Plant Growth Promoting Rhizobacteria and Mycorrhizal Fungi in Sustainable Agriculture and Forestry

Authors: MALLIK, M.A.B - LANGSTON UNIVERSITY; WILLIAMS, ROBERT

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 12/4/2006
Publication Date: 7/15/2008
Citation: Mallik, M.A.B. and Williams, R.D. 2008. Plant growth promoting rhizobacteria and mycorrhizal fungi in sustainable agriculture and forestry. In: Zeng, R.S., Mallik, A.U., Luo, S.M., editors. Allelopathy in Sustainable Agriculture and Forestry. New York: Springer. p. 321 -345.

Interpretive Summary: As plant roots grow in the soil, bacteria and fungi also grow living off the nutrients leaking from the plant roots. Some of these organisms may be harmful to the plant (disease causing bacteria), while other organisms may be beneficial to the plant helping it grow. These beneficial bacteria are called plant growth-promoting rhizobacteria (PGPR) and have been studied for several years. They promote plant growth by providing plant growth regulators, inhibiting disease bacteria, facilitating nutrient uptake, supplying nitrogen, and promoting nodulation in legumes. Several of these bacteria have been isolated and are used to inoculate plants (generally as seed coating) and provide the seedling with resistance to diseases, or stimulate seedling growth, providing earlier crop establishment. Many of these 'biofertilizers' are commercially available, but they are not generally used in general crop production. This may be due to a failure to demonstrate their value under field conditions. By way of review we cover the growth promoting aspects of these bacteria and fungi, provide examples of commercial products, and evaluate their value and potential.

Technical Abstract: Plant-growth promoting rhizobacteria (PGPR) encourage plant growth by producing growth regulators, facilitating nutrient uptake, accelerating mineralization, reducing plant stress, stimulating nodulation, providing nitrogen fixation, promoting mycorrhizal fungi, suppressing plant diseases, and functioning as nematicides and insecticides. Many of the PGPR are fluorescent pseudomonads (Pseudomonas fluorescens), but other bacteria (Bacillus sp., Azotobacter sp., Acetobacter sp., Azospirillum sp.) are known as well. Many of these organisms have been formulated into biofertilizers and are commercially available. However, a disconnect occurs between the demonstration of the growth-promoting activity of these organisms in laboratory and field studies versus their use in commercial production. The reason for this is two-fold. First, there have been inconsistent results between experimental studies and practical field applications where the growth-promoting activities of the rhizobacteria are masked by other environmental and management factors. Second, there is a lack of technology transfer and education, thus limiting the farmers' use of biofertilizers. Here we review the role of rhizobacteria stimulating plant growth and their use as biofertilizers; indicate that the use of biofertilizers may be of more benefit in unproductive and stressful environments; and recommend that commercially available biofertilizers be evaluated in standardized field test.