Spatial and temporal expression analysis of defense-related genes in soybean cultivars with different levels of partial resistance to Phytophthora sojae

The molecular mechanisms and the defense responses associated with partial resistance to Phytophthora 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 correlated with partial resistance to P. sojae, northern blot analysis of seven defense-related genes in 14 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 during infection was monitored in a spatial, time-course infection assay with two soybean cultivars, Conrad (high level of partial resistance) and OX 20-8 (Rps1a, low level of partial resistance). mRNA was isolated for Northern blot analysis from root sections harvested below, at, and above the inoculation site at 0, 6, 12, 24, 48 and 72 h 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 at 72 hai were significantly higher in Conrad with higher levels of partial resistance. Our results suggest that defense responses associated with accumulation of PR1a, MMP, IPER and beta-1,3-endoglucanase (EGL) mRNAs may contribute to the partial resistance response to P. sojae in soybean. (c) 2005 Elsevier Ltd. All rights reserved.