PUTRESCINE AMINOPROPYLTRANSFERASE IS RESPONSIBLE FOR BIOSYNTHESIS OF SPERMIDINE, SPERMINE, AND MULTIPLE UNCOMMON POLYAMINES IN OSMOTIC STRESS-TOLERANT ALFALFA

The biosynthesis of polyamines from the diamine putrescine is not full y understood in higher plants. A putrescine aminopropyltransferase (PA PT) enzyme activity was characterized in alfalfa (Medicago sativa L.). This enzyme activity was highly specific for putrescine as the initia l substrate and did not recognize another common diamine, 1,3-diaminop ropane, or higher-molecular-weight polyamines such as spermidine and s permine as alternative initial substrates. The enzyme activity was inh ibited by a general inhibitor of aminopropyltransferases, 5 '-methylth ioadenosine, and by a specific inhibitor of PAPTs, cyclohexylammonium sulfate. The initial substrate specificity and inhibition characterist ics of the enzyme activity suggested that it is a classical example of a PAPT. However, this enzyme activity yielded multiple polyamine prod ucts, which is uncharacteristic of PAPTs. The major reaction product o f PAPT activity in alfalfa was spermidine. The next most abundant prod ucts of the enzyme reaction using putrescine as the initial substrate included the tetramines spermine and thermospermine. These two tetrami nes were distinguished by thin-layer chromatography to be distinct rea ction products exhibiting differential rates of formation. In addition , the uncommon polyamines homocaldopentamine and homocaldohexamine wer e tentatively identified as minor enzymatic reaction products but only in extracts prepared from osmotic stress-tolerant alfalfa cultivars. PAPT activity from alfalfa was highest in meristematic shoot tip and f loral bud tissues and was not detected in older, nonmeristematic tissu es. Product inhibition of the enzyme activity was observed after sperm idine was added into the in vitro assay for alfalfa PAPT activity. A b iosynthetic pathway is proposed that accounts for the characteristics of this PAPT activity and accommodates a novel scheme by which certain uncommon polyamines are produced in plants.