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 The condensation of decarboxylated SAM and putrescine is catalyzed by spermidine synthase (putrescine aminopropyltransferase) [EC 2.5.1.16]. Further condensation of spermidine with decarboxylated SAM, catalyzed by spermine synthase [EC 2.5.1.22], produces the tetramine, spermine (Flores et al, 1989).
Recently, the assumption that spermidine synthase and spermine synthase are two separate enzymes in plants, has been questioned (Bagga et al, 1997). The putrescine aminopropyltransferase of alfalfa may be responsible not only for spermidine and spermine synthesis, but also the synthesis of the uncommon polyamines, thermospermine, homocaldopentamine and homocaldohexamine (Bagga et al, 1997). Antisense SAM decarboxylase [EC 4.1.1.50] potato plants show markedly altered phenotypes including stunted growth, short internodes, stem branching and small leaves, decreased SAM decarboxylase transcripts, a reduction in SAM decarboxylase activity, reductions in the levels of putrescine, spermidine and spermine, and a marked increase in ethylene production (Kumar et al, 1996). This supports the idea that there is a competitive interaction between polyamine and ethylene biosynthesis (Walden et al, 1997).
Bagga S, Rochford J, Klaene Z, Kuehn GD, Phillips GC 1997 Putrescine aminopropyltransferase is responsible for biosynthesis of spermidine, spermine, and multiple uncommon polyamines in osmotic stress-tolerant alfalfa. Plant Physiol. 114: 445-454. Flores HE, Protacio CM, Signs MW 1989 Primary and secondary metabolism of polyamines in plants. In "Plant Nitrogen Metabolism" (JE Poulton, JT Romeo, EE Conn eds), Rec. Adv. Phytochem, Vol 23, Plenum Press, New York, pp. 329-393. Kumar A, Taylor M, Mad Arif SA, Davies H 1996 Potato plants expressing antisense and sense S-adenosylmethionine decarboxylase (SAMDC) transgenes show altered levels of polyamines and ethylene: antisense plants display abnormal phenotypes. Plant J. 9: 147-158. Walden R, Cordeiro A, Tiburcio AF 1997 Polyamines: small molecules triggering pathways in growth and development. Plant Physiol. 113: 1009-1013.
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