In this article we will discuss about:- 1. Concept of Host Plant Resistance 2. Types of Host Plant Resistance 3. Mechanism 4. Genetics 5. Durable Plant Resistance 6. Induced Plant Resistance 7. Economic Impact.
Concept of Host Plant Resistance:
A plant is neither susceptible to all the phytophagous insects nor any insect species is the pest of all the species of plants. Host range of a particular insect may be wide or narrow whereas some insects like locust are feeders on all types of plants. However, such insects are usually not considered in host plant-insect interactions.
Plant species which are fed by an insect are called host plants while those which are not fed at all are non-host plants. The inability of the insects to attack a non-host plant is termed immunity and such a plant is not considered a host of that insect. The terms host plant and immunity excludes each other.
Plants which are not fed at all would not generally be considered for resistance and, therefore, would be classified as immune. A host plant can be resistant, more or less, but not totally immune. Any degree of host reaction short of immunity is, thus, resistance. Considering all the flora and fauna in nature and host plant-insect interactions, it may be said that immunity is the rule and susceptibility is an exception.
In every plant species there exists a great deal of diversity with respect to the extent of damage done by an insect. Individual plants which show lesser damage are called resistant and those showing more damage are called susceptible, thus, these terms are relative. Host plant resistance is the result of interactions between two biological entities, the plant and the insect under the influence of various environmental factors.
Since the host plant resistance is the result of interactions between the plant and the insect, it is, therefore, assumed that optimum conditions under which a plant species is grown are also favourable enough for the growth and development of the insect so that the plant species is accepted by the insect.
The concept of host plant resistance should, therefore, be developed by comparing the performance of a variety under optimum conditions for the growth and development of the plant in the absence and presence of insect populations capable of causing maximum loss to the host plant.
Painter (1951) described plant resistance as the “relative amount of heritable qualities that influence the ultimate degree of damage done by the insect. In practical agriculture, resistance represents the ability of a certain variety to produce a larger crop of good quality than do ordinary varieties at the same level of insect population”.
Maxwell et al. (1972) extended the definition of Painter (1951) by considering level of insect infestation and environmental conditions. According to them, resistance is “those heritable characteristics possessed by the plant which influence the ultimate degree of damage done by the insect. From a practical point of view, resistance is the ability of certain variety to produce larger yield of good quality than other varieties at the same initial level of infestation and under similar environmental conditions.”
According to Kogan (1982) resistance to insects is the “inheritable property that enables a plant to inhibit the growth of insect populations or to recover from injury caused by populations that were not inhibited to grow. Inhibition of population growth generally derives from the biochemical and morphological characteristics of a plant which affect the behaviour or the metabolism of insects so as to reduce the relative degree of damage these insects can potentially cause.”
In other words, host plant resistance refers to the heritable qualities of a cultivar to counteract the activities of insects so as to cause minimum per cent reduction in yield as compared to other cultivars of the same species under similar conditions.
The emphasis in this definition is not on the absolute yield obtained from the so called resistant variety as compared to the susceptible one, but on the per cent decrease in yield vis-a-vis the yield obtained without the attack of the insect. It means that a cultivar may yield poor but carries the genes for resistance and on the contrary a cultivar may yield good without having any genes for resistance.
Types of Host Plant Resistance:
Host plant resistance to insects may be divided into different categories based on several parameters:
1. Intensity of Resistance:
Interactions between host plants and insects are spread over a wide spectrum of intensity. In terms of the host plant, lesser the population of the insect and/or lesser the damage they cause to the plant, more resistant the plant is likely to be. On the other hand, from the point of view of the insect, interaction varies from totally unsuitable host to completely suitable for growth and development of the insect. Therefore, intensity of resistance is a relative term and should be discussed in relation to a susceptible cultivar of the same species.
Painter (1951) used the following scale to classify degrees of resistance based on intensity:
An immune variety is one which a specific insect will never consume or injure under any known conditions. There are thus few, if any, cultivars immune to the attack of specific insects which are, otherwise, known to attack cultivars of the same species.
(ii) High Resistance:
A variety with high resistance is one which possesses qualities resulting in small damage by a specific insect under a given set of conditions.
