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Title: Spatial and Temporal Expression Anaylysis of Defense-Related Genes in Soybean Cultivars with Different Levels of Partial Resistance to Phytophthora Sojea

item Redinbaugh, Margaret - Peg

Submitted to: Physiological and Molecular Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/21/2005
Publication Date: 11/10/2005
Citation: Vega-Sanchez, M.E., Redinbaugh, M.G., Costanzo, S., Dorrance, A.E. 2005. Spatial and Temporal Expression Anaylysis of Defense-Related Genes in Soybean Cultivars with Different Levels of Partial Resistance to Phytophthora Sojea. Physiological and Molecular Plant Pathology. 66(5):175-182.

Interpretive Summary: It is hypothesized that 'partial' or 'quantitative' resistance to pathogens may be of agronomic importance, because its deployment may avoid problems associated with rapid ability of pathogens to overcome race-specific resistance. While an understanding of mechanisms associated with race-specific resistance is emerging, little is known about mechanisms associate with partial resistance. The expression defense-related 'pathogen response' (PR) genes in soybeans with partial resistance to root and stem rot caused by Phytophthora sojae was examined. There was no difference between partially resistant and fully susceptible soybean lines in the levels of constitutive expression of the PR genes. This is in contrast to results obtained with two other plant-pathogen systems. However, there was higher expression of PR genes in the region above the inoculation site 72 h after inoculation with the pathogen. These results indicate that multiple mechanisms are likely to be associated with partial resistance. The results will be used to identify important times and tissues to be examined by researchers developing global analysis of soybean transcript expression in response to the pathogen.

Technical Abstract: The molecular mechanisms and the defense responses associated with partial resistance to P. sojae in soybean are unknown. In this study, we examined correlations between the expression of defense genes with partial resistance. First, to determine whether constitutive levels of expression of defense-related genes correlate with partial resistance to P. sojae, northern blot analysis of 7 defense-related genes in fourteen cultivars with low, moderate and high levels of partial resistance was performed. Pearson's correlations between mean lesion length and mean constitutive mRNA signals for defense-related genes showed no significant association to partial resistance to P.sojae. These results suggested that mechanisms linked to defense-related mRNA levels expressed during infection might better explain variations in partial resistance to P. sojae in soybean. Second, accumulation of four defense-related transcripts was monitored in a spatial, time-course infection assay with two soybean cultivars, Conrad (high level of partial resistance) and OX 20-8 (Rps 1a, low level of partial resistance). mRNA was isolated for Northern blot analysis from root sections which were harvested below, at, and above the inoculation site at 0, 6, 12, 24, 48 and 72 hours after inoculation (hai) with P. sojae. P. sojae and soybean actin cDNAs were used as probes in the infected root sections to estimate relative proportions of RNA. Differential mRNA accumulation patterns for both soybean and P. sojae actin following P. sojae colonization in the three root sections of Conrad and OX 20-8 suggested that effective lesion-limiting mechanisms occurred primarily in the upper root section. Transcript levels for PR1a, matrix metalloproteinase (MMP) and basic peroxidase (IPER) at the inoculation site; and IPER above the inoculation site were significantly higher in Conrad with higher levels of partial resistance. Our results suggest that defense responses associated with accumulation of mRNA levels of PR1a, MMP, IPER and EGL may contribute to the partial resistance response to P. sojae in soybean. IMPACT STATEMENT: Crops with durable resistance to soil-borne pathogens are needed so that farmers have economically viable, environmentally sustainable ways to control the diseases and losses caused by the pathogens. This research shows that there may be multiple mechanisms working at the molecular level that lead to partial resistance to pathogens in important crops like soybeans. Understanding the nature of the resistance is critical for developing methods for rapidly incorporating these desirable traits into commercial cultivars.