14-3-3 proteins are abundant eukaryotic proteins that interact with many other proteins, thereby modulating their function and thus cell metabolism. The data from mRNA analysis confirm the developmental regulation of 14-3-3 isoform expression in potato plants. In order to test whether or not 14-3-3 protein expression affects plant phenotype and metabolism, transgenic potato plants either overexpressing Cucurbita pepo 14-3-3 or underexpressing endogenous 14-3-3 isoforms were analysed. An increase in tuber number and a decrease in tuber size in the over-expressed transformant was observed; the transgenic plants contain more chlorophyll than the control and they lose it more slowly than the control when transferred to the dark. The 14-3-3-repressed transgenic plants showed a decrease in tuber number and an increase in tuber size; an increase in the fresh weight of the transgenic tubers was also detected. The increased catecholamine level was accompanied by an increased ratio of soluble sugars to starch in overexpressed transformant. The opposite effect was detected in 14-3-3-repressed transgenic plants. All the repressed plants showed significant increases in nitrate reductase (NR) activity, suggesting that the regulation of NR occurs in vivo, and is not isoform-dependent. The increase in NR activity resulted in a significant decrease in nitrate level. The level of sucrose phosphate synthase activity was also significantly increased in all 14-3-3-underexpressed transgenes, and remarkably the increase in enzyme activity was accompanied by respective changes in sucrose levels in the tubers. The most intriguing finding was the significant (2–3-fold) increase in ethylene content in all the 14-3-3-repressed transgenic lines, which probably resulted from a methionine level increase. The substantial increase of ethylene level in the repressed forms might explain the significant shortening of the vegetation period of the analysed transgenic plants.

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