Plant Secretory Structures: Types, Mechanisms, and Functions
Plant Secretory Structures
Secretory Structures: These structures are involved in plant metabolism and plant-environment interactions. They are based on the type of shock and the type of secretion secreted. Functions include:
- Production of compounds (cellulose, callose)
- Disposal of surplus (glands eliminate excess salt or water)
- Accumulation of substances (mineral salts in the form of crystals)
- Attraction of pollinators (nectar)
- Protection against herbivorous animals (defensive hairs)
Secreted Substances: These include ion surplus (removed as salts or accumulated as crystals), polysaccharides (sugars, mucilage), fatty compounds, enzymes, hormones, alkaloids, tannins, terpenes, and resins.
Secretion Mechanisms
Secretion can be passive (osmosis) or active (when product concentration is very high in the cytoplasm). There are three main types:
- Holocrine: The entire cell content is released by cell lysis.
- Granulocrine: Secreted product accumulates in vesicles produced by dictyosomes or endoplasmic reticulum (ER).
- Eccrine: Cytoplasmic molecules are secreted through molecular pumps in the tonoplast or plasma membrane.
Internal Secretory Structures
Intra-secreting cells: These are secretory idioblasts:
- Oleifera Secreting Cells: The most common, containing aroma oils, located in the rhizome, bark, or endosperm. Accumulation occurs via the ER (e.g., cinnamon bark, peanut seed).
- Mucilaginous Cells: Present in cacti and succulents, secretion is granulocrine through the plasma membrane, accumulating under the cell wall. The protoplast degenerates, leaving mucilage.
- Tanniferous Cells: Tannins accumulate in vacuoles, found in the phloem, marrow, or epidermis. Secretion is eccrine.
Secretory Cavities
- Olisigenous Cavities: Can be of schizogenous or lysigenous origin.
- Lysigenous Cavities: Formed by lysis of whole cells (holocrine), surrounded by cells that disintegrate before secretion (e.g., orange and lemon exocarp).
- Schizogenous Cavities: Formed by dissolution of the middle lamella and dilated intercellular spaces. Cells lining the cavities differ in secretory epithelium, secretion is merocrine. Schizogenous channels are surrounded by dense epithelial cells.
Laticifers
Laticifers are tubes formed by living, coenocytic (multinucleated), vacuolated cells that secrete latex. Latex composition includes hydrocarbons, lipids, acids, tannins, mucilage, alkaloids, salts, gums, proteins, vitamins, and starch. Latex can be milky (Carica papaya), transparent (Allium), orange, or yellow. Their role is in excretion or defense. They are found in Pteridophyta, 900 dicot genera, and monocots. They are associated with the phloem or spread throughout plant tissue.
Classification of Laticifers
- Non-articulated: Formed by a single, acellular, extra-long cell with a cell wall, containing heme and pectin. They are multinucleated with active metabolism. Latex accumulates in vacuoles or more, secretion is eccrine.
- Articulated: Laticifer cells are connected by plasmodesmata or perforations. Secretion is holocrine, with protoplasm degeneration.
- Non-anastomosing Articulated: Continuous, formed by dedifferentiation of parenchyma cells (e.g., Allium, Musa, Ipomoea).
- Anastomosing Articulated: Laticifers fuse to form a three-dimensional network, formed by dedifferentiation of parenchyma cells, forming bridges (e.g., Carica papaya).
External Secretory Structures
Glandular Trichomes
These have a unicellular or pluricellular head. Secretion accumulates between the cuticle, which stretches and ruptures, releasing the contents. They store essential oils.
Defensive Hairs
Secretion is eccrine, lipophilic, and accumulates below the cuticle. Upon contact with larvae, a sticky fluid is exuded, immobilizing them.
Urticating Hairs
These trichomes have a central vacuole containing toxins (e.g., acetylcholine, histamine). Body hair is calcified, with vesicles accumulating in the ER, meeting in a vacuole. Dictyosomes deposit silica in the wall.