Secretory Tissues and Structures

Trichomes (hairs) are the most common type of external gland. They can be unicellular or multicellular, with a short stalk or sessile. Based on shape, capitate (headed) and peltate (shield-shaped) glandular trichomes are distinguished (Wagner, 1991; Vitarelli et al., 2015).

Unlike trichomes, emergences involve not only epidermal cells but also deeper tissues. The best‑known example is the stinging hairs of nettle Urtica dioica. They are large unicellular hairs with an expanded base surrounded by a rosette of cells. The tip of the hair ends in a club‑shaped head that breaks off easily upon contact, exposing a sharp point. The cell contains a mixture of toxins, including histamine, acetylcholine, and serotonin (Ribeiro et al., 2021; Evert, 2006). Emergences also occur in some species as defensive outgrowths.

Nectaries are glands that secrete a sugary solution (nectar). They can be floral (on flowers: receptacle, sepals, petals, stamens) and extrafloral (on leaves, stipules, stems). Floral nectaries attract pollinators, while extrafloral nectaries often “pay” for the services of ant bodyguards (Vitarelli et al., 2015; Evert, 2006; Fahn, 1988). Nectaries vary greatly in structure — from simple groups of secretory cells to complex glands with their own vascular tissue. The secretion is released through stomata or cuticular ruptures.

Hydathodes are specialized structures through which the plant exudes liquid water (guttation). Typically, a hydathode consists of a water stoma (permanently open) and the underlying parenchyma — the epithem (Evert, 2006; Beck, 2010). Hydathodes are often located at leaf margins or tips. Guttation usually occurs under conditions of high air humidity and good water supply, when root pressure exceeds transpiration. The exuded water may contain dissolved mineral salts and organic substances.

Idioblasts are single specialized cells scattered among other cells of the ground tissue. They can accumulate various substances (Evert, 2006; Mauseth, 2016; Ribeiro et al., 2021).

Laticifers are living tubular or branched cells containing latex — a milky sap. Latex is an emulsion or suspension of rubber, terpenoids, alkaloids, and other substances in an aqueous phase (Evert, 2006; Beck, 2010). Two types of laticifers are distinguished:

These are large cavities or canals lined by secretory (epithelial) cells. Based on the mode of formation, three types are distinguished (Fahn, 1988; Evert, 2006; Lin, 2023).

Thus, the classification of secretory structures reflects their evolutionary diversity and specialization. The outline and description provided give a basis for further understanding of their ontogeny, physiology, and ecological role (Sections 3–6).

Trichomes are initiated as single initial cells in the protoderm. These cells are distinguished by their larger size, dense cytoplasm, and large nucleus (Dong et al., 2023; Evert, 2006). Development includes the following stages:

Genetic control. Intensively studied in the trichomes of Arabidopsis thaliana. Genes controlling initiation (GLABRA1, TRANSPARENT TESTA GLABRA1), morphogenesis (including branching), and differentiation have been identified. MicroRNAs (miR156, miR172, miR319) play a key role in the post‑transcriptional regulation of these processes, ensuring spatial and temporal control of trichome development (Dong et al., 2023).

Nectaries are initiated as protrusions of the protoderm (or subepidermal layers) in characteristic zones of the flower (receptacle, base of stamens, ovary). Under the influence of hormonal signals, cells actively divide, forming a multicellular “bulge.” Within the primordium, a vascular bundle (xylem and phloem) often differentiates, supplying the gland with water and sugars (Evert, 2006). Nectary cells acquire characteristic features: dense cytoplasm, large nuclei, numerous mitochondria, and a well‑developed endoplasmic reticulum. Nectar is secreted through modified stomata or by cuticular rupture.

Hydathodes develop from the protoderm at leaf margins (often at the tips of teeth). A characteristic feature is the formation, beneath the water stoma, of the epithem — a cluster of large colorless parenchyma cells connected to the endings of vascular bundles (Evert, 2006). The epithem actively participates in water filtration and secretion.

