Spermidine
possesses multiple healthy functions, and soybeans contain
the most abundant spermidine. In this study, spermidine contents of
soybeans from different varieties and production regions in China
were evaluated, and a spermidine synthase gene (speE) was identified by recombinant expression, transcriptional verification,
and sequence analysis. Spermidine contents of soybean samples from
18 varieties ranged 72.38–228.82 mg/kg, and those from 19 production
regions ranged 134.64–242.32 mg/kg. The highest-spermidine
sample GZ was used to clone four predicted speE genes.
Expressing the gene speE5 improved the spermidine
titer by 54% in Bacillus amyloliquefaciens, confirming that speE5 was involved in spermidine
synthesis. Transcriptional verification was performed through a soybean
germination model. Germination for 48 h led to a onefold increase
of spermidine in samples SHX and HB, and corresponding speE5 transcriptional levels were improved by 26-fold and 18-fold, respectively,
further verifying the function of speE5. Finally,
the sequences of the speE5 gene and deduced amino
acids were analyzed, and the conserved sites and catalysis mechanisms
were presented. This study identified an active spermidine synthase
gene from soybean for the first time, which provided an important
gene resource for genetic breeding of spermidine-rich soybean or microbial
cell factory.