Differential expression of distinct soybean resistance genes interacting with Argentinean isolates of Diaporthe phaseolorum var. meridionalis

Soybean Stem Canker (SSC), caused by Diaporthe phaseolorum var. meridionalis (Dpm), is an important disease of soybean in Argentina. There are five known dominant genes that confer resistance to SSC, Rdm1 to Rdm5. Particularly, Rdm2 was identified in cv. Tracy-M and then it was stabilized in the breeding line T2. The Rdm4 gene was first identified in cv. Hutcheson. More recently it was found that this gene was linked to the Rdm5 gene, defining the Rdm4-5 resistance region in Hutcheson. The objective of this work was to analyze the behaviour of the dominant Rdm2, Rdm4 and Rdm5 genes interacting with the CE109 and CE112 local physiological races of Dpm, in different susceptible backgrounds (genotypes RA702 and J77-339). Rdm4 and Rdm5 segregated phenotypically as completely dominant genes in the specific interactions with the CE109 and CE112 isolates, respectively, in both susceptible backgrounds. Similarly, Rdm2 segregated as expected for a complete dominant gene in the specific interaction with the CE109 isolate, in both susceptible backgrounds. However, when interacting with the CE112 isolate, the Rdm2 gene did not segregate as expected for a completely dominant gene, neither in RA702 nor in J77-339 susceptible background. The distorted segregation of the Rdm2 gene was due to incomplete penetrance. To the best of our knowledge this is the first report documenting changes in the degree of penetrance of a soybean resistance gene (Rdm2) depending upon the physiological race of Dpm which interacts with and the genetic background in which the Rdm gene is being expressed.