subject: Plant Defense Against Herbivory - Medical Disposables Products - China Gifts& Health Care Products [print this page] Evolution of defensive traits Evolution of defensive traits
Timeline of plant evolution and the beginnings of different modes of insect herbivory
Main article: Evolutionary history of plants
The earliest land plants evolved from aquatic plants around 450 million years ago (Ma) in the Ordovician period. These early land plants had no vascular system and required free water for their reproduction. Vascular plants appeared later and their diversification began in the Devonian era (about 400 Ma). Their reduced dependence on water resulted from adaptations such as protective coatings to reduce evaporation from their tissues. Reproduction and dispersal of vascular plants in these dry conditions was achieved through the evolution of specialized seed structures. The diversification of flowering plants (angiosperms) during the Cretaceous period is associated with the sudden burst of speciation in insects. This diversification of insects represented a major selective force in plant evolution, and led to selection of plants that had defensive adaptations. Early insect herbivores were mandibulate and bit or chewed vegetation; but the evolution of vascular plants lead to the co-evolution of other forms of herbivory, such as sap-sucking, leaf mining, gall forming and nectar-feeding.
Records of herbivory
Viburnum lesquereuxii leaf with insect damage; Dakota Sandstone (Cretaceous) of Ellsworth County, Kansas. Scale bar is 10 mm.
Our understanding of herbivory in geological time comes from three sources: fossilised plants, which may preserve evidence of defence (such as spines), or herbivory-related damage; the observation of plant debris in fossilised animal faeces; and the construction of herbivore mouthparts.
Long thought to be a Mesozoic phenomenon, evidence for herbivory is found almost as soon as fossils which could show it. Within under 20 million years of the first fossils of sporangia and stems towards the close of the Silurian, around 420 million years ago, there is evidence that they were being consumed. Animals fed on the spores of early Devonian plants, and the Rhynie chert also provides evidence that organisms fed on plants using a "pierce and suck" technique. Many plants of this time are preserved with spine-like enations, which may have performed a defensive role before being co-opted to develop into leaves.
During the ensuing 75 million years, plants evolved a range of more complex organs - from roots to seeds. There was a gap of 50 to 100 million years between each organ evolving, and it being fed upon. Hole feeding and skeletonisation are recorded in the early Permian, with surface fluid feeding evolving by the end of that period.
A Plain Tiger Danaus chrysippus caterpillar making a moat to block defensive chemicals of Calotropis before feeding
Co-evolution
Herbivores depend on plants for food, and have evolved mechanisms to obtain this food despite the evolution of a diverse arsenal of plant defenses. Herbivore adaptations to plant defense have been likened to offensive traits and consist of adaptations that allow increased feeding and use of a host plant. Relationships between herbivores and their host plants often results in reciprocal evolutionary change, called co-evolution. When a herbivore eats a plant it selects for plants that can mount a defensive response. In cases where this relationship demonstrates specificity (the evolution of each trait is due to the other), and reciprocity (both traits must evolve), the species are thought to have co-evolved. The "escape and radiation" mechanism for co-evolution presents the idea that adaptations in herbivores and their host plants have been the driving force behind speciation, and have played a role in the radiation of insect species during the age of angiosperms. Some herbivores have evolved ways to hijack plant defenses to their own benefit, by sequestering these chemicals and using them to protect themselves from predators.
Types
Plant defenses can be classified generally as induced or constitutive. Constitutive defenses are always present in the plant species, while induced defenses are synthesized or mobilized to the site where a plant is injured. There are wide variations in the composition and concentration of constitutive defenses and these range from mechanical defenses to digestibility reducers and toxins. Most external mechanical defenses and large quantitative defenses are constitutive, as they require large amounts of resources to produce and difficult to mobilize.
