Submitted to: Journal of Forestry Science - Honduras
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/29/1999
Publication Date: N/A
Citation: Interpretive Summary: Soybean yield losses due to Phytophthora (PR) root rot occur regularly. This disease is potentially devastating in soybean varieties that do not have resistance. Two forms of resistance (whole-plant or race-specific resistance and root or partial resistance) are important for PR control. Soybean gene(s) known as "Rps" confer race-specific resistance to different tinfectious strains (races) of the PR fungus. Some of the new PR races are not controlled by the Rps1-k gene widely used in breeding programs for resistance. Development of improved technology to identify factors influencing partial resistance in soybean progeny is needed to effectively combine the two forms of resistance in soybean varieties. Since manganese (Mn) deficiency and disease severity has been documented with several other crops, a study was undertaken to identify the role of Mn in soybeans inoculated with the PR pathogen. This soybean study was unique since it involved evaluations of Mn activity (Mn2+ vs. Mn4+ or reduced vs. oxidized form of Mn) in resistant and susceptible plants inoculated with the PR pathogen, whereas roles of Mn activity have been emphasized in healthy vs. diseased tissue in other crops. Manganese activity in PR inoculated soybeans identified slight increases in the Mn4+ or oxidized fraction in inoculated hypocotyl tissue for both susceptible and resistant soybeans; however total Mn (Mn2+ and Mn4+) was consistently higher in inoculated than control and wounded tissues regardless of the disease reaction; total Mn accumulation was also influenced by race of the pathogen and soybean resistance gene interaction. This is the first report of an increase in total Mn accumulation in inoculated tissues. Information from this soybean study provides new insight about the role of Mn in disease resistance.
Technical Abstract: The relationship of manganese deficiency and disease severity has been documented with several crops. Roles of Mn2+ and Mn4+ have been emphasized in healthy and diseased tissue, respectively. To evaluate the role of Mn in Phytophthora root rot, soybean isolines with the rps and Rps1-k genes were inoculated with P. sojae race 2 or 25. Hypocotyl tissue was analyzed for Mn content and oxidation state using micro-X-ray absorption near edge structure spectroscopy (XANES). The fraction of Mn4+ in inoculated tissue was only slightly elevated in both susceptible and resistant tissue. Total Mn was higher in inoculated than in control and wounded tissue regardless of the disease reaction; however, Mn accumulation was influenced by race of P. sojae and Rps gene interaction. Soybeans with the Rps1-k gene (susceptible to race 25 and resistant to race 2) had significantly higher Mn (4-fold) than non-inoculated plants, whereas, soybeans with the rps gene (susc. to both race 2 and 25) had significantly higher Mn (4-fold) than the controls when inoculated with race 2 but only a 2-fold increase when inoculated with race 25. This is the first study documenting differences in total Mn accumulation between non-inoculated and inoculated tissue.