(iii) Low Resistance:
A low level of resistance indicates the possession of qualities which cause a variety to show lesser damage or infestation by an insect than the average for the crop under consideration.
A susceptible variety is one which shows average or more than average damage caused by an insect.
(v) High Susceptibility:
A variety shows high susceptibility when much more than average damage is done by the insect under consideration.
These terms are relevant to express resistance vis-a-vis screening of varieties under field conditions and have nothing to do with the mechanism of resistance. An intermediate level of resistance is sometimes referred to as moderate resistance.
2. Ecological Resistance:
Sometimes a plant or a variety may be classified as resistant due to unfavourable environmental conditions for the insect and no heritable trait is involved. In this case, there may be differential impact of the environment on the host and on the insect which affects the expression of resistance. Painter (1951) called this type of resistance as pseudoresistance, which refers to apparent resistance resulting from transitory characters in potentially susceptible host plants.
Pseudoresistance is generally classified into three broad categories:
(i) Host Evasion:
Under some circumstances, a host may pass through the most susceptible stage quickly or at a time when number of insects is less. Some varieties evade injury by early maturing. Late planting of an early maturing variety or other special experiments will indicate whether true resistance is present or not.
(ii) Induced Resistance:
This term may be used for increase in resistance temporarily as a result of some changed conditions of plants or environment, such as change in the amount of water or nutrient status of the soil. Such induced resistance may be of great significance especially in the field of horticulture, but should not be confused with inherent differences in resistance which exist between varieties or individual plants.
Escape refers to the absence of infestation or injury to the host plant because of transitory circumstances such as incomplete infestation. Thus, an uninfested plant located in a susceptible population does not necessarily mean that it is resistant. Even under very heavy infestation, susceptible plants will occasionally escape and only studies of their progenies will establish their true expression of resistance or susceptibility.
The terms host evasion and escape seem to be synonyms but critical analysis reveals that host evasion pertains to whole population of the host and insect is absent or insignificant while escape pertains to one or a few individuals in the presence of insects causing damage to other plants.
3. Evolutionary Concept:
Resistance to an insect is evolved either due to long host plant and insect association at the gene centers or due to pleiotropic effects of genes which are present as a result of selective forces unrelated to the insect. Based on these factors host plant resistance to insects can be divided into sympatric and allopatric resistance.
(i) Sympatric Resistance:
Sympatric resistance may be defined as those heritable qualities possessed by an organism which influence the ultimate degree of damage done by a parasitic species having a prior continuous, coevolutionary history with that species of organism. This type of resistance evolves at original home of plants and insects. Association at the gene centres results in natural selection for resistance in plants. The resistance is evolved as a result of gene-for-gene nature of coevolution of plants and herbivores.
(ii) Allopatric Resistance:
Allopatric resistance may be defined as those heritable qualities possessed by an organism which influence the ultimate degree of damage done by a parasitic species having no prior continuous coevolutionary history with that species of organism. The resistance to insects in plants is evolved in the absence of insects to which the host is resistant. Allopatric resistance is not the result of coevolution, but rather due to fortuitous, pleiotropic effects of genes which are present as a result of selective forces unrelated to the pest insect.
Though not essential, in general, sympatric resistance is governed by major genes and allopatric resistance is polygenic in nature.
4. Genetic Resistance:
Genetic resistance may be grouped under various categories:
I. Number of Genes:
(i) Monogenic Resistance:
When resistance is controlled by single gene, it is called monogenic resistance.
(ii) Oligogenic Resistance:
When resistance is governed by few genes, it is called oligogenic resistance.
(iii) Polygenic Resistance:
When resistance is governed by many genes, it is referred to as polygenic resistance. The term horizontal resistance is also used to denote the resistance governed by polygenes.
II. Major or Minor Genes:
(i) Major Gene Resistance:
The resistance controlled by one (monogenic) or a few (oligogenic) major genes is called major gene resistance. This is also called vertical resistance. Major genes have a strong effect and these can be identified easily.
(ii) Minor Gene Resistance:
When resistance is controlled by a number of minor genes, each contributing a small effect, it is called minor gene resistance. This is also referred to as horizontal resistance. In certain crops, the cumulative effect of minor genes is expressed when the plants grow older and this phenomenon is termed as adult resistance, mature resistance or field resistance.
