What and why is rhizobacterial plant growth promotion?
The term 'plant growth promotion' (PGP) is often used to describe increased plant growth followed by increased crop yield. Many microbes especially, rhizobacteria (bacteria from rhizosphere) are known for their PGP properties. Rhizobacteria live in the soil surrounding immediately to the root regions (rhizome) and soil adhered to root system (roots/root hairs). Enhanced plant growth and subsequent yield can be obtained by inoculating seedlings, leaves, seeds or roots with plant growth promoting rhizobacteria (PGPR). There are various ways of inoculation of above mentioned plant parts; PGPR inoculum is a living culture as liquid, solid pellets, spray or mixed with carrier material such as lignite.
The term PGPR was first time introduced by Joseph Kloepper and colleagues in 1980 to describe the significance of root colonizing rhizosphere bacteria that stimulate plant growth by the production of plant growth hormones. In the definition of PGPR, Kloepper et al have specially mentioned rhizobacteria to be root colonizing: it means that colonization of roots is crucial for the establishment of positive successful relationship between rhizobacteria and their plant host. Many rhizobacteria such as rhizobia are host specific and colonize roots of only particular host plant roots and unable to colonize roots of other plant species. The identification, approach, colonization and establishment of host and rhizobacteria interrelationship is in fact a myriad of complex reactions at cellular and molecular level involving host-bacteria signaling and expression of certain genes. Root colonization process, in other words is vital for beneficial rhizobacterial association with appropriate host plant/s. Root colonization tests are routinely carried out in laboratories by using specific genetic, biochemical or phenotypic markers to determine viable count, activity and trace the fate of bioinoculants or sometimes to study complexity of plant-microbe interactions in the soil and rhizosphere region. Now-a-days it is possible to observe rhizobacteria in the rhizosphere environment in their living state by employing specialized microscopical methods.
However, the use of bacterial inoculants to increase the yield has been a century old practice, even before the terms like rhizosphere or plant growth promotion were properly understood. In 1895, the first rhizobial inoculant (nitragin) was commercialized by Nobble and Hiltner to increase crop yield. In 1897, Phosphobaktrin inoculum of Bacillus megaterium var phosphaticum, the first phosphate solubilizing (PS) inoculum was marketed in Soviet Union. This was followed by commercialization of Azotobacter containing inocula Azotogen and Azotobaktrin in India, Europe, USA, Belgium, New Zealand, Soviet Union, Australia and Netherlands. Until now PGPR inoculants have been used to promote yield of many vegetable, ornamental and fruit plants such as radish, beet, potato, sugarcane, tea plantations, canola, fern, orchids, squash, chrysanthemum, roses, cucumber, pepper, tomato, tobacco, melons, coriander, cotton, pine, spruce, bamboo and nearly all leguminous, non-leguminous pulses and cereal grain crops.
Both the beneficial and deleterious types of rhizobacteria are present in the rhizosphere and therefore microbe-plant interactions may prove beneficial or deleterious. However, only the interactions that maintain plant health and favour plant growth are considered under PGP. In 1904, Hiltner introduced and explained the concept of 'rhizosphere' and also found that in this region rhizobacterial activities are more concentrated. It was established that rhizosphere environment provides structural support, water and nutrients to the plant and microbes and it is thus an ideal habitat for thriving of bacterial relatives of host plants. It is now known that many different mechanisms are responsible for plant growth promotion like production of growth regulators, hormones, and vitamins, decomposition of organic substrates and products of nutrient cycling. The term 'plant growth promotion' has now been utilized in broader sense, in that antagonistic and biocontrol activities of rhizobacteria against their deleterious competitors are too considered. Biocontrol mechanisms such as production of secondary metabolites as antibiotics, cyanides, chitinases, and glucanases like enzymes, toxins, and micronutrient scavenging systems like siderophores are some of them to be utilized in antagonism and subsequent disease control in the rhizosphere region. A single or more than one mechanism may be operated in favour of host plant by a rhizobacterium. The growth promoting properties improve plant growth and yield while as biocontrol properties protect plant from phytophathogens, hence keep them healthy and free from diseases. Today, PGPR formulations are used either singly with particular property aimed for specific improvement or mixed/combined formulation consisting of more than one PGPR in synergistic composition for a large scale applications.
However, the rate of utilization of recently developed PGPR formulations is very low as compared to traditional rhizobial inoculations and chemical fertilizers or pesticides. The adverse effects of chemical pesticides and fertilizers are already evident in the various forms of air, water and land pollution, recalcitrance of chemical formulations or permanent loss of soil fertility. Still farmers worldwide do not refrain from using chemicals to support crop growth and productivity. To encourage the application of PGPR fertilizers technological barriers between farming community, scientists and industries needs to be broken. The knowledge of PGPR strains, their activities, characteristics, compatibility with native flora and fauna and most important their comparable growth and yield promotion with chemical fertilizers must be extended to the farmers. Utilization of PGPR biocultures for crop application means development and management of sustainable agriculture; in other words a suitable alternate for organic and ecofriendly farming.
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