Polyamines, ubiquitous organic aliphatic cations, have been
implicated in a myriad of physiological and developmental processes in many
organisms, but their in vivo functions remain to be determined. We
expressed a yeast S-adenosylmethionine decarboxylase gene (ySAMdc;
Spe2) fused with a ripening-inducible E8 promoter to specifically
increase levels of the polyamines spermidine and spermine in tomato fruit
during ripening. Independent transgenic plants and their segregating lines were
evaluated after cultivation in the greenhouse and in the field for five
successive generations. The enhanced expression of the ySAMdc gene resulted in
increased conversion of putrescine into higher polyamines and thus to
ripening-specific accumulation of spermidine and spermine. This led to an
increase in lycopene, prolonged vine life, and enhanced fruit juice quality.
Lycopene levels in cultivated tomatoes are generally low, and increasing them
in the fruit enhances its nutrient value. Furthermore, the rates of ethylene
production in the transgenic tomato fruit were consistently higher than those
in the nontransgenic control fruit. These data show that polyamine and ethylene
biosynthesis pathways can act simultaneously in ripening tomato fruit. Taken
together, these results provide the first direct evidence for a physiological
role of polyamines and demonstrate an approach to improving nutritional
quality, juice quality, and vine life of tomato fruit.
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