III. Biotype Reaction:
(i) Vertical Resistance:
This type of resistance is effective against certain specific biotypes of the insect but not against others. It is also called specific resistance. Vertical resistance is qualitative as the frequency distribution of resistance and susceptible plants are discontinuous.
(ii) Horizontal Resistance:
This type of resistance is effective against all the known biotypes of the insect. It is also called nonspecific resistance. Horizontal resistance is quantitative as the degree of resistance depends on the number of minor genes each contributing a small effect.
IV. Multitrophic Interactions:
Interaction among host plants, insect pests and their natural enemies (tritrophic interaction) leads to effective defense and attack at each level.
On this basis, two types of plant resistance have been recognised:
(i) Intrinsic Resistance:
Here the plant alone produces defense through physical means (trichomes or toughness) or through production of chemicals (toxins or digestibility reducers) or both (glandular trichomes or resins).
(ii) Extrinsic Resistance:
Here the natural enemies (third trophic level) of insect pests (second trophic level) benefit the host plants (first trophic level) by reducing the pest abundance.
Mechanism of Resistance:
Painter (1951) grouped mechanisms of resistance into three main categories, viz., nonpreference, antibiosis and tolerance.
Nonpreference refers to the response of the insect to the characteristics of the host plant which make it unattractive to the insect for feeding, oviposition or shelter. As the term ‘nonpreference’ pertains to the insect and not to the host plant, Kogan and Ortman (1978) proposed the term antixenosis to describe the host plant properties responsible for nonpreference. Antixenosis signifies that the plant is considered an undesirable or a bad host. Antixenosis may result from certain morphological characteristics or the presence of allelochemicals in the host plant.
Under certain circumstances the nonpreference response of the insect can be quite important, especially when light infestations cause severe damage, e.g., infestation by insect vectors of plant diseases or insects which sever growing parts or peduncles of the plants such as stem borer infestations resulting in white heads. In field plantings, nonpreferred varieties frequently escape infestation and even when insects are caged on nonpreferred hosts, they lay fewer eggs and thereby develop smaller populations than those caged on susceptible varieties.
Antibiosis refers to the adverse effect of the host plant on the biology (survival, development or reproduction) of the insects and their progeny infesting it. All these adverse physiological effects of permanent or temporary nature following ingestion of a plant by an insect are attributed to antibiosis.
The insects feeding on resistant plants may manifest antibiotic symptoms varying from acute or lethal to subchronic or very mild. The most commonly observed symptoms in insects include larval death in first few instars, abnormal growth rates, disruption in conversion of ingested food, failure to pupate, failure of adults to emerge from pupae, abnormal adults, inability to concentrate food reserves followed by failure to hibernate, decreased fecundity, reduction in fertility, restlessness and abnormal behaviour.
These symptoms may appear due to various physiological processes, viz., presence of toxic substances, absence or insufficient amount of essential nutrients, nutrient imbalances, presence of antimetabolites and enzymes adversely affecting food digestion and utilization of nutrients.
Tolerance refers to the ability of the host plant to withstand an insect population sufficient to damage severely the susceptible plants. It is generally attributable to plant vigour, regrowth of damaged tissues, resistance to lodging, ability to produce additional branches, utilization of non-vital parts by insects and compensation by growth of neighbouring plants.
However, tolerance has no adverse effect on the insect pest. The ability of tolerant varieties to support insect infestation for longer periods without loss in yield or quality than the susceptible varieties enables them to frequently escape insect damage through compensation by the plants.
As tolerance is not likely to provide a high level of resistance, it could be useful in combination with other mechanisms of resistance. Moreover, tolerant varieties do not depress insect populations nor do they provide any selection pressure on the insects. Thus, these can prove very useful to prevent the development of insect biotypes.
Although the above widely recognised classification of mechanisms appears to provide a generally acceptable breakdown of the phenomenon of host plant resistance, however, some overlap may occur between antixenosis and antibiosis, and a problem may arise in the separation of these two mechanisms.
