Use of Plant Growth Promoting Bacteria

Under natural environmental conditions, successful plant growth developments and higher crop yields depend mainly on the genetic make-up of crops, nutrients availability, the presence of certain useful microorganisms and the absence of pathogens in the surrounding soil. Some beneficial soil bacteria and fungi act directly by providing plant growth enhancing products by atmospheric nitrogen fixation process, while others act indirectly. Indirectly acting soil bacteria are those bacteria that inhibit the growth of harmful pathogenic soil microorganisms and thus protect the plants from them. This process is called antibiosis. It occurs as either the beneficial strain takes up all the resources and leaves no resources for the growth of pathogens or it produces and releases a compound that stops the growth of phytopathogenic organisms.

Although chemical fertilizers increase the crop yields up to a significant level, but their cost has been increasing rapidly. Moreover, they have many negative effects both on environment and human health. The toxic chemicals present in fertilizers are absorbed by plants and enter the food chain through vegetables and cereals. The most harmful effect of fertilizers on human health is the seepage of chemicals into the ground water which is then extracted in drinking water. This water can contain high levels of nitrates and nitrites that cause blue baby syndrome. Mercury, lead, cadmium and uranium are very toxic metals that have been found in fertilizers and can cause problems in kidneys, lungs and liver and can also cause cancer depending upon their quantities consumed by humans.

So there is a dire need to develop alternate sources of fixed nitrogen including the development of diazotrophic microorganisms such as Rhizobium, Frankia, Azospirillum, Azotobacter and Cyanobacteria.

Phytopathogens are serious threat to agricultural productivity and may cause losses to crop yield from 25 to 100%. It is a very huge loss of productivity. For most of the bacterial diseases, plants show no symptoms for a prolonged period of time. Under these conditions, the entire crop can be destroyed. Most of the chemicals that are used to kill phytopathogens are harmful to humans and animals. They persist in the natural environment. So the need of the hour is that to replace these hazardous chemicals with biological control agents that are environment friendly.

One approach is to develop transgenic plants that are resistant to one or more of them. Alternatively, some plant growth promoting bacteria can act as biocontrol agents to prevent the damage caused by pathogens. Plant growth promoting bacteria produce a variety of substances that limit or stop the growth of phytopathogens. These include siderophores, antibiotics, small molecules and different types of enzymes. This approach is still at the initial stage of development but appears to have considerable potential.

Much of the recent genetic research is being directed at creating microbial strains that have plant growth promoting activity. This research has focused on the following areas of study:

● The Molecular study of Nitrogen Fixation:

Under this topic it has been investigated that whether it is possible to increase the level of microbial nitrogen fixation and reduce the dependency on chemical fertilizers.

● Root Nodule Formation:

Symbiotic bacteria play their role in root nodule formation. The aim of this research is to produce genetically engineered bacteria that can outcompete the naturally occurring symbiotic bacteria.

● Microbial Synthesis of Iron-withdrawing Compounds (Siderophores):

It is aimed to produce useful strains that prevent the growth of phytopathogenic microorganisms.

● Microbial Phytohormone Production:

Pathways are being studied to develop strains that will produce selected phytohormones that stimulate the rapid plant growth.

● Engineering Better Strains of Bacteria:

This type of research is aimed to develop better biocontrol strains of bacteria that can replace chemical pesticides.

● Lowering the Ethylene Levels in Plants by using Bacteria:

The purpose of this study is to lessen the high levels of ethylene by bacteria and thereby decreasing the damage to the plant from different environmental stresses such as drought, flooding, salt stress and the presence of pathogens.