Host Plant Resistance could support sustainable IPM in GM cotton systems by reducing the need to apply insecticides against emergent pests or other secondary pests. Cotton (Gossypium sp.) is a major crop in many countries around the world and its fiber is a major raw material for apparel, bed linen, and many other products. About 35 million ha of cotton are planted in the world each year, producing about 26 million tones of lint.
The word ‘cotton’ refers to four separate species in the genus Gossypium that are grown for the fibers covering the epidermis of their seeds: G. arboreum, G. barbadense (Pima cotton), G. herbaceum, and G. hirsutum (Upland cotton). This review will focus on G. hirsutum cotton, as it comprises around 95% of global cotton production.
Challenges to Pest Management
Arthropod pests have likely affected cotton since it was domesticated at least 3,000 years ago. A large number of arthropod species have been described as cotton pests, but only less than 40 of them are considered key pests of the crop. They directly decrease yield or reduce fiber quality, and their management is a key challenge for cotton growers worldwide.
Potential losses up to 40% occur from invertebrate pests alone in cotton. Significantly, even after the implementation of control measures, it is estimated that losses of about 12% occur to invertebrate pests. The economic implications of invertebrate pests encompass both crop losses and the costs of control, which mainly consist of insecticides and their application.
In many cotton systems, the primary pests are lepidopterans such as Helicoverpa or Heliothis sp., Earias sp., and Pectinophora sp. The capacity to manage these pests without spraying insecticides would strongly support IPM approaches. Genetic modification (GM) of cotton containing genes to express the protein(s) from the bacteria Bacillus thuringiensis (Bt), which are highly effective at killing the larvae of some lepidopterans, was introduced in the mid-1990s and greatly reduced pesticide use.
Bt-cotton is highly efficacious against target pests, at the same time having a negligible effect on non-target insects and causing little or no harm to most other organisms, including people.
Globally, 25 million hectares were planted in 2013 to Bt-cotton, representing 68% of all cotton grown in the world. Including other crops, 76 million hectares were planted to genetically engineered crops producing insecticidal proteins from Bt.
However, GM cotton is not a ‘perfect’ solution. Firstly, target pest species may become resistant, requiring the implementation of strategies to reduce this risk. This risk is especially high for cultivars expressing a single Bt protein. Several of these genes, therefore, need to be stacked to delay the development of resistance in the target insect population.
However, HPR traits may help support resistance management for the Bt-cottons as Carrière et al. (2004) and Williams et al. (2011) reported that the presence of the terpenoid gossypol, which provides resistance to a range of cotton pests, can contribute to delaying the development of insect resistance against Cry proteins. Secondly, Bt-cotton crops can sometimes provide a more favorable environment for other pests that are not susceptible to the Bt proteins.
The sucking bug complex in particular was historically controlled co-incidentally by insecticides applied against lepidopteran pests. Consequently, with dramatically reduced pesticide use against lepidopteran pests the sucking pest complex has increased in importance in most Bt-cotton systems. These ‘emergent’ pests may require targeted control, which creates further issues as control options are often limited and the less expensive options, such as pyrethroids or organophosphates, are disruptive of natural enemy populations.
Use of these compounds against sucking pests ultimately leads to an increase in risks of secondary pests outbreaks, such as spider mites, aphids, or whitefly. These secondary pests then require control, hence, selecting them for pesticide resistance. In Australia for example, spider mites have become resistant to both organophosphates and pyrethroids. Although insecticide applications have greatly decreased with the adoption of Bt-cotton, even with the presence of some important outbreaks caused by secondary pests, some specific situations have been reported with increases in the number of applications required due to these outbreaks.
Among the key pests that are challenges in Bt-cotton systems are the sucking bugs, spider mites, thrips, silverleaf whitefly, and aphids. Sucking bugs are currently considered the primary pest in many of the Bt-cotton growing regions such as Australia, China, India, and the United States, and in most seasons will require targeted control. The sucking bug complex comprises primarily of Adelphocoris sp., Lygus sp., Creontiades dilutus and C. pacificus, mealybugs (Phenacoccus solenopsis, Pseudococcus corymbatus, Pulvinaria maxima, and Saissetia nigra), and the green vegetable bug (Nezara viridula).
These species feed on young squares and bolls, causing their abortion or damage to developing bolls. Spider mites (predominantly Tetranychus urticae) feed on the underside of leaves by sucking out the contents of the mesophyll cells, resulting in reduced yield and fiber quality. Thrips (predominantly Frankliniella sp. and Thrips sp.) are able to damage cotton seedlings and therefore cause a delay in plant growth and maturity, sometimes reducing yield when the attack is severe.
Conversely, later in the season thrips are also considered beneficial insects as they are key predators of spider mites. Silverleaf whitefly (Bemisia tabaci) secretes honeydew which contaminates lint, causing difficulties in the mill when the fiber is processed. The development of silverleaf whitefly populations resistant to a wide range of insecticides exacerbates the problem. Cotton aphids (Aphis gossypii) cause a similar damage to the lint as they excrete honeydew when they feed on the plants. They are vectors for viruses and their feeding distorts plant growth and causes a reduction in photosynthetic activity.
Available Sources and Traits for Host Plant Resistance
Controlling these ‘emergent’ sucking pests with pesticides poses a risk to successful IPM approaches, and at the same time undermines the value of GM technology, as Bt-cotton facilitates the control of non-target pests by their natural enemies. HPR could support sustainable IPM in GM cotton systems by reducing the need to apply insecticides against emergent pests or other secondary pests. Cultivars resistant to key emergent or secondary pests would require less pesticide applications, thus reducing costs, increasing the population of beneficial insects, and helping the environment.