Antixenosis refers to undesirability, i.e., avoidance by insect whereas antibiosis refers to unsuitability, i.e., adverse effects on the insect after feeding on the host plant. However, sometimes it becomes difficult to separate the two mechanisms unless the insect-plant relationship is fully examined. For example, Eruca sativa Lam. (taramira) is not a preferred host of mustard aphid, Lipaphis erysimi (Kaltenbach).
The growth and development of this insect was observed to be slower on E.sativa as compared to that on Brassica species in confinement. The mechanism appeared to be antibiosis, but it had been found that the poor development was due to reduced feed uptake because of the presence of certain allelochemicals in E. sativa, indicating antixenosis.
Genetics of Host Plant Resistance:
Information on the number of genes involved in resistance of plants to a particular insect pest has great practical significance in identifying diverse sources of resistance and using these for breeding broad-based resistant plants. The inheritance of Hessian fly resistance in wheat has been most thoroughly investigated and there are 32 genes identified that are dominant or partially dominant for resistance.
The resistance to first brood European corn borer in corn inbreeds has been found to result from one gene pair; two or more gene action controlling resistance has been variously reported as dominant or partially dominant, primarily additive or having a significant epistatic component. The inheritance of resistance in corn to the corn leaf aphid, Rhopalosiphum maidis (Fitch), is determined by many genes with varying degrees of dominance and additivity.
The resistance to brown planthopper and green leafhopper in rice is simply inherited. Nine dominant [Bph-1, Bph-3, Bph-6, Bph-9, Bph-10, Bph-14, Bph-16, Bph-17 and Bph-18 (t)] and 12 recessive (bph-2, bph-4, bph-5, bph-7, bph-8, bph-11, bph-12, bph-13, bph-15, bph-19(t), bph-20 and bph-21) genes for brown planthopper resistance have been identified from rice varieties. Ten dominant (Glh-1, Glh-2, Glh-3, Glh-5, Glh-6, Glh 7, Glh-9(t), Glh-11(t), Glh-12(t) and Glh-13(t)) and three recessive (glh-4, glh-8 and glh-10) genes for green leafhopper resistance have also been identified.
The resistance to whitebacked planthopper, Sogcitellci furcifera (Horvath), has been found to be governed by seven dominant genes designated as Wbph-1, Wbph-2, Wbph-3,Wbph-5, Wbph- 6, Wbph-7(t) and Wbph-8(t) and one recessive gene, wbph-4. The resistance of barley to the greenbug is attributed to two dominant genes. A single incomplete dominant gene or dominant genes at more than one locus, account for resistance to greenbug in sorghum (Table 9.2).
The significance of genetic analysis of resistance is evident from the success of host plant resistance programme in rice for the brown planthopper (BPH). The first BPH-resistant variety with Bph-1 gene, IR26, was released in 1973. It was widely accepted in Indonesia, Philippines and Vietnam, but became susceptible in 1976-77 due to the development of biotype 2 of BPH. IR36 with bph-2 gene replaced IR26 and is still widely grown.
Meanwhile, when a biotype capable of damaging IR36 appeared in small pockets in the Philippines and Indonesia, IR56 and IR60 with Bph-3 gene for resistance were released. IR66 with bph-4 gene for resistance was released in 1987, while IR68, IR70, IR72 and IR74, all with Bph-3 gene were released in 1988. These varieties are now extensively grown in tropical and subtropical rice-growing countries.
Durable Plant Resistance:
Various strategies can be adopted to prolong the useful life of the resistant varieties or to develop varieties with different genes so that the farmers may have access to new varieties when the resistance of the current varieties breaks down.
Major Gene Resistance:
Several strategies can be employed to maintain varietal resistance and to prolong the useful life of major genes.
Some of the strategies are:
(i) Sequential Release of Varieties:
The sequential release of varieties with major genes involves the incorporation of a single major gene into commercial varieties. This strategy has been successfully followed for controlling the brown plant hopper (BPH) of rice in Asia. Widespread outbreaks of BPH occurred in 1973-74 in several rice-growing countries.
The rice varieties with Bph-1 gene for resistance such as IR26, IR28 and IR30 were released in several countries such as Indonesia, Vietnam and the Philippines. By 1977-78, a new biotype capable of attacking these varieties appeared and varieties with Bph-2 gene such as IR36, IR38 and IR42 were released.
