Update:
Tests have been conducted to further elucidate the role of two mechanisms that may explain the innate immunity response (IIR) as a means of broad-based resistance to disease. Differences have been detected between a known resistant and susceptible line for the two mechanisms (1) pathogen-associated molecular pattern (PAMP), and (2) reactive oxide species (ROS) or super oxides. However, these differences are not always significant or consistent.

The main goal has been to identify QTLs that control the IIR. Thus far, four QTLs have been identified that are associated with ROS production and in one case, explained 80% of the genetic variation of the immunity response between a resistant line and susceptible line. Still, in an evaluation of mapping populations of soybean rust and soybean aphid, the QTLs did not distinguish between the progeny containing high and low ROS levels.

It has been found that many genes, even hundreds, are activated in an IIR response, namely the PAMPs system of chitin and flagelin22. The differentially expressed genes were classified in different functional categories of signal transduction and stress response. In one case, a SCN-resistant line showed a higher fold induction than the susceptible line for these set of genes.

Additional results and details of the study:
In order to confirm the QTL identified in our publication, we screened the innate immunity response of 25 different soybean genotypes, which were used to generated the soybean Nested Associate Mapping (NAM) populations. This analysis has now led to the selection of parents of specific RIL populations that we intend to utilize for further QTL mapping.

We worked with George Graef, Univ. of Nebraska, to test some of his mapping populations in the hope of mapping additional innate immunity QTLs. The innate immunity responses of six soybean genotypes, which show different resistance levels to sclerotinia, were analyzed after treatment with flagellin 22 (flg22) or chitin. The innate immunity response was analyzed through the measurement of the reactive oxygen species (ROS) and nitric oxide (NO) production, which are hallmarks of innate immunity. Our analysis showed that flg22 induced stronger ROS production than chitin treatment and also that the production of either ROS or NO was genotype dependent. Likewise, we observed that the genotype Vinton81 produced more ROS triggered by flg22 than the other five genotypes; in contrast Williams 82 produced more ROS after chitin treatment. When NO production was measured, we observed that Corsoy 79 was the genotype that produced more NO in response to either treatment.

In order to map QTLs associated with ROS and NO production, we selected a mapping population derived from a cross of Vinton 81 (strong innate immunity triggered by flg22) vs Williams 82 (lower innate immunity triggered by flg22). Preliminary data shows that the variation in the ROS production is quantitative inherited and this variation can be mapped. Although we do not see strong differences in the NO production between these two parental lines, we are analyzing the NO production across the entire mapping population. These data show quantitative variation across the mapping population that we are now trying to utilize for QTL mapping.

The relative sensitivity of soybean fungal pathogens exposed to hydrogen peroxide was tested to indicate the potential sensitivity to ROS produced in soybean in response to pathogen attack. Results indicated that germination fell off significantly at 0.005M H2O2 for all three fungi but there was still some germination of P. pachyrhizi in 0.01M H2O2 but none at 0.05M.

The purpose of the project was to determine if the soybean plant's own immune system could be enhanced or harnessed through genetic manipulation to improve disease resistance - as opposed to breeding in specific resistance genes, which is laborious and is often limited in ability to resist specific diseases. It was thought that harnessing the plant's immune system might prove more durable and offer broad resistance against numerous diseases. This project, over approximately four years, identified some of the key mechanisms that may regulate the soybean plant's immune system. However, efforts to identify the specific genes within a cluster of multiple genes (i.e. QTLs) behind each mechanism for each specific disease or pathogen proved much more difficult. At project completion, prospective QTLs were identified but for those QTLs that were tested against certain pathogens, there were no consistent positive results. Much more testing would be required to determine if a soybean plant's immune system can be effectively harnessed for improved disease resistance.