Iron deficiency chlorosis (IDC) is a wide-spread problem strongly affecting soybean production in North Dakota. The characteristic yellowing of plant leaves suffering from IDC is caused by a lack of chlorophyll formation due to poor function of iron-requiring enzymes involved in chlorophyl biosynthesis. North Dakota soils normally contain more than enough iron for plant function, however much of the iron is not in soluble form needed by the plant. A reduction in iron solubility at high soil pHs caused by high levels of CaCO3 (lime) in the top-soil is the main cause of IDC; white the iron is there, it isn’t available to the plant. High lime soil is common in North Dakota, and IDC is exacerbated by salinity which is also becoming more and more common.
Despite a decades-long recognition of the problem, few effective solutions are available for farmers. One option involves applying iron fertilizer furrow at planting. But only red chelate fertilizers such as Soygreen (EDDHA) are effective, and these fertilizers are almost impractically expensive for most farmers at a cost of ~$18/acre at the recommended rate – which often needs to be exceeded to alleviate IDC. Some genes in soybeans that confer resistance have been identified (Mamidi et al. 2014), and varieties that incorporate these genes can help, though traits to improve IDC tolerance are often not sufficiently incorporated into commercial varieties with other desirable traits such as weed control.
The ability of microbes to solubilize iron, making insoluble iron available to the plant, has long been recognized (Crowley et al. 1988) and represents a new opportunity to combat IDC. Bacteria such as pseudomonads can release siderophore compounds which are effective in solubilizing iron and can improve the iron nutrition of plants (Vansuyt et al. 2007). Such microbes could be cultured and deployed with rhizobia as inoculants to combat IDC. Further, by isolating these microbes from North Dakota soils, the likelihood they will be persistent and effective when growers applying them in our environmental conditions would be enhanced (a “tailored inoculant” approach).
In this study we aim to build on previous work to assess the potential of the soybean microbiome as a new tool to combat IDC. In FY22, in a study that analyzed four fields in Eastern ND with varying levels of IDC, we observed a significant correlation in the structure of the soybean root and rhizosphere microbiome with the IDC level of the soil (see FY22 report). We hypothesize that unique groups of microbes that are enriched under IDC conditions could help alleviate IDC in soybeans when cultured and used as inoculants along with root nodule forming rhizobia. We have already optimized a greenhouse assay that will be suitable for measuring growth potential of microbiome members, and a plate screening assay that can identify siderophore producers (microbe-produced iron solubilizing molecules that function like Fe-chelating fertilizers). With this study we aim to utilize these resources to attempt to identify individual microbes or groups of microbes, cultures from the IDC soybean microbiome that could have a beneficial effect to soybean plants grown under IDC conditions.