These were widely grown for about a decade when a new biotype appeared. Varieties with Bph-3 gene, IR68, IR70, IR72 and IR74 were then released and are now widely grown. This strategy has also been followed for resistance to the Hessian fly in wheat.
(ii) Gene Pyramiding:
Pyramiding of major genes aims to combine two or more major genes into the same variety. Varieties with two or more major genes are likely to have a longer useful life as the development of new biotypes will be slower. It has been postulated that a simultaneous release of varieties with two genes for resistance to the Hessian fly of wheat would result in better durability than a sequential release.
Pyramiding of two Bt genes through genetic engineering has also been proposed to prolong the useful life of transgenic resistance. Similarly, combining genes encoding a toxin and a repellent may offer longer lasting resistance than either approach alone.
(iii) Rotation of Varieties:
The process of adaptation of an insect can be interrupted by growing a resistant variety in one season and another resistant variety with a different gene during the next season. This strategy was followed to protect the rice crop from tungro virus epidemics in South Sulawesi, Indonesia, in 1970s.
Varieties with one gene for green leafhopper (vector for virus) resistance were planted in one season and another variety with a different gene for resistance was planted in the next season. This strategy has-been very effective in prolonging the useful life of the vector-resistant varieties.
(iv) Multiline Varieties:
The multiline approach envisages the incorporation of several major genes into an isogenic background and the mixing of these lines to form a multiline variety. This strategy was successfully employed for breeding oats with crown rust resistance. However, the effectiveness of this strategy in developing insect-resistant varieties is little known.
(v) Varietal Mixtures:
This strategy employs the use of varietal mixtures consisting of 80-90 per cent resistant plants and 10-2-0 per cent susceptible plants of similar varietal background. Such varietal mixtures exert lower selection pressure on the insect as they are able to survive and reproduce on the susceptible plants.
Polygenic resistance is a quantitative trait that is governed by a large number of genes, each with a small contribution to resistance. The level of resistance is not generally high and it does not exert strong selection pressure on the insect, hence a virulent biotype rarely if ever develops and the resistance is more durable.
However, parents with polygenic resistance are generally landraces with poor agronomic traits. In the process of selecting plants with better agronomic traits in crosses involving such parents, not all the polygenes are transferred and the level of resistance is diluted. Alfalfa germplasm resistant to the spotted alfalfa aphid has been developed through mass selection for polygenic variation.
Induced resistance is the qualitative or quantitative enhancement of a plant’s defense mechanisms against pests in response to external physical or chemical stimuli. Induced resistance results in change in a plant that produces a negative effect on herbivores. This is a non-heritable resistance where host plants are induced to impart resistance to tide over pest infestation. Induced resistance offers considerable promise to increase the levels of resistance to insect pests.
Plant allelochemical production may be induced by any injury to the plant, such as herbivore feeding or even through the autolysis of plant cells. Mechanical disruption of plant tissues whether by shaking or rubbing can also affect the plants physicochemically and thus, the development of insects associated with them.
These mechanical disruptions evoke phytochemical responses which have been classified as:
(i) Cellular chemical changes,
(ii) Changes in cells adjacent to the damaged tissues, and
(iii) Generalized changes apparent in a plant part or the entire plant.
Many studies of induced responses have indicated changes in the levels of tannins and phenols, which are products of shikimic acid pathway. The relative activity of the enzyme phenylalanine ammonia lyase (PAL) can determine the production of phenolics, including lignin. Hence, PAL activity is considered an important indicator of induced resistance. Herbivore damage also affects the concentration of available nitrogen as well as other important nutrients in foliage.
Wounding plant tissues may induce changes in protein, lipid and phenol metabolism. Phytochemicals produced by damaged plants may be detrimental in insects. Mechanically wounded tomato leaves have been found to stimulate the release of a proteinase inhibitor inducing factor (PIIF) into vascular transport system of damaged plants.
The feeding of larvae of Spodopterci littorcilis (Boisdual) on damaged potato leaves decreased by nine fold within 8h after damage, and within 24h, leaves adjacent to initially damaged leaves promoted similar adverse effects on larval feeding. Wounding induced the oxidation of plant phenols to produce toxic quinones and synthesis of mono-and diphenols. Phenol levels also increased following damage by lygus bugs, Lygus disponsi Linnavuori, to Chinese cabbage, sugar beet and cotton.