Induced defenses include secondary metabolic products, as well as morphological and physiological changes. An advantage of inducible, rather than constitutive defenses, is that increased variability increases the effectiveness of the defenses. This advantage comes from the suggestion that if herbivores can choose among different plants and plant tissues, they may avoid eating plants that have both constitutive and induced defenses.
Chemical defenses
Persimmon, genus Diospyros, has a high tannin content which gives immature fruit, seen above, an astringent and bitter flavor.
The evolution of chemical defenses in plants is linked to the emergence of chemical substances that are not involved in the essential photosynthetic and metabolic activities. These substances, secondary metabolites, are organic compounds that are not directly involved in the normal growth, development or reproduction of organisms, and often produced as by-products during the synthesis of primary metabolic products. These secondary metabolites play a major role in defenses against herbivores.
Secondary metabolites are often characterized as either qualitative or quantitative. Qualitative metabolites are defined as toxins that interfere with an herbivore metabolism, often by blocking specific biochemical reactions. Qualitative chemicals are present in plants in relatively low concentrations (often less than 2% dry weight), and are not dosage dependent. They are usually small, water soluble molecules, and therefore can be rapidly synthesized, transported and stored with relatively little energy cost to the plant. Qualitative allelochemicals are ususally effective against non-adapted specialists and generalist herbivores.
Quantitative chemicals are those that are present in high concentration in plants (5 40% dry weight) and are equally effective against all specialists and generalist herbivores. Most quantitative metabolites are digestibility reducers that make plant cell walls indigestible to animals. The effects of quantitative metabolites are dosage dependent and the higher these chemicals proportion in the herbivore diet, the less nutrition the herbivore can gain from ingesting plant tissues. Because they are typically large molecules, these defenses are energetically expensive to produce and maintain, and often take longer to synthesise and transport.
Types of chemical defenses
Plants have developed many secondary metabolites involved in plant defense, which are collectively known as antiherbivory compounds and can be classified into three sub-groups: nitrogen compounds (including alkaloids, cyanogenic glycosides and glucosinolates), terpenoids, and phenolics.
Alkaloids are derived from various amino acids. Over 3000 known alkaloids exist, examples include nicotine, caffeine, morphine, colchicine, ergolines, strychnine, and quinine. Alkaloids have pharmacological effects on humans and other animals. Some alkaloids can inhibit or activate enzymes, or alter carbohydrate and fat storage by inhibiting the formation phosphodiester bonds involved in their breakdown. Certain alkaloids bind to nucleic acids and can inhibit synthesis of proteins and affect DNA repair mechanisms. Alkaloids can also affect cell membrane and cytoskeletal structure causing the cells to weaken, collapse, or leak, and can affect nerve transmission.
Cyanogenic glycosides are stored in inactive forms in plant vacuoles. They become toxic when herbivores eat the plant and break cell membranes allowing the glycosides to come into contact with enzymes in the cytoplasm releasing hydrogen cyanide which blocks cellular respiration. Glucosinolates are activated in much the same way as cyanogenic glucosides, and the products can cause gastroenteritis, salivation, diarrhea, and irritation of the mouth.
The terpenoids, sometimes referred to as isoprenoids, are organic chemicals similar to terpenes, derived from five-carbon isoprene units. There are over 10,000 known types of terpenoids. Most are multicyclic structures which differ from one another in both functional groups, and in basic carbon skeletons. Monoterpenoids, continuing 2 isoprene units, are volatile essential oils such as citronella, limonene, menthol, camphor, and pinene. Diterpenoids, 4 isoprene units, are widely distributed in latex and resins, and can be quite toxic. Diterpenes are responsible for making Rhododendron leaves poisonous. Plant steroids and sterols are also produced from terpenoid precursors, including vitamin D, glycosides (such as digitalis) and saponins (which lyse red blood cells of herbivores).
