Copyright © 2012 Bernard R. Glick. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The worldwide increases in both environmental damage and human population pressure have the unfortunate consequence that global food production may soon become insufficient to feed all of the world's people. It is therefore essential that agricultural productivity be significantly increased within the next few decades. To this end, agricultural practice is moving toward a more sustainable and environmentally friendly approach. This includes both the increasing use of transgenic plants and plant growth-promoting bacteria as a part of mainstream agricultural practice. Here, a number of the mechanisms utilized by plant growth-promoting bacteria are discussed and considered. It is envisioned that in the not too distant future, plant growth-promoting bacteria (PGPB) will begin to replace the use of chemicals in agriculture, horticulture, silviculture, and environmental cleanup strategies. While there may not be one simple strategy that can effectively promote the growth of all plants under all conditions, some of the strategies that are discussed already show great promise.
The use of plant growth promoting bacteria may prove useful in developing strategies to facilitate plant growth under normal as well as diverse abiotic stress conditions. The application of microbes with the aim of improving nutrients availability for plants is an important practice and necessary for sustainable agriculture. During the past couple of decades, the use of microbial inoculants for sustainable agriculture has increased tremendously in various parts of the world. Significant increases in growth and yield of agronomically important crops in response to inoculation with plant growth promoting (PGP) microbes have been repeatedly reported. The actual biodiversity of PGP microbes belong to different groups including Actinobacteria, Bacteroidetes, Balneolaeota Firmicutes, Proteobacteria and Spirochaetes. PGP bacteria are naturally occurring soil bacteria that aggressively colonize plant roots and benefit plants by providing growth promotion either directly by solubilization of phosphorus, potassium and zinc; production of indole acetic acids, gibberellic acid, cytokinin; biological nitrogen fixation or in-directly by production of ammonia, hydrogen cyanide, siderophore and biocontrol against different plant pathogens. In this review, we have discussed method of isolation, characterization, identification and biodiversity of bacteria associated with crops and further mechanisms of plant growth promotion under the normal as well as diverse abiotic stress conditions.
Abstract Plants acquire phosphorus from soil solution as phosphate anion. It is the least mobile element in plant and soil contrary to other macronutrients. It precipitates in soil as orthophosphate or is absorbed by Fe and Al oxides through legend exchange. Phosphorus solubilizing bacteria play role in phosphorus nutrition by enhancing its availability to plants through release from inorganic and organic soil P pools by solubilization and mineralization. Principal mechanism in soil for mineral phosphate solubilization is lowering of soil pH by microbial production of organic acids and mineralization of organic P by acid phosphatase. Use of phosphorus solubilizing bacteria as inoculants increases P uptake. These bacteria also increase prospects of using phosphatic rocks in crop production. Greater efficiency of P solubilizing bacteria has been shown through co-inoculation with other beneficial bacteria and mycorrhiza. This article incorporates the recent developments on microbial P solubilization into classical knowledge on the subject.
The greatest issue affecting the sustainability of broad acre cropping both environmentally and economically is the requirement of fertilizers. These are based on mined phosphorous or other mineral ores, ammonia produced through the Harbour-Bosch process and industrially manufactured potash. As global demand for fertilizers increases, the costs associated with the production for each of these major nutrients increases. Biofertilizers such as plant growth promoting bacteria (PGPB) are a possible biotechnology that could alleviate the need for addition of increasing amounts of fertilizers. These bacteria naturally occur in soils and aggressively colonize around plant roots and have been shown to have plant growth promoting effects. PGPB are known to influence plant growth by various direct and indirect mechanisms; while some can affect plant physiology directly by mimicking synthesis of plant hormones, others increase mineral availability and nitrogen content in soil. Here we review the previously characterized modes of action for enhancement of plant growth by PGPB such as nitrogen fixation, nutrient solubilization and production of auxins and enzymes, as well as discussing more recent proposed modes of action such as secondary metabolites.