Abstract

Utilization of plant growth-promoting bacteria colonizing roots is environmentally friendly technology instead of using chemicals in agriculture, and understanding of the effects of their colonization modes in promoting plant growth is important for sustainable agriculture. We herein screened the six potential plant growth-promoting bacteria isolated from Beta vulgaris L. (Rhizobium sp. HRRK 005, Polaromonas sp. HRRK 103, Variovorax sp. HRRK 170, Mesorhizobium sp. HRRK 190, Streptomyces sp. HRTK 192, and Novosphingobium sp. HRRK 193) using a series of biochemical tests. Among all strains screened, HRRK 170 had the highest potential for plant growth promotion, given its ability to produce plant growth substances and enzymes such as siderophores and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, respectively, concomitantly with active growth in a wider range of temperatures (10–30 °C) and pH (5.0–10.0). HRRK 170 colonized either as spots or widely on the root surface of all vegetable seedlings tested, but significant growth promotion occurred only in two vegetables (Chinese cabbage and green pepper) within a certain cell density range localized in the plant roots. The results indicate that HRRK 170 could function as a plant growth promoter, but has an optimum cell density for efficient use.

Highlights

  • Plants associate and form different relationships with diverse microorganisms in nature, including with plant growth-promoting bacteria that efficiently colonize plant roots

  • These results suggest that the effect of plant growth-promoting bacteria on plant growth is correlated with the number, dispersion, and colonization of infecting cells associated with their biofilm formations in plant tissues

  • We screened six potential plant growth-promoting bacterial strains, which were previously isolated from sugar beet, and selected Variovorax sp

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Summary

Introduction

Plants associate and form different relationships with diverse microorganisms in nature, including with plant growth-promoting bacteria that efficiently colonize plant roots. Noirot-Gros et al [15] reported that the formation of bacterial biofilms, in which microbial cells live in self-synthesized extracellular polymeric substances, resulted in different formations, dispersion, and colonization patterns, followed by plant growth-promoting efficiency. These results suggest that the effect of plant growth-promoting bacteria on plant growth is correlated with the number, dispersion, and colonization of infecting cells associated with their biofilm formations in plant tissues. Yields of sugar beet, which are important as both a source of sugar and in crop rotation, are persistently high, presumably because of the interactions with useful microorganisms such as plant growth-promoting bacteria [6]. The results demonstrate that HRRK170 is a plant growth promoter with beneficial biochemical properties, but has an optimum cell density for its full function

Bacterial Strains and Growth Conditions
Biochemical Characteristics
Morphological Observation of HRRK170 by Scanning Electron Microscopy
Effect of HRRK170 on the Plant Growth
Tissue Localization of HRRK170 in Plant Roots
Calculation of HRRK170 Cell Density Using Color Development by GUS Staining
Statistical Analysis
Bacterial Growth Under Different Temperatures and pH Values
Effect of HRRK170 on the Growth of Vegetable Seedlings
Plant Tissue Localization of HRRK 170
Evaluation of HRRK 170 Cell Density Localized in the Plant Roots
B Chinese
Discussion
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