A review of issues related to measuring colonization of plant roots by bacteria

Abstract

Root colonization by introduced bacteria is an important step in the interaction of beneficial bacteria with the host plant. Investigators attempting to measure root colonization by bacteria must face several issues. A clear concept or definition of root colonization should be stated in each research summary, as several different definitions have been proposed. We consider true root colonists to be those bacteria that colonize roots in competitive conditions, i.e., natural field soils. Different methods of processing root samples are required if one is measuring external root colonization alone, internal colonization alone, or both. Given that most beneficial bacterial strains currently under investigation as root colonists are members of taxa naturally found in soils, a marking system is required to differentiate the introduced strain from members of the indigenous rhizosphere community. Spontaneous antibiotic resistance, immunological approaches, and foreign DNA sequences are among the marking systems that have been used and each has some possible advantages and disadvantages. More research is needed in the development and comparison of marking systems. The design of experiments to measure root colonization should take into account several statistical issues. One must decide what constitutes the sample unit for each replication of a given treatment, e.g., whole root systems or root segments. Consideration should also be given to how best to express the estimated population of root colonists (e.g., cfu/g fresh or dry weight root, cfu/cm root, or cfu/surface area root). Statistical analysis by standard analysis of variance tests should be used whenever possible to separate treatment means of colonization levels; however, one must determine that the underlying assumptions of these tests are correct for each experiment. Finally, in quantification of populations on roots, one will almost certainly encounter replications with no bacteria, i.e., zeros. There are several options for how to calculate treatment means when one or more replications is a zero, and the implications of these options are discussed. Key words: bioluminescence, genetic markers, plant growth-promoting rhizobacteria, rhizosphere bacteria, root, colonization.

Screening of bacterial strains for developing effective pesticide-tolerant plant growth-promoting microbial consortia from rhizosphere soils of vegetable fields of eastern Uttar Pradesh, India

“…Out of 50, 14 microbial strains were selected according to morphological character such as colonies margin, shape, colour and gram staining (Aneja 2003;Kloepper and Beauchamp 1992). Fourteen bacterial strains were also selected for various biochemical properties, e.g.…”

Influence of an Elevated Atmospheric CO 2 Content on Soil and Rhizosphere Bacterial Communities Beneath Lolium perenne and Trifolium repens under Field Conditions

“…They enhance plant growth by several mechanisms, including (i) the control of phytopathogens by antibiotic production [16,48], by competition for mineral nutrients [44], by secretion of enzymes that can lyse fungal cells [34], and by induction of plant resistance [64]; (ii) the contribution to plant mineral nutrition by secretion of plant-utilizable siderophores that bind iron [6] or by solubilizing minerals such as phosphorus [21]; (iii) nitrogen fixation [13]; and (iv) the production of phytohormones (for a review, see [21]). To influence the plant, root colonization by associative rhizobacteria is considered as a factor of primary importance [31]. Among others, bacterial chemotaxis response to root exudation is involved in this process.…”

Biological control of tomato bacterial spot by seed microbiolization

“…The stability of the control by the microorganism is usually associated with the performance of multiple mechanisms of action (Guetsky et al, 2001) and with an aggressive colonization of the tissues of the host plant (Kloepper & Beauchamp, 1992). In the present study, the mechanisms of action of the DFs1420 isolate were not studied here but some of its capabilities are already known.…”