Cloning and Characterization of a GmSAGT 1 Gene Whose Transcriptional Expression Is Associated with the Level of Resistance to Soybean Downy Mildew

We have cloned a GmSAGT 1 gene coding for a bi-functional enzyme of serine/alanine glyoxylate aminotransferase from a soybean variety Zaofeng5 resistant to soybean downy mildew (SDM). The GmSAGT1 expressed in Escherichia coli exhibited serine glyoxylate aminotransferase (SGT) and alanine glyoxylate aminotransferase (AGT) activity that produced 20-fold more glycine than that of the control. The amount of hydrogen peroxide (H 2 O 2 ) dramatically increased in E. coli expressing GmSAGT1, and bacterial growth was severely inhibited. Real-time polymerase chain reaction (RT-PCR) showed that GmSAGT 1 could be detected in Zaofeng5 prior to salicylic acid (SA) treatment and increased by fivefold after SA induction. In contrast, it was not detectable in susceptible variety Heinong10 prior to and only slightly increased after SA induction. Fifteen soybean varieties belonging to highly resistant (HR), medium resistant (MR), resistant (R), susceptible (S), and highly susceptible (HS) category were used for real-time quantitative RT-PCR (qRT-PCR) analysis, which showed the average expression level of GmSAGT 1 in HR group was 13 times of that in HS group. Northern blot analysis indicated that the expression level of GmSAGT 1 in HS, S, MR, and R group was 5, 22, 39, and 65 % of that in HR, respectively. Transgenic tobacco expressing GmSAGT 1 increased resistance to a fungal pathogen Rhizoctonia solani . Our results have demonstrated that higher expression of GmSAGT 1 gene is closely correlated to increased SDM resistance.