Date of Graduation

Summer 2019

Degree

Master of Natural and Applied Science in Agriculture

Department

College of Agriculture

Committee Chair

Michael Burton

Keywords

soybean microsymbionts, lablab microsymbionts, non-rhizobial endophytes, 16S ribosomal RNA (rRNA) and nifH gene high-throughput sequencing, Bradyrhizobium, Nitrobacter, Tardiphaga

Subject Categories

Agriculture | Environmental Microbiology and Microbial Ecology

Abstract

Biological nitrogen fixation by rhizobia in the root nodules of legumes is a significant source of agricultural nitrogen in global crop production systems. The influence of and interaction of factors involved in nodule endophyte selection remain poorly understood. In the present study, the influences of crop rotation (soybean-legume vs. cotton-legume) and recalcitrant soil organic matter (compost amendment) on the relative distribution of endophytic bacteria in the root nodules of greenhouse-grown soybean and lablab were investigated by extracting, amplifying, and sequencing 16S ribosomal RNA (rRNA) and nifH genes. Neither preceding crop nor compost amendment were found to have an influence on microsymbiont selection at the level of genus. In both crops and in all treatments, Bradyrhizobium spp. were the dominant rhizobial symbionts, accounting for 95.9% of all recovered 16S rRNA sequences from root nodules, suggesting strong selection exhibited by both soybean and lablab. Likewise, the genera Nitrobacter and Tardiphaga, close relatives of Bradyrhizobium, were present in all root nodules, accounting for an average of 2.9% and 1.0% of nodule sequences, respectively. Previously reported non-rhizobial endophytes were present only inconsistently and at low abundances if at all, suggesting that they may not play a significant role in plant growth as nodule endophytes. These findings indicate that the isolation, characterization, and subsequent inoculation of seeds with non-rhizobial species may not be sufficient to establish their role as endophytes. Their relative abundance in the root nodules should be regarded an important means of certifying a suspected endophyte.

Copyright

© Scott David McElveen

Open Access

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