Idioblasts may differentiate directly from ground meristem cells or by transdifferentiation (dedifferentiation and subsequent specialization) of already formed parenchyma cells (Fahn, 1988; Evert, 2006). The initial idioblast cell is often recognized by its larger nucleus and denser cytoplasm. During growth, storage substances (mucilage, oils) or crystals accumulate in vacuoles. The walls of idioblasts may thicken and lignify, and the protoplast often dies at maturity (e.g., crystal idioblasts). In some species (e.g., lemon), the formation of essential‑oil idioblasts occurs lysigenously: a group of cells accumulates secretion, then their protoplasts break down, releasing the contents into the resulting cavity (Lin, 2023; Turner et al., 1998, cited in Beck, 2010).

Laticifer ontogeny differs depending on the type (Evert, 2006; Beck, 2010).

The formation of these structures is closely linked to programmed cell death (PCD) (Lin, 2023; Evert, 2006).

Most secretory cells are living parenchyma cells with dense cytoplasm, a large nucleus, well‑developed endoplasmic reticulum (ER), and Golgi apparatus (Evert, 2006; Beck, 2010). However, during maturation, some may lose their protoplast (e.g., in lysigenous cavities) or, conversely, remain alive and function for a long time (laticifers, epithelium of resin ducts).

Epithelial cells lining schizogenous or lysigenous cavities are specialized secretory cells. In conifer resin ducts, the epithelium may be uniseriate, and the cells have dense cytoplasm and thickened, often lignified walls (Evert, 2006; Fischer et al., 2019). In the essential‑oil canals of Apiaceae, the epithelium is thin‑walled and actively synthesizes and secretes essential oils.

In places of intensive transport of substances (e.g., in nectaries, hydathodes, at the interface of phloem with storage parenchyma), transfer cells are found (Evert, 2006; Beck, 2010). Their distinctive feature is numerous cell wall ingrowths (increasing the surface area of the plasma membrane) that facilitate active transport of ions and molecules. These ingrowths are usually located on the side facing the vascular bundle or the secretory cavity.

Idioblasts: Secretion (essential oil, mucilage, tannin, crystals) accumulates in the vacuole. The cell may remain alive or die.

Laticifers: Latex is contained in the cytoplasm (in non‑articulated laticifers — in the syncytium) and vacuoles. Laticifers are living cells.

Schizogenous cavities and canals: The space between cells (the cavity) forms by separation of epithelial cells. Secretion is released into the cavity across the plasma membrane and cell wall (eccrinally or granulocrinally). The cavity may be isolated from other tissues by a layer of compact cells or by thickened, suberized walls of the epithelial cells (Ribeiro et al., 2021; Lin, 2023).

Lysigenous cavities: Form by complete breakdown of cells; the secretion is a mixture of autolysis products and secretions from surrounding cells. The cavity is also often surrounded by living cells that may participate in further secretion (Evert, 2006).

Resin ducts of conifers: Vertical resin ducts run through the entire wood, connecting with horizontal (radial) ducts, forming a three‑dimensional network. The secretion (oleoresin) accumulates in the central duct lumen (Evert, 2006; Fischer et al., 2019).

Glandular trichomes: Secretion is released into the subcuticular space — between the cell wall of the secretory cells and the cuticle. As a result, the cuticle is lifted and stretched, forming a “bubble.” When the cuticle ruptures, the secretion flows out. In peltate trichomes, the cuticle may remain intact, and secretion is released through special pores (Wagner, 1991; Evert, 2006).

Nectaries: Secretion may be released through modified stomata or pores in the cuticle. In some nectaries, the cuticle periodically ruptures and then regenerates (Beck, 2010).

Hydathodes: Water is released through permanently open water stomata (hydathodes). Beneath the stoma is the epithem — loose parenchyma through which water is filtered from the endings of vascular bundles (Evert, 2006).

Cutinization and suberization of walls: To protect living cells from toxic secretions or to prevent reabsorption of secreted substances, the walls of secretory cells are often impregnated with cutin or suberin (e.g., in the epithelium of resin ducts, in the sheath of crystal idioblasts). This isolates the secretory compartment from the rest of the apoplast (Fahn, 1988; Evert, 2006).

Thickened secondary walls: In some types of secretory cells (e.g., in crystal idioblasts and sclereids), the walls are greatly thickened and lignified, providing mechanical strength and protection against crushing.

Pores and plasmodesmata: For communication between secretory cells and surrounding tissues, as well as to ensure outflow or inflow of substances, walls contain numerous pores (simple, bordered) or plasmodesmata (especially characteristic of laticifers and epithelial cells) (Evert, 2006; Chano et al., 2015).