Phenolics, sometimes called phenols, consist of an aromatic 6-carbon ring bonded to a hydroxy group. Some phenols have antiseptic properties, while others disrupt endocrine activity. Phenolics range from simple tannins to the more complex flavonoids that give plants much of their red, blue, yellow, and white pigments. Complex phenolics called polyphenols are capable of producing many different types of effects on humans, including antioxidant properties. Some examples of phenolics used for defense in plants are: lignin, silymarin and cannabinoids. Condensed tannins, polymers composed of 2 to 50 (or more) flavonoid molecules, inhibit herbivore digestion by binding to consumed plant proteins and making them more difficult for animals to digest, and by interfering with protein absorption and digestive enzymes. Silica and lignins, which are completely indigestible to animals, grind down insect mandibles (appendages necessary for feeding).
In addition to the three larger groups of substances mentioned above, fatty acid derivates, amino acids and even peptides are used as defence. The cholinergic toxine, cicutoxin of water hemlock, is an polyyne derived from the fatty acid metabolism. -N-Oxalyl-L-,-diaminopropionic acid as simple amino acid is used by the sweet pea which leads also to intoxication in humans. The synthesis of fluoroacetate in several plants is an example for the use of small molecules to disturb the metabolism of the herbivore, in this case the citric acid cycle.
Mechanical defenses
The thorns on the stem of this raspberry plant, serve as a mechanical defense against herbivory.
Plants have many external structural defenses that discourage herbivory. Depending on the herbivore physical characteristics (i.e. size and defensive armor), plant structural defenses on stems and leaves can deter, injure, or kill the grazer. Some defensive compounds are produced internally but are released onto the plant surface; for example, resins, lignins, silica, and wax cover the epidermis of terrestrial plants and alter the texture of the plant tissue. The leaves of holly plants, for instance, are very smooth and slippery making feeding difficult. Some plants produce gummosis or sap that traps insects.
A plant's leaves and stem may be covered with sharp spines or trichomes- hairs on the leaf often with barbs, sometimes containing irritants or poisons. Plant structural features like spines and thorns reduce feeding by large ungulate herbivores (e.g. kudu, impala, and goats) by restricting the herbivores' feeding rate, or by wearing down the molars as in pears. The structure of a plant, its branching and leaf arrangement may also be evolved to reduce herbivore impact. The shrubs of New Zealand have evolved special wide branching adaptations believed to be a response to browsing birds such as the moas. Similarly, African Acacias have dense thorns on the outside, but none in the middle of the crown, which is comparatively safe from herbivores such as giraffes. Young Acacias, which would be vulnerable due to their proximity to the ground, display a profusion of thorns gradually decreasing with age.
Coconut palms protect their fruit by surrounding it with multiple layers of armour.
Trees such as coconut and other palms, may protect their fruit by multiple layers of armour, needing efficient tools to break through to the seed contents, and special skills to climb the tall and relatively smooth trunk.
Thigmonasty
Thigmonastic movements, those that occur in response to touch, are used as a defense in some plants. The leaves of the sensitive plant, Mimosa pudica, close up rapidly in response to direct touch, vibration, or even electrical and thermal stimuli. The proximate cause of this mechanical response is an abrupt change in the turgor pressure in the pulvini at the base of leaves resulting from osmotic phenomena. This is then spread via both electrical and chemical means through the plant; only a single leaflet need be disturbed.
This response lowers the surface area available to herbivores, which are presented with the underside of each leaflet, and results in a wilted appearance. It may also physically dislodge small herbivores, such as insects. Thigmonasty is not only useful in discouraging herbivores, however. For instance the venus flytrap makes use of it to catch its own food.
Mimicry and camouflage
Some plants mimic the presence of insect eggs on their leaves, dissuading insect species from laying their eggs there. Because female butterflies are less likely to lay their eggs on plants that already have butterfly eggs, some species of neotropical vines of the genus Passiflora (Passion flowers) contain physical structures resembling the yellow eggs of Heliconius butterflies on their leaves, which discourage oviposition by butterflies.