Sources of Resistance in Gossypium sp.
The first step to improve HPR to invertebrate pests is to identify the resistance traits that can be incorporated into elite cotton cultivars through breeding. These traits can be found in the cotton genetic pool or created through molecular techniques. Therefore, the availability of gene pools with enough variability to include some genotypes with high levels of HPR is essential. The genus Gossypium comprises about 50 species with a high genetic diversity between them.
It appeared between 10 and 15 million years ago and diversified in three different centers of origin: Africa–Arabia, Australia, and Central America. The genus can be divided into eight diploid genome groups (2n = 26 chromosomes), as well as five allotetraploid species (2n = 52). Of these, only four species are grown commercially (G. arboreum, G. barbadense, G. herbaceum, and G. hirsutum).
The African G. herbaceum and the Indian G. arboreum are both diploids while the American G. barbadense and G. hirsutum are both allotetraploids. The diversity within the cultivated species has declined due to domestication and breeding for increased productivity. Despite this lack of diversity, especially in G. hirsutum, there has been research to identify HPR traits to key pests. The bollworm complex has been excluded from the table as this review focuses on the management of emergent or secondary pests in Bt-cotton systems.
Plant Defense Mechanisms
Host plant resistance against herbivorous invertebrate pests is generally defined as “the sum of genetically inherited qualities that results in a plant of one cultivar or species being less damaged by a pest arthropod than a susceptible plant lacking these qualities”. Among its benefits as a pest control measure, HPR is durable, easy to use, environmentally friendly, and compatible with other management practices.
On the other hand, breeding for HPR is generally a slow and difficult process that has mostly been overlooked in preference to the use of chemical control of pests. In recent times, breeding for HPR is becoming a more feasible alternative due to several facts: the reduction in the impact of the Lepidopteran pests by Bt-cotton, increasing pest resistance to insecticides, enactment of strict environmental regulations on insecticides and their use, and advances in molecular technologies.
Plant defense mechanisms have been traditionally classified into three main categories: antixenosis or non-preference mechanisms, that prevent or deter the herbivore from feeding on the plant; antibiosis mechanisms, that affect the insects performance and survival by a physical or chemical trait; and tolerance, that represents the plant’s ability to compensate for herbivore damage and yield productivity.
Currently, tolerance is usually regarded as a plant defense strategy separate from resistance. Resistance is to cover “those plant traits that reduce the extent of injury done to a plant by a herbivore” as in practice antixenosis and antibiosis are often difficult to separate.
Resistance mechanisms or categories can also be direct (e.g., antibiosis, leaf morphology) and indirect (e.g., the attraction of natural enemies of the herbivore), and they can be expressed constitutively (e.g., leaf morphology) or be induced following a cascade of processes after some damage is caused by the herbivory (e.g., induced chemical responses). All of these mechanisms are unusually controlled polygenetically, but a number of cases of single-gene resistance have also been reported.
HPR Traits Available in Cotton
Traits providing HPR in cotton can include one or several defense mechanisms functioning in a complex way. Some of the morphological traits provide a mechanical barrier to the pest, such as trichomes or hairs on leaves, while others influence the general growing habit and appearance of the plant, such as okra leaf or red coloration of the plant or even the microclimate conditions present on the leaf, such as in okra leaves.
There is also a wide array of chemical compounds used by cotton plants to defend themselves from herbivores, such as flavonoids, tannins, and particularly terpenoids such as gossypol. The latter is produced by plants of the genus Gossypium and has been shown to be toxic to many pests that affect cotton. The application of HPR traits is complex as different traits can operate at the same time to provide a given level of resistance.
A number of reviews focused on HPR traits in cotton are available. In the present review, HPR traits will be discussed from the point of view of the genetic source providing the resistance and the prospects for the incorporation of these traits in commercial cultivars.
Traits for direct resistance mechanisms are frequently targeted in HPR breeding because they usually have major effects and they are also easier to identify and select for. On the other hand, traits for indirect HPR are not as simple to identify and are rarely targeted. Traits for both constitutive and induced HPR can play a major role in controlling HPR, but constitutive mechanisms are more usually targeted as once they are identified, plants carrying them can be selected without having to perform a bioassay.
For that reason, traits for constitutive morphological resistance, such as a high leaf hair density or thickness are often initially targeted in breeding programs. Other traits for constitutive HPR, such as constitutive chemical compounds, can also be relatively simple to target.
However, the initial identification of the specific compounds involved in the resistance is often more challenging than identifying morphological HPR traits. Antibiosis traits can have the biggest impact on HPR and are probably the most successfully used in cotton, both in breeding for secondary pests and in main pests (Bt-cotton). However, identifying antibiosis is not as straightforward as other HPR traits such as morphological traits, often requiring the use of bioassays.
The history of cotton production is linked with the history of the emergence of new pests. In recent times, these emergence events have generally been related to the use of insecticides and/or the emergence of Bt-cottons. However, there are few examples of successful deployment of HPR traits to the emergent pests or linked secondary pests in cotton cultivars.
Recent research indicates that there is significant scope to improve HPR in cotton, especially against key secondary pests. This review outlines sources of germplasm and the opportunities to improve HPR in cotton against invertebrate pests in GM cotton systems. Unfortunately, traits providing a high level of HPR sometimes have other undesirable effects. Therefore, it is necessary to use caution when introgressing these HPR traits into elite cultivars. Modern techniques can also help to identify and expedite the process of incorporating HPR traits into elite germplasm.
Source: Trapero, C., Wilson, I. W., Stiller, W. N., & Wilson, L. J. (2016). Enhancing integrated pest management in GM cotton systems using host plant resistance. Frontiers in plant science, 7, 500.