Thus, the structural organization of secretory tissues represents a complex system in which cellular specialization, features of intercellular spaces, and barrier functions of walls ensure efficient synthesis, compartmentalization, and, when necessary, release of biologically active substances. Understanding these structural foundations is necessary for analyzing the physiology of secretion (Section 5) and the influence of external factors (Section 6).

Water deficit and intense solar radiation often act together, especially in arid and semi‑arid regions. Plants respond to these stresses with a range of adaptations, including an increase in the density of glandular trichomes and an increase in the concentration of essential oils and other protective substances (Evert, 2006; Mauseth, 2016).

High temperatures. Overheating increases the evaporation of volatile terpenes, which can mask the true rate of their synthesis. However, in some species (e.g., eucalyptus), essential oil release increases sharply upon heating, which helps cool the leaves through evaporation and creates a protective aerosol screen (Fahn, 1988; Evert, 2006).

Low temperatures. In temperate‑zone plants, an increase in the content of tannins and phenolic compounds in the vacuoles of parenchyma cells (bark, wood) is often observed in autumn. These substances act as antiseptics, preventing the development of pathogens in weakened tissues, and at the same time increase frost resistance by stabilizing cell membranes. In conifers, resin ducts in the wood fill with resin in winter, which solidifies and seals transport pathways; secretion resumes in spring (Chano et al., 2015; Evert, 2006).

As mentioned above, UV‑B induces the accumulation of flavonoids and phenolic acids in epidermal and subepidermal cells. These compounds not only absorb UV but also neutralize free radicals formed under UV exposure. In many plants (e.g., some species of Sedum, Sempervivum), under UV‑B, anthocyanins accumulate in the epidermis, giving the leaves a reddish color; anthocyanins also serve as effective UV filters (Evert, 2006; Mauseth, 2016). In glandular trichomes, UV activation may enhance the synthesis of terpenes with antioxidant properties (Wagner, 1991).

As noted in Section 5, salinization stimulates the activity of salt glands in halophytes (Caperta et al., 2020; Beck, 2010). With increasing NaCl concentration in the soil, the density of salt crystals excreted on the leaf surface increases, indicating enhanced secretion. Na^\+ and Cl^- ions are actively removed from the mesophyll, preventing toxic damage. In some species (e.g., Limonium sinuatum), the amount of proline and other osmotica in the cytoplasm also increases under salinization, but gland function remains the main detoxification mechanism (Caperta et al., 2020).

In coniferous trees, bark damage by insects (bark beetles) or mechanical wounding triggers the formation of traumatic resin ducts in the cambial zone and young xylem (Chano et al., 2015; Fischer et al., 2019; Fahn, 1988). This process is controlled by signaling molecules, primarily jasmonic acid (JA) and ethylene. Under their influence, cambial and parenchyma cells reorient their divisions, forming tangential rows of epithelial cells that line a schizogenous cavity. The secreted resin contains high concentrations of mono‑, sesqui‑, and diterpenes, which are toxic to insects and pathogenic fungi. Induction of resinosis is systemic: damage to one part stimulates a defensive response around the entire stem circumference and even in parts above and below (Chano et al., 2015).

Upon pathogen attack (fungi, bacteria), idioblasts and parenchyma cells containing tannins and phenols are often activated in the infection zone. These compounds have pronounced antimicrobial properties. In addition, plants can synthesize phytoalexins — low‑molecular‑weight antimicrobial substances that are often terpenoids or phenols. For example, in pines under fungal infection, the synthesis of stilbenes is enhanced in the resin duct epithelium (Chano et al., 2015; Evert, 2006).

In some plants (e.g., tomato, tobacco, cotton), leaf damage by caterpillars or aphids leads to an increase in the density of glandular trichomes on newly formed leaves (induced defense). The signal is jasmonic acid, which activates the expression of genes controlling trichome development (Wagner, 1991; Dong et al., 2023; Vitarelli et al., 2015). An increase in the number of glands raises the content of repellent and toxic substances (e.g., nicotine in tobacco, α‑thujone in thuja). However, there is a trade‑off: the formation of additional trichomes requires energy and resources that could otherwise be used for growth and reproduction. Therefore, enhanced secretion is often triggered only under real threat (Wagner, 1991).