One of the most exciting reports is about the damage to popular and sugar maple tree foliage that increased the total phenol content of foliage of adjacent, non-connected trees, suggesting that plants are capable of communicating through wounding. Therefore, mild wounding via defoliation, abrasion or infection appears to elicit a general plant response that is beneficial to the plant but detrimental to the insect.
The potential of the use of plant growth regulators (PGRs) to induce resistance in plants has recently attracted considerable attention. The PGR (2-chloroethyl trimethyl-ammonium chloride (CCC), limits fecundity or survival of the cabbage aphid, Brevicoryne brassicae (Linnaeus) and green peach aphid, Myzus persicae (Sulzer) on treated susceptible cultivars of Brussels sprout.
PGRs affect the chemical bases of insect-plant interactions in following two ways:
(i) PGRs can alter the nutritional quality of the host plant by reducing the amount of available protein or amino acids;
(ii) PGRs can trigger the biosynthesis of allelochemicals in the plant which will be both toxic and feeding deterrent to herbivorous insects.
Economic Impact of Insect Resistant Plants:
There are several examples which reveal distinct advantages to farmers by growing insect- resistant varieties. The Hessian fly, Mayetiola destructor (Say), used to be a serious pest of wheat in USA, but its incidence was reduced from nearly 100 per cent to below 1 per cent in certain areas by the cultivation of resistant varieties.
By 1974, nearly 6.5 million ha were planted to Hessian fly- resistant wheat cultivars and by 1980, more than 28 Hessian fly-resistant varieties had been released to farmers in USA. The cultivation of resistant varieties of wheat has saved the growers at least $10 million annually in production losses from Hessian fly and wheat stem saw fly, Cephus cinctus Norton.
The significance of resistance in this case is even more prominent because alternate control measures against these pests are ineffective or impractical, owing to large area involved and relatively small per unit area value of the crop. The yield losses from wheat stem saw fly alone could exceed 75 per cent when resistant varieties are not used.
Similarly, the cultivation of European corn borer-resistant maize has made it possible a reduction in the use of insecticides by about 22,000 tonnes per year against this insect and has increased maize yields considerably. The resistant maize inbreds grown in mid western U.S. reduced losses by the borer from $350 million in 1949 to $10 million in the 1960s. The use of resistant varieties to the spotted alfalfa aphid, Therioaphis trifolii f.maculata (Buckton) has also led to an annual saving of about 300 tonnes of insecticides in USA.
The aphid-resistant varieties saved growers at least $35 million annually in the southwestern U.S. during the 1960s. It has been estimated that about 319,000 tons of insecticides (approximately 37% of the total insecticides applied during 1960s) were saved annually through planting of insect resistant cultivates of alfalfa, barley, maize and sorghum in USA.
The success in developing insect-resistant rice cultivars has been outstanding and cultivars resistant to brown planthopper, green leafhopper, yellow stem borer, striped stem borer and gall midge have been developed and are extensively grown. As an example, a rice variety 1R36, which is resistant to all the above five insects, is planted annually in over 10 million ha of riceland of the world.
Its cultivation alone has yielded an additional income of one billion dollars annually to rice growers and processors. Rice varieties resistant to the brown planthopper and green leafhopper are planted over 200 million ha of riceland in Asia. Dr Gurdev S. Khush was awarded the 1996 World Food Prize for developing IR36 and other rice varieties.
Despite the cost and time of development, the ultimate returns from insect-resistant varieties are quite impressive. It has been estimated that the costs for development of cultivars resistant to Hessian fly, wheat stem sawfly, spotted alfalfa aphid and European corn borer were about $9.3 million.
However, the total savings to cultivators using these varieties were about $308 million annually. Thus, after 10 years of use, these varieties provided a net saving of about $3 billion, i.e. a 300: 1 return on each research dollar invested. The total estimated global value of insect resistant cultivars is approximately $ 2.0 billion.
This is in addition to the other ecological advantages such as pest suppression in adjacent susceptible crops, absence of secondary pest outbreaks, and reduced mortality of beneficial arthropod populations and minimum disturbance of the agroecosystem.