The repertoire of secondary metabolism (involving the production of co
mpounds not essential for growth) in the plant kingdom is enormous, bu
t the genetic and functional basis for this diversity is hard to analy
se as many of the biosynthetic enzymes are unknown. We have now identi
fied a key enzyme in the ornamental plant Gerbera hybrida (Asteraceae)
that participates in the biosynthesis of compounds that contribute to
insect and pathogen resistance. Plants transformed with an antisense
construct of gchs2, a complementary DNA encoding a previously unknown
function(1,2), completely lack the pyrone derivatives gerberin and par
asorboside. The recombinant plant protein catalyses the principal reac
tion in the biosynthesis of these derivatives: GCHS2 is a polyketide s
ynthase that uses acetyl-CoA and two condensation reactions with malon
yl-CoA to form the pyrone backbone of the natural products. The enzyme
also accepts benzoyl-CoA to synthesize the backbone of substances tha
t have become of interest as inhibitors of the HIV-1 protease(3-5). GC
HS2 is related to chalcone synthase (CHS) and its properties define a
new class of function in the protein superfamily. It appears that CHS-
related enzymes are involved in the biosynthesis of a much larger rang
e of plant products than was previously realized.