Indirect defenses
The large thorn-like stipules of Acacia collinsii are hollow and afford shelter for ants, which in return protect the plant against herbivores.
Another category of plant defenses are those features that indirectly protect the plant by enhancing the probability of attracting the natural enemies of herbivores. Such an arrangement is known as mutualism, in this case of the "enemy of my enemy" variety. One such feature are semiochemicals, given off by plants. Semiochemicals are a group of volatile organic compounds involved in interactions between organisms. One group of semiochemicals are allelochemicals; consisting of allomones, which play a defensive role in interspecies communication, and kairomones, which are used by members of higher trophic levels to locate food sources. When a plant is attacked it releases allelochemics containing an abnormal ratio of volatiles. Predators sense these volatiles as food cues, attracting them to the damaged plant, and to feeding herbivores. The subsequent reduction in the number of herbivores confers a fitness benefit to the plant and demonstrates the indirect defensive capabilities of semiochemicals. Induced volatiles also have drawbacks, however; some studies have suggested that these volatiles also attract herbivores.
Plants also provide housing and food items for natural enemies of herbivores, known as iotic defense mechanisms, as a means to maintain their presence. For example, trees from the genus Macaranga have adapted their thin stem walls to create ideal housing for an ant species (genus Crematogaster), which, in turn, protects the plant from herbivores. In addition to providing housing, the plant also provides the ant with its exclusive food source; from the food bodies produced by the plant. Similarly, some Acacia tree species have developed thorns that are swollen at the base, forming a hollowing structure that acts as housing. These Acacia trees also produce nectar in extrafloral nectaries on their leaves as food for the ants.
Most plants have endophytes, microbial organisms that live within them. While some cause disease, others protect plants from herbivores and pathogenic microbes. Endophytes can help the plant by producing toxins harmful to other organisms that would attack the plant, such as alkaloid producing fungi which are common in grasses such as tall fescue (Festuca arundinacea).
Leaf shedding and colour
There have been suggestions that leaf shedding may be a response that provides protection against diseases and certain kinds of pests such as leaf miners and gall forming insects. Other responses such as the change of leaf colours prior to fall have also been suggested as adaptations that may help undermine the camouflage of herbivores. Autumn leaf color has also been suggested to act as an honest warning signal of defensive commitment towards insect pests that migrate to the trees in autumn.
Costs and benefits
Defensive structures and chemicals are costly as they require resources that could otherwise be used by plants to maximize growth and reproduction. Many models have been proposed to explore how and why some plants make this investment in defenses against herbivores.
Optimal defense hypothesis
The optimal defense hypothesis attempts to explain how the kinds of defenses a particular plant might use reflect the threats each individual plant faces. This model considers three main factors, namely: risk of attack, value of the plant part, and the cost of defense.
The first factor determining optimal defense is risk: how likely is it that a plant or certain plant parts will be attacked? This is also related to the plant apparency hypothesis, which states that a plant will invest heavily in broadly effective defenses when the plant is easily found by herbivores. Examples of apparent plants that produce generalized protections include long-living trees, shrubs, and perennial grasses. Unapparent plants, such as short-lived plants of early successional stages, on the other hand, preferentially invest in small amounts of qualitative toxins that are effective against all but the most specialized herbivores.
The second factor is the value of protection: would the plant be less able to survive and reproduce after removal of part of its structure by a herbivore? Not all plant parts are of equal evolutionary value, thus valuable parts contain more defenses. A plant stage of development at the time of feeding also affects the resulting change in fitness. Experimentally, the fitness value of a plant structure is determined by removing that part of the plant and observing the effect. In general, reproductive parts are not as easily replaced as vegetative parts, terminal leaves have greater value than basal leaves, and the loss of plant parts mid-season has a greater negative effect on fitness than removal at the beginning or end of the season. Seeds in particular tend to be very well protected. For example, the seeds of many edible fruits and nuts contain cyanogenic glycosides such as amygdalin. This results from the need to balance the ef
