Endogenous Production of Geosmin in Table Beet

in HortScience

The earthy flavor of table beet is due to an aromatic terpene derivative called geosmin. It has been hypothesized that geosmin presence in beet is due to geosmin-producing bacteria such as Streptomyces spp. that exist in the soil. However, recent findings suggest that beet may produce geosmin endogenously without microbial influence. The purpose of this study was to determine whether such endogenous production of geosmin occurred in beet by making use of an aseptic tissue culture (TC) environment to remove potential microbial influences on geosmin production. Four table beet accessions (‘Bull’s Blood’, ‘Touchstone Gold’, ‘W364B’, and ‘Pacemaker III’) were grown in three separate TC experiments and in the greenhouse and measured for geosmin concentration via gas chromatography–mass spectrometry (GC-MS). Sequencing of 16S ribosomal RNA was used to identify potential microbial contaminants in TC. Operational taxa units (OTUs) classification resulted in RNA sequences with homology to bacterial RNA of either chloroplast (98%) or mitochondria (2%) origin. Other OTUs identified were considered within the range of sequencing error. In 15 of the 16 TC–grown samples used for the 16S rRNA aseptic validation and in all the greenhouse-grown plant samples, geosmin was detected. Geosmin concentrations from bulked beet tissue of each accession were higher in the TC environment than the greenhouse environment. The lack of microbial detection in the TC environment and the subsequent identification of geosmin from beets grown in the aseptic environment is a strong indication that geosmin is produced endogenously by beets. This finding raises several interesting questions about the functional significance of this molecule for Beta vulgaris.

Contributor Notes

We thank the University of Wisconsin Biotechnology Center DNA Sequencing Facility for providing DNA extraction, library construction, and the bioinformatic pipeline services. The technical assistance and advice from Andy Witherell and Amy Charkowski are gratefully acknowledged.

Corresponding author. E-mail: ilgoldma@wisc.edu.

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    Table beet accession ‘Touchstone Gold’ grown in tissue culture (TC) and greenhouse. (A) Example of TC beets in culture. (B) Germination of beet multigerm seed ball in TC. (C) Greenhouse-grown beet tissue rinsed for geosmin concentration analysis.

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    Geosmin concentration (μg·kg−1) of bulked beet leaf tissue (n = 3) of open pollinated (‘Bull’s Blood’ and ‘Touchstone Gold’), hybrid (‘Pacemaker’), and inbred (‘W364’) beets grown in either aseptic tissue culture (TC) or greenhouse conditions (GH). LGC Root is beetroot tissue from a low-geosmin beet population and prepared for analysis under the same conditions as beet accession leaf tissue.

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