Exogenous spermidine promotes the formation of the closing layer at potato tuber wounds by inducing polyamine synthesis and phenylpropanoid metabolism

Spermidine (Spd) is an essential polyamine in plants. Although there are reports that exogenous Spd can enhance the synthesis of endogenous polyamines and induce plant resistance, it remains unknown whether exogenous Spd affects the formation of the potato tuber wound closing layers by modulating polyamine synthesis and phenylpropanoid metabolism. This study found that Spd enhanced the gene expression and activity of arginine decarboxylase (ADC), ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (SAMDC), leading to an increase in the levels of Put, Spd, and Spm. Additionally, Spd increased the activity of phenylalanine ammonia-lyase (PAL), 4-coumaroyl-CoA ligase (4CL), and cinnamyl alcohol dehydrogenase (CAD), and elevated the levels of five phenolic acids and three lignin monomers. Furthermore, Spd increased the gene expression and activity of peroxidase (POD), diamine oxidase (DAO), and polyamine oxidase (PAO), resulting in an increase in the level of H₂O₂. The aforementioned phenolic acids and lignin monomers are oxidized and cross-linked under the combined action of POD and H₂O₂ to form suberin polyphenols (SPP) and lignin, which are deposited on the wound surface. The competitive inhibitor D-arginine (D-Arg) from ADC and the specific inhibitor guazatine (GZ) from PAO can delay the deposition of SPP and lignin by inhibiting polyamine synthesis, reducing the levels of phenolic acids and lignin monomers, and inhibiting POD activity and H₂O₂ production. In conclusion, exogenous Spd can promote the deposition of the closing layer on potato tuber wounds by promoting endogenous polyamine synthesis and phenylpropanoid metabolism.