SOIL: A Habitat for Beneficial Microorganisms

Brenda Tubana, Ham, Jong, Fultz, Lisa

Jayvee Cruz, Brenda Tubaña, Lisa Fultz and Jong Ham

Soil has many functions in our ecosystem, with the most notable being a medium for plant growth. Soil holds and supplies many plant-essential nutrients and is populated by multitudes of microorganisms, including bacteria and fungi, that are influential on plant growth.

The rhizosphere, from the Greek word rhizo or rhiza, meaning root and sphere, is the zone where the soil is under the influence of the plant’s roots. It is the region where the roots generate and release simple and complex sugars, growth regulators, amino acids, enzymes, etc., and is the site where intense microbial activity occurs. The diversity of microorganisms is important in maintaining soil quality and in enhancing plant growth. Plants are healthier and become more stress-tolerant when grown on soils inhabited by diverse microbial communities.

Plant growth-promoting rhizobacteria is a group of microorganisms that actively colonize the rhizosphere and exert beneficial effects on plant growth. These beneficial rhizobacteria affect plant growth and survival through direct and indirect mechanisms. The direct mechanisms include fixation of atmospheric nitrogen — an essential nutrient to plants — and production of various enzymes and phytohormones. The indirect mechanisms include production of antibiotic and siderophore, parasitism to pathogens and scavenging of nutrients from the soil. All of these affect plant growth and tolerance to stressful conditions in the field, such as salinity and flooding.

Silica solubilizing bacteria are rhizobacteria that can transform silica to orthosilicic acid, which is a plant-usable form of silicon. Silicon is a beneficial nutrient with an outstanding role in enhancing plant defense mechanisms against stress from insects and pathogens. Silicon is accumulated in the form of silica gel and is deposited in the shoot and leaf epidermal cells, which can strengthen the mechanical structure and protective layers of plants. Another way silicon improves plant defense is through induction of systemic resistance inside the plant.

An LSU AgCenter research project was designed to identify and isolate silica solubilizing bacteria from soil and document the plant-growth promoting contributions of these bacteria. Soil samples were taken from sugarcane, rice, soybean, corn and wheat fields across Louisiana. From these soil samples, 130 bacteria were isolated with 20 isolates capable of solubilizing silica. These isolates also solubilize phosphorus and produce multiple plant growth-promoting compounds.

A suitable carrier is needed to facilitate the distribution of silica solubilizing bacteria isolates during application. This study examined survival of silica solubilizing bacteria in different carriers, including the byproducts from sugarcane production (bagasse) and rice production (rice hulls). The highest population of silica solubilizing bacteria was sustained for 150 days with a bagasse plus soil mixture carrier. Not only does this study suggest that the potency of these bacteria as a biostimulant can last for several months but it also brings value to bagasse.

Further work with one of the isolates revealed its ability to colonize root tissues of two-week old rice seedlings, which indicates that the application of this biostimulant is possible through seed treatment. The process involved inoculating the roots of rice seedlings with silica solubilizing bacteria tagged with a protein that fluoresces green under a microscope.

The availability of a practical means of applying biostimulants is essential to scale up this technology. The outcomes of this study are important and foundational for developing bio-based approaches to improve crop performance. The availability of bio-based production technologies such as biostimulant application to soil and plants is essential to keeping the crop industry sustainable, productive and friendly to the environment.

Jayvee Cruz is a graduate student in the School of Plant, Environmental, and Soil Sciences. Brenda Tubaña holds the Jack E. and Henrietta Jones Professorship and Lisa Fultz is an associate professor in the School of Plant, Environmental, and Soil Sciences. Jong Ham is a professor in the Department of Plant Pathology and Crop Physiology.

(This article appears in the summer 2021 issue of Louisiana Agriculture.)

A close-up of a soil sample with worms.

This sample of soil and tillage radish roots is entangled with mycelia and earthworms. Soil provides a niche for different organisms that promote nutrient cycling and plant growth. Photo by Daniel Forestieri

Eight square dishes with various samples in them.

Different inoculant carriers for silica solubilizing bacteria consisting of silicate slag, fresh and burned bagasse, fresh and burned rice hull, soil and their mixes. Photo by Jayvee A. Cruz

Woman looking into microscope.

These scientists are analyzing protein-tagged silica solubilizing bacteria in rice roots. Photo by Brenda Tubaña

View of green dots under a microscope.

Bright green dots inside the rice roots indicate the presence of the protein-tagged silica solubilizing bacteria. Photo by Jayvee A. Cruz

8/28/2021 2:07:21 PM
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