Authors: Deepak V Pawar, Kishor U Tribhuvan , Jyoti Singh
1ICAR-NRCPB, I.A.R.I, New Delhi-12
For many years, man has worked to improve agricultural productivity by using soil microbes. These microbes can be cultivated on a large scale and made to assist plant growth. This process is known as microbial fermentation. Microbes function as both providers and defenders, either by converting important macromolecules into forms usable by plants as biofertilizers or control weeds, pests, and diseases as bioherbicides and bioinsecticides.
Biofertilizers
Plants have a limited ability to extract phosphate and nitrogen from the environment and need microbes to absorb them. These microbes serve as biofertilizers in “nutrient recycling” and help plants gather energy sources in exchange for food in the form of byproducts. This helps plants develop bigger root systems. The fungus Penicillium bilaii produces an organic acid that converts phosphates into forms useful to plants. A biofertilizer from this organism is applied either by coating seeds with the fungus or applying it directly into the ground. In legumes, the bacterium Rhizobium lives in nodules found in roots. These nodules can take nitrogen from the air and turn it into its available form and transfer the nutrient directly into the plant.
Biofertilizers have been found to:
- increase crop yield
- replace chemical nitrogen and phosphorus
- stimulate plant growth
- activate the soil biologically
- restore natural soil fertility
- protect against drought and some soil borne diseases
Fermentation methods have also developed bioinsecticides which were based on the insecticidal proteins of bacteria, fungi, and viruses. Bioinsecticides do not persist long in the environment, have short shelf lives and are effective even in small quantities. They are also safe to humans and animals and affect only a single species of insect. However, bioinsecticides work slowly and their efficacy can depend on the timing of application. Since most bioinsecticide agents are living organisms, their success is also affected by environmental factors and other microbial competitors present in the environment.
Bacteria-based bioinsecticides
A widely used bioinsecticide, the bacterium Bacillus thuringiensis , or Bt, produces a protein poisonous to insects. After ingestion, the toxin creates ulcers in the insect’s stomach causing the insect to die. However, Bt is very selective and affects only specific species of insect pests and
does not harm humans, birds, fish, or other beneficial insects.
Fungi-based bioinsecticides
Fermentation technology is also used to mass produce fungi-based bioinsecticides. Their spores are harvested, packaged and applied to insect-infested fields. These spores use enzymes to break through the surface of the insects’ bodies and, once inside, begin to grow and cause death. One bioinsecticide, Bb, is based on the action of Beauveria bassiana, a fungus found worldwide in soils and plants. These Bb bioinsecticides have many advantages. The fungus does not grow in warm-blooded organisms, does not harm plants, and does not survive long in bodies of water. Its spores can also withstand harsh conditions and is inactivated by ultraviolet rays.
Virus-based bioinsecticides
An example of a virus-based bioinsecticide is the Baculovirus. It affects insect pests like corn borers, potato beetles, flea beetles, and aphids. One particular strain is being used to control Bertha army worms, which attack canola, flax, and vegetable crops.
Bioherbicides
Weeds are a constant problem for farmers and if left uncontrolled, can reduce crop yields significantly. Farmers fight weeds with tillage, hand weeding, synthetic herbicides, or a combination of all techniques. The use of bioherbicides is another way of controlling weeds without the hazards of synthetic herbicides. Bioherbicides are made up of microorganisms and certain insects that can target specific weeds. The microbes possess invasive genes that can attack the defense genes of the weed and kill it. Some bioherbicides also contain pathogens with genes that can cause fatal diseases to a specific weed only. This specificity of the microbes makes such bioherbicides very useful. Bioherbicides can also survive in the environment long enough for the next growing season and are cheaper than synthetic pesticides.
Bioherbicides and Striga
Sub-saharan Africa is home to sorghum and corn, as well as Striga, a weed that can wipe out important cereals, lower crop yields and increase the cost of production. Using bioherbicides together with genetic modification of certain cereals, scientists have lowered Striga parasitism and have increased corn and sorghum harvests. For instance, the sorghum seeds can be inoculated with the fungus Fusarium through a coating of Arabic gum. The preparation for this takes up to 14 days and is conducted by village women. Another approach against Striga is the new hybrid maize Ua Kayongo, whose seeds are coated with Strigaway herbicide. It has Imazapyr resistance (IR-maize), which is based on a naturally-occurring herbicide resistance in maize and which was incorporated into Kenyan maize varieties by African plant breeders at International Maize and Wheat Improvement Center (CIMMYT) and the Kenya Agricultural Research Institute (KARI).
Conclusion
Microorganisms can either work symbiotically with plants to help in plant nutrition or they can work alone, or with other species, in battling weeds and pests. Although microorganisms are often labeled dangerous, they can be crucial in saving crops, increasing yields, and protecting soils.
References:
EduGreen. 2015. Biofertilizers. http://edugreen.teri.res.in/explore/bio/ferti.html
US Environmental Protection Agency. 2015. What are BioPesticides http:// www.epa.gov/pesticides/biopesticides/whatarebiopesticides.html
About Author / Additional Info:
I am PhD research scholar, pursuing PhD at IARI, New Delhi in the discipline of Molecular Biology and Biotechnology. I am working on blast disease resistance in O. sativa