Cysteine Protease Inhibitors Reduce Enzymatic Browning of Potato by Lowering the Accumulation of Free Amino Acids

Enzymatic browning is a major issue
affecting the quality of processed
potato (Solanum tuberosum L.). To understand
the molecular mechanism of browning, transcriptional analyses were
performed by employing potatoes that differed in browning. Coexpression
analysis indicated that 9 out of 15 upregulated genes in browning-less
groups encoded for potato protease inhibitors (StPIs). In addition, gene otology analysis showed that the enriched terms
were mainly involved in protease inhibitors. Overexpression of cysteine StPI 143 and StPI 146 individually reduced
browning and lowered protease activities and tyrosine and total free
amino acid (FAA) contents, but they could not decrease polyphenol
oxidase activity. Moreover, supplementing exogenous tyrosine or total
FAAs into transgenic potato mash to wild-type amounts promoted mash
browning, browning with total FAAs, more than with tyrosine, resembling
wild-type levels. These results implied that cysteine StPIs reduced browning via lowering the accumulation of FAAs in addition
to tyrosine. Our findings have enriched the knowledge about the roles
and mechanisms of protease inhibitors in regulating enzymatic browning
of potato, which provide new ways for controlling potato browning.