Iron is an essential element for living organisms that plays critical roles in the process of bacterial growth and metabolism. However, it remains to be elucidated whether piuB encoding iron-uptake factor is involved in iron uptake and pathogenicity of Xanthomonas axonopodis pv. glycines (Xag). To investigate the function of piuB, we firstly generated a piuB deletion mutant (ΔpiuB) by homologous recombination. Compared with the wild-type, the piuB mutant exhibited significantly reduced growth and virulence in host soybean. The mutant displayed markedly increased siderophore secretory volume, and its sensitivity to Fe³⁺, Cu²⁺, Zn²⁺ and Mn²⁺ was significantly enhanced. Additionally, the H₂O₂ resistance, exopolysaccharide yield, biofilm formation, and cell mobility of ΔpiuB were significantly diminished compared to that of the wild-type. The addition of exogenous Fe³⁺ cannot effectively restore the above characteristics of ΔpiuB. However, expressing piuB in trans rescued the properties lost by ΔpiuB to the levels in the wild-type. Taken together, our results demonstrated that PiuB is a potential factor for Xag to assimilate Fe³⁺, and is necessary for Xag to be pathogenic in host soybean.
Iron ; Glycine max ; Virulence ; Xanthomonas axonopodis/genetics* ; Hydrogen Peroxide

The WRKYs are a group of plant-specific transcription factors that play important roles in defense responses. In this study, we silenced 2 GmWRKY33B homologous genes using a bean pod mosaic virus (BPMV) vector carrying a single fragment from the conserved region of the GmWRKY33B genes. Silencing GmWRKY33B did not result in morphological changes. However, significantly reduced resistances to Pseudomonas syringae pv. glycinea (Psg) and soybean mosaic virus (SMV) were observed in the GmWRKY33B-silenced plants, indicating a positive role of the GmWRKY33B genes in disease resistance. Kinase assay showed that silencing the GmWRKY33B genes significantly reduced the activation of GmMPK6, but not GmMPK3, in response to flg22 treatment. Reverse transcriptase PCR (RT-PCR) analysis of the genes encoding prenyltransferases (PTs), which are the key enzymes in the biosynthesis of glyceollin, showed that the Psg-induced expression of these genes was significantly reduced in the GmWRKY33B-silenced plants compared with the BPMV-0 empty vector plants, which correlated with the presence of the W-boxes in the promoter regions of these genes. Taken together, our results suggest that GmWRKY33Bs are involved in soybean immunity through regulating the activation of the kinase activity of GmMPK6 as well as through regulating the expression of the key genes encoding the biosynthesis of glyceollins.
Glycine max/genetics* ; Disease Resistance/genetics* ; Biological Assay ; Dimethylallyltranstransferase ; Gene Silencing