PLANT PHYSIOLOGY , Vol 114, Issue 2 445-454, Copyright © 1997 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Putrescine Aminopropyltransferase Is Responsible for Biosynthesis of Spermidine, Spermine, and Multiple Uncommon Polyamines in Osmotic Stress-Tolerant Alfalfa
S. Bagga, J. Rochford, Z. Klaene, G. D. Kuehn and G. C. Phillips
Department of Agronomy and Horticulture (S.B., J.R., Z.K., G.C.P.), and Department of Chemistry and Biochemistry (G.D.K.), Molecular Biology Program, New Mexico State University, Las Cruces, New Mexico 88003
The biosynthesis of polyamines from the diamine putrescine is not fully
understood in higher plants. A putrescine aminopropyltransferase (PAPT)
enzyme activity was characterized in alfalfa (Medicago sativa L.). This
enzyme activity was highly specific for putrescine as the initial substrate
and did not recognize another common diamine, 1,3-diaminopropane, or
higher-molecular-weight polyamines such as spermidine and spermine as
alternative initial substrates. The enzyme activity was inhibited by a
general inhibitor of aminopropyltransferases, 5[prime]-methylthioadenosine,
and by a specific inhibitor of PAPTs, cyclohexylammonium sulfate. The
initial substrate specificity and inhibition characteristics of the enzyme
activity suggested that it is a classical example of a PAPT. However, this
enzyme activity yielded multiple polyamine products, which is
uncharacteristic of PAPTs. The major reaction product of PAPT activity in
alfalfa was spermidine. The next most abundant products of the enzyme
reaction using putrescine as the initial substrate included the tetramines
spermine and thermospermine. These two tetramines were distinguished by
thin-layer chromatography to be distinct reaction products exhibiting
differential rates of formation. In addition, the uncommon polyamines
homocaldopentamine and homocaldohexamine were tentatively identified as
minor enzymatic reaction products but only in extracts prepared from
osmotic stresstolerant alfalfa cultivars. PAPT activity from alfalfa was
highest in meristematic shoot tip and floral bud tissues and was not
detected in older, nonmeristematic tissues. Product inhibition of the
enzyme activity was observed after spermidine was added into the in vitro
assay for alfalfa PAPT activity. A biosynthetic 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.
