Physiological and molecular characterization of strategy-I responses and expression of Fe-transporters in Fe-deficient soybean

Abstract

Iron (Fe) deficiency is one of the major nutritional disorders that affect soybean production. This study aimed to better understand the adaptive responses and gene characterization in Fe-deficient soybean. The experiment was conducted on 14-day-old soybean plants in a soil pot under Fe sufficiency and bicarbonate-induced Fe deficiency. In soybean, Fe deficiency caused severe root and shoot morphological retardation, which was accompanied by the reductions in chlorophyll score, Fv/Fm (quantum yield efficiency of PSII), Pi_ABS (photosynthetic performance index) as well as an increase in electrolyte leakage and cell death. This is in accordance with the lower Fe status in the root and shoot of Fe-deficient plants. Further, the expression of GmIRT1 (Fe-regulated transporter) was significantly induced, while the members of NRAMP (natural resistance-associated macrophage proteins) transporters were either downregulated (GmNRAMP2a, GmNRAMP2b, GmNRAMP5a, GmNRAMP7) or constitutively expressed (GmNRAMP5b, GmNRAMP6a, GmNRAMP6b) in roots of Fe-deficient soybean relative to controls. Furthermore, the activity of Fe-chelate reductase and the expression of GmFRO2 (ferric reductase oxidase) in roots of Fe-deficient soybean were both significantly reduced, indicating the impairment of the ability of soybean plants to maintain Fe availability in the rhizosphere. However, plants showed a decrease in rhizospheric pH and upregulation of GmATPase4 in roots, indicating that rhizosphere acidification is one of the adaptive approaches that soybean plants possess in response to Fe-deficiency. In bioinformatics analysis, soybean Fe-uptake genes showed partnership with Arabidopsis FRO2, FRU, NRAMP1, bHLH38, and bHLH39 genes. In addition, Fe-starved plants demonstrated elevated H2O2 and O2•– in roots which were consistent with the inefficient oxidative defense of antioxidant enzymes. These findings could aid in the development of Fe-efficient soybean by breeding or genome editing techniques.