A Soybean β-Expansin Gene GmEXPB2 Involved in Root System Architecture Responses to Abiotic Stresses in Transgenic Arabidopsis

Root system architecture determines the relative distribution of plant roots in different soil layers and may respond plastically to different abiotic stresses including phosphorus (P) deficiency, iron (Fe) deficiency, salt stress, and water deficiency, but its response mechanism is still unclear. Previously, we cloned and characterized a vegetable soybean β-expansin gene, GmEXPB2, from a Pi starvation-induced soybean cDNA library. This gene was mainly expressed in roots, and was highly induced by Pi deficiency. In this study, we found that GmEXPB2 was also induced by Fe deficiency, salt stress or drought stress. Arabidopsis was transformed with a construct containing the GUS reporter gene driven by GmEXPB2 promoter for the gene localization analysis or the 35S promoter to drive the overexpression of the Open Reading Frame (ORF) of GmEXPB2 for the gene function analysis. Five-day-old seedlings were harvested and incubated in a GUS staining solution for three hours at 37 °C, and the expression pattern of GmEXPB2 under different abiotic stresses was confirmed by GUS staining in transgenic Arabidopsis. Three overexpression lines were selected using q-PCR analysis for further study. Seven-day-old seedlings were transplanted and subjected to different abiotic stresses for ten days. The overexpression lines of GmEXPB2 showed significant increases in primary and lateral root length and lateral root number under low P and water deficiency, while slightly increase under low Fe stress, but no difference under salt stress. Taken together, our results suggest that GmEXPB2, a root β-expansin gene, is involved in root system architecture responses to abiotic stresses and plays an important role in regulating adaptive changes of the root system architecture.  The use of GmEXPB2 gene expression may be an effective tool to identify root characteristics, and have a great potential for improving crop productivity under stress environments.