly Psychoda phalaenoides and Psychoda grisescens-overnight. However, little is known about the genes underlying the production of many Arum maculatum volatile organic compounds, and their influence on variation in pollinator attraction rates. Therefore, we performed de novo transcriptome sequencing of Arum maculatum appendix and male floret tissue collected during anthesis and postanthesis, from 10 natural populations across Europe. These RNA-seq data were paired with gas chromatography-mass spectrometry analyses of floral scent composition and pollinator data collected from the same inflorescences. Differential expression analyses revealed candidate transcripts in appendix tissue linked to malodourous volatile organic compounds including indole, p-cresol, and 2-heptanone. In addition, we found that terpene synthase expression in male floret tissue during anthesis significantly covaried with sex- and species-specific attraction of Psychoda phalaenoides and Psychoda grisescens. Taken together, our results provide the first insights into molecular mechanisms underlying pollinator attraction patterns in Arum maculatum and highlight floral chamber sesquiterpene (e.g. bicyclogermacrene) synthases as interesting candidate genes for further study.

Colleters, Extrafloral Nectaries, and Resin Glands Protect Buds and Young Leaves of Ouratea castaneifolia (DC.) Engl. (Ochnaceae)

Buds usually possess mechanical or chemical protection and may also have secretory structures. We discovered an intricate secretory system in Ouratea castaneifolia (Ochnaceae) related to the protection of buds and young leaves. We studied this system, focusing on the distribution, morphology, histochemistry, and ultrastructure of glands during sprouting. Samples of buds and leaves were processed following the usual procedures for light and electron microscopy. Overlapping bud scales protect dormant buds, and each young leaf is covered with a pair of stipules. Stipules and scales possess a resin gland, while the former also possess an extrafloral nectary. Despite their distinct secretions, these glands are similar and comprise secreting palisade epidermis. Young leaves also possess marginal colleters. All the studied glands shared some structural traits, including palisade secretory epidermis and the absence of stomata. Secretory activity is carried out by epidermal cells. Functionally, the activity of these glands is synchronous with the young and vulnerable stage of vegetative organs. This is the first report of colleters and resin glands for O. castaneifolia. We found evidence that these glands are correlated with protection against herbivores and/or abiotic agents during a developmental stage that precedes the establishment of mechanical defenses.