Increased Production of Secondary Compounds Under Iron Deficiency and Alkaline Stress in Soybean (Glycine max).

Alkaline soils comprise 30% of the earth and have low plant-available iron concentrations. Thus, alkaline stress can cause iron deficiency chlorosis (IDC). In the North-Central U.S., IDC causes soybean yield losses of $260 million annually. No studies have determined whether the physiological and molecular responses to IDC resulting from alkaline stress are equivalent to those resulting from low iron supply. There is genetic variation for severity of IDC symptoms, but the specific basis for IDC tolerance is not known. IDC tolerant and sensitive soybean lines resulting from 10 cycles of recurrent selection for improved IDC tolerance on high-pH soils in Nebraska provide a contrast to identify specific factors associated with IDC. Because plant responses to low iron supply include increased expression of certain iron uptake genes, we compared gene expression profiles 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.