Year of Publication

2016

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Agriculture, Food and Environment

Department

Plant and Soil Sciences

First Advisor

Dr. Luke A. Moe

Abstract

Plant growth promotion can be enhanced by soil- and rhizosphere-dwelling bacteria by a several different methods. One method is by promoting nutrient acquisition from soil. Phosphorus is an essential nutrient that plants obtain from soil, but in many cases it is immobilized in forms that are not available for plant uptake. Bacteria can solubilize insoluble soil phosphates by secreting gluconic acid. This chemical is produced from glucose by the activity of the bacterial enzyme glucose dehydrogenase, which requires a coenzyme called PQQ. In this dissertation, I studied the regulation of the glucose dehydrogenase enzyme and the PQQ coenzyme in the model rhizosphere-dwelling bacteria Pseudomonas putida KT2440 according to differences in bacterial growth conditions. I also extended this study to the bacteria in environmental samples. Broccoli rhizosphere soil samples were collected to study the diversity of the PQQ-producing phosphate solubilizing bacteria in rhizosphere soil and if PQQ was a universal phosphate-solubilizing factor among rhizosphere-dwelling bacteria.

While GDH-dependent phosphate solubilization has been observed in numerous bacteria, little is known concerning the mechanism by which this process is regulated. This study indicated that GDH specific activity and PQQ levels vary according to growth condition, with the highest levels of both occurring when glucose is used as the sole carbon source and under conditions of low soluble phosphate. Under these conditions, however, PQQ levels limit in vitro phosphate solubilization. GDH specific activity data correlated well with gcd gene expression data, and the levels of expression of the pqqF and pqqB genes mirrored the levels of PQQ synthesized, suggesting that one or both of these genes may serve to modulate PQQ levels according to the growth conditions. The pqq gene cluster (pqqFABCDEG) encodes at least two independent transcripts, and expression of the pqqF gene appears to be under the control of an independent promoter and terminator.

Due to the significance of PQQ in mineral phosphate solubilization, I isolated phosphate-solubilizing bacteria from broccoli rhizosphere soil using culture dependent methods and screened the PQQ-producing phosphate solubilizing bacteria to study how the phosphate-solubilizing ability and PQQ-producing capacity has correlated among the rhizosphere-dwelling bacteria. A positive correlation were observed between the two parameters, and a similar changing pattern was observed for them. Several new genera of PQQ-producing phosphate-solubilizing bacteria were isolated and identified by 16S rRNA gene sequencing analysis.

Digital Object Identifier (DOI)

https://doi.org/10.13023/ETD.2016.408

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