Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/31/2011
Publication Date: 4/8/2011
Citation: Paul, C., Hill, C.B., Hartman, G.L. 2011. Quantification of Phakopsora pachyrhizi DNA for Assessing Partial Resistance in Soybean. Plant Disease. 95:DOI: 10.1094/PDIS-10-10-0729. Interpretive Summary: Soybean rust is caused by a fungus that produces large amounts of spores that are wind dispersed. The disease was detected in the continental U.S.A. in 2004. Single gene resistance in soybean to the fungus has been documented, but few studies have characterized partial resistance in soybean. To facilitate this process, a molecular technique (quantitative polymerase chain reaction) was used to compare digital visual disease assessment to fungal DNA. Results demonstrated that the molecular quantification of fungal DNA to assess colonization was successful to distinguish relative soybean rust resistance among soybean genotypes. In addition, this study showed that this assay could distinguish two kinds of resistances, incomplete or partial resistance, more precisely than traditional assessment methods. The accuracy of this technique will significantly facilitate mapping quantitative trait loci controlling partial resistance to soybean rust in soybean. This information will be useful to scientists working on improving rust resistance in soybean.
Technical Abstract: Soybean rust, caused by the fungus Phakopsora pachyrhizi (H. Sydow & Sydow), was detected in the continental U.S. in 2004. Single gene resistance in soybean to P. pachyrhizi has been documented, but few studies have characterized partial resistance in soybean. To facilitate this process, quantitative polymerase chain reaction (QPCR) was used to compare digital visual disease assessment to fungal DNA (FDNA) from QPCR assays. Genotype and isolate were the most significant (P < 0.0001) sources of variation for all the digital assessments and for FDNA. Significant correlations were found between FDNA and number of lesions (r = 0.65, P < 0.0001), lesion area (r = 0.69, P < 0.0001), number of uredinia (r = 0.80, P < 0.001) and uredinia per lesion (r = 0.77, P < 0.0001). Number of lesions was significantly correlated with lesion area (r = 0.5, P = 0.01), number of uredinia (r = 0.67, P < 0.0001) and uredinia per lesion (r = 0.45, P = 0.04). Quantity of FDNA on Williams 82, PI 084674, PI 462312, and PI 506764 appeared to be relatively parallel with colonization on Williams 82 consistently higher and colonization on PI 506764 consistently lower than the others 4 d after inoculation, whereas colonization on UG5 began to decline rapidly 8 d after inoculation. Effect of removal of urediniospores prior to QPCR analysis indicated that relative amounts of fungal DNA measured by QPCR 16 d after inoculation was not changed by the removal of urediniospores. This study showed that QPCR assay can distinguish resistance in soybean genotypes expressing incomplete or partial resistance more precisely than traditional assessment methods. QPCR technique will significantly facilitate mapping quantitative trait loci (QTL) controlling partial resistance to soybean rust in soybean.