How to develop insect-pest resistant plants?

like 891

Plants are attacked by a variety of insect-pests (herbivores). Some insects suck the nutritious juice from phloem of the host plant (sucking insects-hoppers, whitefly, aphids, thrips etc.), some eat or chew the tissues (chewing insects-caterpillars) and some others bore the plant parts (fruit fly, beetles etc.). Damage by such insect-pests may cause severe crop loss and/or poor food quality. For instance, an outbreak of brown plant hopper (BPH) in rice during 1970s caused economic havoc in South East Asia. Insect-pests can be managed by application of chemicals (pesticides) that would kill them; but this approach is not desirable because traces of pesticides (residues) may stay in the food items or in the surroundings, which would cause food and environmental safety concerns. Therefore, it is important to develop crops that could inherently withstand pest attack and would require no or only minimum application of chemicals to protect them. Scientists have long been exploring this possibility and found that some plants in a species do resist the insect attack, which is generally referred to as “Host Plant Resistance”. In history, the earliest report on discovery of host plant resistance to insects dates back to 1792 when a wheat variety called ‘Underhill’ was found to resist attacks of Hessian fly in New York. Plants have different mechanisms to defend themselves from insect-pests—constitutive or induced defenses. Host plant resistance is mostly of constitutive defense mechanism and is more exploited in development of crop plants resistant to insect-pests.

Host plant characteristics, as defined by Painter—those characters that enable a plant to avoid, tolerate or recover from attacks of insects under conditions that would cause greater injury to other plants of the same species, have evolved through insect-plant interactions. These characteristics are grouped under three categories – antixenosis (non-preference), antibiosis and tolerance. Antixenosis refers to plant traits (morphological, physical or structural and biochemical) that keep the insects away from it so that no colonization takes place. This happens by causing disturbances in the insect behaviour (settling, mating, oviposition, feeding and food ingestion). Pubescence, tissue hardness, repellents and antifeedents or feeding deterrents contribute to antixenosis. Antibiosis operates after the insects have colonized and started utilizing the plant. It affects insect’s growth, development, reproduction and survival. Under antibiosis, insects cannot gain weight, turn restless and finally may die. Toxins, growth inhibitors and nutritional imbalances are some of the antibiotic factors. Glandular trichomes which produce secondary metabolites contribute to antibiosis in plants. Tolerance refers to plant’s ability to grow, reproduce, repair injury and yield satisfactorily under insect-pest attack. Only moderate level of resistance is exercised in tolerance (horizontal) while the antixenosis/antibiosis offer high level of resistance (vertical) and exert heavy pressure on the insect population. Due to this pressure, the insect-pest population adapts (called as biotype) and defeats the host resistance. Emergence of biotypes has been a serious consequence of exploiting vertical resistance against insect-pests in crops. Therefore, tolerance is considered a safer mechanism to exploit but is very challenging to study it.

How do we find if a plant has resistance against an insect-pest? We need to ‘infest’ the plant with insects artificially and monitor its response using standard procedures. As exemplified by the complex nature of resistance categories, resistance can be visualized in many different forms (phenotypes). However, using a simple measure such as ‘seedling survival after insect infestation,’ geneticists report ‘major genes’ that confer vertical resistance against insect-pests in plants. These genes act in ‘dominant or recessive’ manner and have been exploited in cultivar development. For instance, to combat the outbreak of BPH in rice, three genes (Bph1, bph2 and Bph3) were incorporated into the cultivars one after the other, which could curb the severity of the pest. Today, developments in plant molecular biology and genomics have enabled us to expand our knowledge on molecular basis of plant’s responses to insects. A number of genes, their functions, and defense pathways are being unravelled, which would help developing pest resistant cultivars that can be grown free of pesticides and offer us a healthy food.  

ct-resistant genetically modified (GM) crops have been one of the major successes of applying plant genetic engineering technology to agriculture; cotton (Gossypium hirsutum) resistant to lepidopteran larvae (caterpillars) and maize (Zea mays) resistant to both lepidopteran and coleopteran larvae (rootworms) have become widely used in global agriculture and have led to reductions in pesticide usage and lower production costs. Plants have been developed with short sequences of genes from Bt (Bacillus thuringiensis-this bacterium produce proteins that kill certain insects with alkaline digestive tracts) to express the crystal protein Bt makes. With this method, plants themselves can produce the proteins and protect themselves from insects without any external Bt and/or synthetic pesticide sprays. Using this technology, Bt corn, potato and cotton were developed.

 

Article by

Dr. Kadirvel Palchamy,

Senior Scientist (Genetics)

ICAR-Indian Institute of Oilseeds Research

Rajendranagar, Hyderabad-5000030

Telangana State (INDIA)

E-mail: kadirvel.palchamy@icar.gov.in

Posted By : ScienceIndia Administrator