Secretory Structures In Plants

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Plant chemicals can be classed as primary or secondary metabolites, depending on whether or not they have an essential role in plant metabolism and arc universally present in all plants. Primary metabolites include sugars, amino acids. nucleic acids and the chlorophylls. Secondary metabolites make up all the remaining plant chemicals from alkaloids to phenols.

Essential oils and other secondary plant metabolites are found in a wide range of plant species including annual, biennial or perennial herbaceous plants, evergreen or dcciduous shrubs and trees. The ecological and evolutionary role of these secondary metabolites has been associated w ith defence against animals, healing of plant organ wounds, protection from harmful insects, resistance to microbial attacks and attraction of insccts and animals tbr pollination. Several species and varieties of plants, mostly those of commercial interest, were investigated systematically and in depth. Recently, various studies concerned with secretory structures and factors influencing their development have been undertaken by research groups in biological and pharmacological departments.

Secretion is a common feature of living cells and involves the discharge of substances to the exterior (exotropic secretion) or into special intercellular cavities {endotropic secretion). These are specialised cells and the secreted material may contain various salts, latex, waxes, fats, flavonoids, sugars, gums, mucilages, essential oils and resins. It has been assumed that these products are biosynthesised in situ and direct evidence for the biosynthetic capacity of gland cells has become available relatively recently with the development of procedures for gland isolation. These methods have yielded definitive proof of the presence of enzymes specifically w ithin gland cells.

Trichomes present on the leaf surface and other secretory tissues can be examined using light, scanning and transmission electron microscopy which enables detailed observation of major stages in the development of secretory ceils, including their membrane system and nuclei, the overall size of the gland and the amount of material released into the subcuticular cavity.

Essential oils, with or without accompanying resins and gums, are most commonly found in spccial secretory structures either on the surface of the plant or within the plant tissues. The type of structure is family or species specific. This can be useful in identification of plant material and verifying the authenticity of the plant source in the case of suspected adulteration.

Secretory cells

The most simple secretory structure is a single secretion-containing cell where it is only the actual content that distinguishes it from adjacent non-secrctory cells. However, it may also be larger than the other cells or have a thick cuticularized lining. This ccll type is found in many different plant tissues: in the leaf parenchyma of lemongrass {Andropogon spp,), bay (Laurtts nobilis), citronella (Cymbopogon nonius and C. winterianus) and patchouli (Pogostemon patchouli), in the seed coal of cardamom {Elettaria cardamomum), in the rhizome of ginger (Zingiber officinale) [Plates 42-44] and turmeric (Curcuma longa), in the fruit wall of pepper (Piper nigrum), capsicum and chillies (Capsicum annum), in the perisperm and embryo of nutmeg (Myristica fragrans) [Plates 63 & 64], in the bark of cassia (Cassia angustifolia) and cinnamon (Cinnamomum zeylanicum) and in the root of valerian (Valeriana officinalis).

osmophores

Osmophores are areas of flower tissues with secretory cells differing structurally from the adjacent cells (e.g. isodiametric cells in orchids).

Secretory cavities

These cavities are more or less spherical structures that can be formed in two ways: the parenchyma cells can separate one from another leaving intercellular spaces called lumina or lacuna, or an actual cell can disintegrate leaving a cav ity within the tissue. These spaces are lined with secretory cells or an epithelium that produces the essential oils. In high oil yielding plants several layers of these secretory cells are formed. The cavities continually enlarge and some become tilled with cells with thin, convoluted walls which also store the oil produced from within their plastids. Included in this group are fruits and leaves of plants in the Citrus family (C. aurantifolia, C. aurantium, C. bergamia. C. sinensis, C. Union) [Plates 20-22] as well as Eucalyptus spp. [Plates 34 & 35] and buchu leaves. Citrus peel oils are confined in oblate to sphcrical-shaped oil cavities (glands), sometimes called oils sacs, that are located irregularly in the exocarp of the fruit [Plate 22]. These cavities have no walls and are embedded at different depths in the flavedo (the coloured outer portion or skin of the fruit). The glands of grapefruit lie deeply in the flavedo and those of mandarin are likely to be nearer the surface. Fruits and leaves of these plants arc covered by a thick cuticle which is waterproof and also the primary means of water conservation. Being shiny and reflective it is capable of deflecting some of the excess solar radiation in tropical and subtropical regions; it also reflects ultraviolet light, thereby protecting the DNA from the mutagenic effects of sunlight. It is an excellent protection against fungi and bacteria since they have no enzymes capable of digesting cutin. Secretory cavities arc also present in the flow:er buds of cloves (Syzygium aromaticum) [Plates 23-26], the fruit walls of pimento {Pimento dioica), and in the elongated cavities in the bark of myrrh {Commiphora molmol), benzoin (Styrax benzoin)and frankincense (Boswellia spp.) [Plates 36 & 37],

Secretory di cts

Ducts are elongated cavities. They can often branch to create a network extending from the roots through the stem to the leaves, flowers and fruits. They are composed of an epithelium which surrounds a central cavity. Several predisposed cells within the parenchyma undergo asynchronous division and in doing so they expand the initial space in the middle where the cells are all adjacent to form a cavity. Some of these cells forming the wall of the cavity will change into secretory epithelial cells. The oils are biosynthesised within their leucoplasts and move via the endoplasmic reticulum into the cavity. These cavities then become joined to form ducts. They can be found in all of the Umbelliferae family including anise (Pimpinella an is ton), fennel (Foeniculum vulgare), dill (Anethum graveolens), coriander {Conundrum sativum), cumin id

