ARS | Publication request: Genotypic variation in sulfur assimilation and metabolism of onion (Allium cepa L.) III. Characterization of sulfite reductase

Submitted to: Phytochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 26, 2012
Publication Date: November 1, 2012
Citation: Mcmanus, M.T., Joshi, S., Leung, S., Albert, N., Pither-Joyce, M., Shaw, M., Mccallum, J., Searle, B., Shigyo, M., Jakse, J., Havey, M.J. 2012. Genotypic variation in sulfur assimilation and metabolism of onion (Allium cepa L.) III. Characterization of sulfite reductase. Phytochemistry. 83:34-42.

Interpretive Summary: DNA sequences corresponding to a ferredoxin-sulfite reductase (SiR) were cloned from bulb onion and the expression of the gene and activity of the enzyme were characterised with respect to sulfur (S) supply. Cloning, mapping and expression studies revealed that onion has a single functional SiR gene and expresses an unprocessed pseudogene (f-SiR). Northern and quantitative PCR analysis revealed differences in expression pattern between the SiR gene and the pseudogene. Western analysis using antibodies raised to a recombinant SiR revealed that the enzyme is present in chloroplasts and phylogenetic analysis has shown that the onion protein groups with lower eudicots. In hydroponically-grown plants, levels of SiR transcript were significantly higher in the roots of S-sufficient when compared with S-deficient plants of the pungent cultivar 'W202A' but not the less pungent cultivar 'Texas Grano'. In these same treatments, a higher level of enzyme activity was observed in the S-sufficient treatment in leaves of both cultivars before and after bulbing. In a field trial with and without sulfur fertilisation, a statistically significant increase in SiR activity was observed in the leaves of the pungent cultivar 'Kojak' in response to added S, but not in the less pungent cultivar 'Encore'. These results are of interest to onion breeders, producers, and consumers for the development and production of less pungent onions.

Technical Abstract: Genomic and cDNA sequences corresponding to a ferredoxin-sulfite reductase (SiR) have been cloned from bulb onion (Allium cepa L.) and the expression of the gene and activity of the enzyme characterised with respect to sulfur (S) supply. Cloning, mapping and expression studies revealed that onion has a single functional SiR gene and also expresses an unprocessed pseudogene (f-SiR). Northern and qPCR analysis revealed differences in expression pattern between the SiR gene and the pseudogene. Western analysis using antibodies raised to a recombinant SiR revealed that the enzyme is present in chloroplasts and phylogenetic analysis has shown that the onion protein groups with lower eudicots. In hydroponically-grown plants, levels of SiR transcript were significantly higher in the roots of S-sufficient when compared with S-deficient plants of the pungent cultivar 'W202A' but not the less pungent cultivar 'Texas Grano'. In these same treatments, a higher level of enzyme activity was observed in the S-sufficient treatment in leaves of both cultivars before and after bulbing. In a factorial field trial with and without sulfur fertilisation, a statistically significant increase in SiR activity was observed in the leaves of the pungent cultivar 'Kojak' in response to added S but not in the less pungent cultivar 'Encore'.