IX. BIOLOGICALLY ACTIVE MATERIALS IN SOIL

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1 IX. BIOLOGICALLY ACTIVE MATERIALS IN SOIL Required Readings: P. P. Motavalli,* R. J. Kremer, M. Fang, and N. E. Means Impact of genetically modified crops and their management on soil microbially mediated plant nutrient transformations. Journal of Environmental Quality 33: Dunfield, K.E. and J.J. Germida Impact of genetically modified crops on soil- and plantassociated microbial communities. Journal of Environmental Quality 33: Shenker, M., Y. Hadar and Y. Chen Kinetics of iron complexing and metal exchange in solutions by rhizoferrin, a fungal siderophore. Soil Science Society of America Journal 63: Nardi, S., M. Tosoni, D. Pizzeghello, M.R. Provenzano, A. Cilenti, A. Sturaro, R. Rella and A. Vianello Chemical characteristics and biological activity of organic substances extracted from soils by root exudates. Soil Science Society of America Journal 69: Suggested Readings: Mandava, N. B Chemistry and biology of allelopathic agents. p In A. C. Thompson (ed.), The Chemistry of A1lelopathy. American Chemical Society, Washington, D.C. Lynch, J. M Products of soil microorganisms in relation to plant growth. CRC Critical Reviews in Microbiology 5:

2 Soils contain hundreds of compounds derived from plant, animal or microbial sources, the majority of which have never been isolated and identified. All components in soil may be considered, in a general and indirect sense, to be biologically active because in some way they may influence the total biological activity in the soil. Clay minerals, for example, bind many compounds on their surfaces and thus influence their activity. However, the clay mineral itself is considered biologically inactive. A biologically active compound is defined as one that has a direct physiological effect on a plant, animal, or another microorganism. Many known compounds with biological activity are found only in trace amounts in soil. Other compounds, such as fu1vic acids extracted by classical methods, may show biological activity but their chemical structure may have been severely altered during the extraction process from soil. For a compound to be of ecological significance in soil it is essential that it be biologically active in the form and in the concentration actually found in the soil. The production and activity of many biologically active compounds is highly localized in the soil. Also some compounds are active in different ways depending on their concentration. For example, auxin stimulates root growth at low concentrations but when concentrations are increased, a point is reached where it becomes inhibitory. Many compounds may interact in a synergistic or antagonistic manner so that a test for a single compound in soil with a specific biological response often cannot be achieved. The combined effect of location, concentration, and interaction of response makes the study of biologically active compounds in soil extremely difficult. The focus of this chapter will be the investigation of the range and activity of soil compounds which exhibit biological activity. Some of the classes of compounds that exhibit biological activity are discussed below.

3 Soil Fulvic And Humic Acids. Although it is impossible to provide exact chemical structures for fulvic and humic acids, research has shown that these large, rather nondescript compounds can have a stimulatory effect on plant and microbial activity. Root growth and total dry matter production in peanuts, for example, is stimulated by 800 parts per million of fulvic or humic acids. Inhibitory effects have also been observed when concentrations are increased. Plant Growth Regulators. Plant hormones, such as ethylene, auxins, cytokinins, and gibberellin, can be produced by microorganisms in soil, especially within the plant root rhizosphere. These hormones then influence plant growth. The addition of specific substrates that are immediate precursors in the synthetic pathway of a plant hormone can stimulate hormone concentrations above those normally found in soil. In this way a particular physiological plant response can be created. Vitamins. Microorganisms produce vitamins that may play a significant role in plant/microbial interactions. Antibiotics,. Antibiotics are produced by microorganisms in the plant root rhizosphere or during the degradation of a rich energy source such as plant residues. The production of an antibiotic by one microorganism against another gives it a selective advantage within the microenvironment in which it is competing for nutrients and energy. Extracellular Enzymes. Many microorganisms excrete extracellular enzymes that remain active in a free state. Plant and microbial cell remnants can also exhibit enzyme activity that is no longer associated with a living cell. These enzymes are involved in the transformation and cycling of nutrients in the soil. Plant and microbial pathogens may also secrete extracellular enzymes capable of degrading cell wall materials thus lysing and killing the cell.

4 Nucleic Acids. The introduction of foreign genes into a microbial cell that is then introduced into the soil may result in the production of a specific compound with biological activity. It is often desirable (e.g. in the case of an infectious disease causing gene) that nucleic acid not be transferred among microorganisms. However, in ecological terms, exchange of genetic material can also be considered advantageous in that it spurs the development of new metabolic capabilities not previously known. It is not always the organism that is the focus of genetic exchange studies. The fate of nucleic acid, itself, may be of primary interest and its fate in soil must be carefully evaluated. Phytotoxins and/or Allelopathic Compound. A wide range of specific compounds has been isolated from soil that inhibits plant growth. Microorganisms commonly produce these compounds when a large amount of plant residues are added to the soil. The allelopathic response may be due to a single compound, e.g. juglone from walnut trees, or a result of a host of rather nonspecific compounds acting in a synergistic manner, e.g. small organic acids. The source of the phytotoxin or allelochemical may be either microbial or plant. Pesticides. Biological control of specific insect pests of crops has been developing rapidly. Corn rootworm control has been achieved by isolating the gene that causes the production of a specific compound that is toxic to the larvae. The gene has been introduced into a microbial species that can effectively colonize the corn root and thus lend protection to the plant. Many proteins (the Bt proteins) have been isolated from soil microorganisms and bioengineered to be produced in plants offering a continuous means of biocontol. Other microorganisms in soil have served as a source of genes for creating specific herbicide tolerant agronomic crops (e.g. Roundup Ready soybeans). There are undoubtably other natural compounds with pesticide properties produced by microorganisms in soil.

5 The fate of biologically active compounds in soil is of tremendous practical utility. Such compounds become ecologically significant only if concentrations become high enough to cause a response. Many soil factors can affect the fate of a biologically active compound once it is produced. Some compounds may be volatile and temperature will greatly effect their concentration in soil. The ability to bind to soil colloids, such as clays, may inhibit the activity of some compounds, while trapping in soil other compounds so that they can build up to high levels. Moisture will stimulate microbial and plant growth and thus stimulate the rate of production of a biologically active compound. At the same time, the increased microbial activity may result in rapid degradation so that the presence of the biologically active compound in soil is transient. Lack of oxygen will stimulate the production of some compounds, such as the small organic acids that elicit allelopathic response, while inhibiting the production of others. Anaerobic conditions may also aid the stabilization of compounds in soil. The need for further information on the identification, production, and fate of biologically active compounds in soil has become a productive area of research. This reemergence of interest, which was first expressed due to the early discoveries of antibiotics produced by soil microorganisms, is a result of the realization that there exists in soil a large number of natural products of plant, animal, or microbial origin that can be utilized for industrial or agricultural purposes. The research that will lead to these products is just beginning.