(Cumimim cyminum) [Piatcs 27-30], parsley {Petroselinum crispurn) and angelica (Angelica archangelica). In the ease of celery (Apium graveoiens) they can branch to create a network extending from the roots, through the stem to the leaves, then to the flowers and finally to the fruits where they are known as virtae (sing, vitta). They are also present in the Pinaceae [Plates 13-15], Compositae, Hypericaceae and Coniferae families. The resin ducts in the xylem of Coniferae can reach 4-10 cm in length with between 2 and 7 ducts per leaf.

glandl i. ar trichomes

Glandular trichomes arc modified epidermal hairs and can be found covering leaves, stems, and even parts of flowers such as the calyx in many plants of the Labiatae family. These include basil (Ocimum has i Hewn) [Plates 5 & 6], lavender (Lavandula spp.) [Plate 53 & 54], marjoram [Piatcs 61 & 62] and oregano (Origanum spp.) [Piatcs 68-72], mint (Mentha spp.) [Plates 73 & 74] and thyme (Thymus sp.). The secretory cells are attached by a single stem or basal cell in the epidermis. The outer surface of the gland is heavily cutinized. A toughened cuticle in which no pores or perforations are present, usually completely covers the trichome [Plate 7]. The essential oils accumulate in subcuticular spaces and it is thought that they diffuse outwards through the cuticle. The glandular cells differ from normal plant cells in that they have a very large nuclcus and a dense protoplasm thai lacks a large central vacuole. There are numerous plasmodesmata across the walls of the gland cells especially between the stalk cell and the collecting cell. In the very young gland the intracellular organisation is almost identical to that of the adjacent cells but as the secretory cells develop complex changes occur. The membrane system progressively degenerates and in the fully-developed glands only a fine granular cytoplasm remains.

Cells in the multicellular heads usually have nuclei with double the normal numbers of chromosomes (endopolvploidy) and in fully-mature glands, mitochondria are the most abundant organelles which might reflect a high energy requirement. There is also a we 11-developed endoplasmic reticulum. Essential oils accumulate in the subcuticular cavity [Plates 32. 70 & 92]. All these changes in the glands occur at a very early stage of the leaf development and the glandular hairs are fully-formed by the lime the leaves are about 5 mm long. The formation and transformation of the essential oils, however, takes place continuously from the formation of the gland until senescence. The essential oil biogenetic precursor isopentenyl pyrophosphate is. with ligh probability, synthesised in the leucoplasts.

There are two main types of glands that can exhibit minor variations:

(i) Peltate glands with one basal cell, a short stalk and a large six- to eight- celled head [Plates 61,84 & 86].

(ii) Capitate trichomes with either -

a) one basal cell, a short monocellular stalk and a two-cellular head [Plates 72 & 85], or b) one basal cell, a multicellular stalk and a small globose, unicellular head [Plate 67].

In every species there arc distinctive variations in I he size and shape of the glands. For example, yarrow (Achillea millefolium) secretory trichomes of the floret produce azulene and occur mainly in the corolla lobes of the ray and disk florets and also on the leaves. When a floret reaches 0.25 mm. fully-mature trie homes are present having developed from protodermal cells. They have 10 cells including a pair of basal cells, a pair of stalk cells and 3 pairs of glandular cells. In Cannabis sativa a variety of glandular trichomes occur, including bulbous (small glands with a single-celled head and a unicellular stalk), capitate-sessile (a gland with a head of 8 or more cells on a very short stalk) and capitate-stalked (similar to previous type but with a substantial multicellular stalk). These glands are considered to be the main location of the narcotic cannabinoids.

It is possible to isolate an individual gland from the leaf surface and analyse its content for chemical composition and presence of specific enzymes.

Epidermal cells

Essential oils obtained from (lowers are not usually secreted by glandular hairs but merely diffuse through the cytoplasm, the cell walls and the cuticle to the outside. The yield of essential oils from these species is generally very low. Examples include rose (Rosa spp.), 0.075% (w/v), acacia (Acacia spp.) 0.084% (w/v) and jasmine (Jasminum spp.) 0.04% (w/v).

Buds of a number of plant species (e.g. Aesculus, A In us. Betula, Populus, Primus and Rhamnus) also secrete lipophilic substances, mainly flavonoid aglycones mixed with essential oils. Secretion here occurs from epidermal cells which are covered by a cuticle. The secreted material is first eliminated into a space formed between the outer w alls of the cells and the cuticle covering them, forming a blister that subsequently bursts.

Conclusion

Plants have provided man for hundreds of years with many of the basic and important materials required for day-to-day living, including oxygen, food, clothing and timber, as well us being a source of compounds such as oils, resins, rubbers, gums, dyes, pesticides and drugs. Plants can be considered as biochemical factories that have been evolving their programmes over the last 400 million years. Researchers have been elucidating chemical pathways and various industrial applications of these products, and anatomical description is one of the most important aspects of economical uses of these plants. It is also important to highlight the need to safeguard and conserve the diversity of our global flora and to contribute w ith research into careful exploitation of plant resources.

PLATE 3

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  • laura
    What is secretory structure in plant?
    2 years ago

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