Defense-related gene expression in soybean leaves and seeds inoculated with Cercospora kikuchii and Diaporthe phaseolorum var. meridionalis

Soybean leaves and detached seeds were inoculated with either Cercospora kikuchii or Diaporthe phaseolorum var. meridionalis in order to characterize the defense responses of these tissues to fungal infection. By comparing and contrasting the expression of 15 soybean defense-related genes in the two tissues during a 48 h post infection period, we found that pathogen concentrations and gene expression patterns were substantially different for inoculated leaves and seeds of the susceptible cultivar Holladay, but for each tissue a generally similar pattern of expression was observed with either pathogen treatment. In leaves inoculated with C. kikuchii or D. phaseolorum, 14/15 or 8/15 defense-related genes were significantly upregulated, respectively, including the notably strong upregulation of PR3 (chitinase 1) and PR10 (ribonuclease-like protein) by both pathogens. In leaves, the pathogen concentrations declined during the 48 hpi period. In seeds inoculated with either C. kikuchii or D. phaseolorum, 7/15 or 6/15 genes were significantly upregulated, but PR3 and PPO (polyphenol oxidase) were not significantly upregulated or not detected with either pathogen treatment, while PR10 was significantly upregulated only in seeds inoculated with D. phaseolorum. Pathogen concentrations in seeds did not decline, but increased over the 48 hpi period. Our results suggest that differences between the numbers and types of defense genes upregulated in the two inoculated tissues may explain, at least partially, the decline in leaves and the increase in seeds of the pathogen quantities.

Research highlights
► Soybean leaves and detached seeds were inoculated with either Cercospora kikuchii or Diaporthe phaseolorum var. meridionalis in order to characterize the defense responses of these tissues to fungal infection.
► By comparing and contrasting the expression of 15 soybean defense-related genes in the two tissues during a 48 h post infection period, we found that pathogen concentrations and gene expression patterns were substantially different for inoculated leaves and seeds of the susceptible cultivar Holladay, but for each tissue a generally similar pattern of expression was observed with either pathogen treatment.
► In leaves inoculated with C. kikuchii or D. phaseolorum, 14/15 or 8/15 defense-related genes were significantly upregulated, respectively, including the notably strong upregulation of PR3 (chitinase 1) and PR10 (ribonuclease-like protein) by both pathogens.
► In leaves, the pathogen concentrations declined during the 48 hpi period. In seeds inoculated with either C. kikuchii or D. phaseolorum, 7/15 or 6/15 genes were significantly upregulated, but PR3 and PPO (polyphenol oxidase) were not significantly upregulated or not detected with either pathogen treatment, while PR10 was significantly upregulated only in seeds inoculated with D. phaseolorum.
► Pathogen concentrations in seeds did not decline, but increased over the 48 hpi period.
► Our results suggest that differences between the numbers and types of defense genes upregulated in the two inoculated tissues may explain, at least partially, the decline in leaves and the increase in seeds of the pathogen quantities.