Expression Profiling of Iron Deficiency Chlorosis (IDC) in Soybean (Glycine max): Similarities and Differences Between Low Iron Supply and Alkaline Stress

Alkaline soils comprise 30% of the earth and have low plant-available iron concentration, and can cause iron deficiency chlorosis (IDC). In the North-Central U.S., IDC causes soybean yield losses of $260 million annually. However, it is not known whether molecular responses to IDC are equivalent to those resulting from low iron supply. IDC tolerant and sensitive soybean lines from recurrent selection on high-pH soils in Nebraska provide a contrast to identify specific factors associated with IDC. We compared gene expression under alkaline and low iron conditions by sequencing RNA from roots of IDC tolerant and sensitive soybean lines grown hydroponically. We found both substantial overlap and substantial differences in differentially expressed genes when comparing iron deficiency and alkaline stress. Classical iron uptake genes were upregulated by both Fe deficiency and alkaline stress, including ferric-chelate reductase and ferrous transporter genes, however, their gene products did not function well at alkaline pH. In addition, genes in the phenylpropanoid synthesis pathway were upregulated in both alkaline and low Fe conditions. These genes lead to the production of fluorescent root exudate compounds, such as coumarins. Fluorescence of nutrient solution increased with alkaline treatment, and was higher in the IDC tolerant line. We hypothesize that root coumarin exudates become essential at alkaline pH where the classical iron uptake system does not function well. We are performing metabolomic profiling to identify these compounds. This work could result in new strategies to screen for IDC tolerance, and provide breeding targets to improve crop alkaline stress tolerance.