BIOSYNTHESIS OF CYCLIC DITERPENE HYDROCARBONS IN RICE CELL-SUSPENSIONS - CONVERSION OF 9,10-SYN-LABDA-8(17),13-DIENYL DIPHOSPHATE TO 9-BETA-PIMARA-7,15-DIENE AND STEMAR-13-ENE
Rs. Mohan et al., BIOSYNTHESIS OF CYCLIC DITERPENE HYDROCARBONS IN RICE CELL-SUSPENSIONS - CONVERSION OF 9,10-SYN-LABDA-8(17),13-DIENYL DIPHOSPHATE TO 9-BETA-PIMARA-7,15-DIENE AND STEMAR-13-ENE, Archives of biochemistry and biophysics, 330(1), 1996, pp. 33-47
The biosynthesis of diterpene hydrocarbons with enzyme extracts from r
ice cell suspension cultures was investigated to verify proposed pathw
ays and intermediates in the production of the momilactone and oryzale
xin phytoalexins. Diterpene synthase activity in cells treated with ch
itin to elicit the phytoalexin response was compared with the activity
in untreated cells using the acyclic substrates [1-H-3](E,E,E)- and [
1-H-3](E,Z,E)-geranylgeranyl diphosphates (GGPPs 4-OPP and 11-OPP) as
well as the bicyclic substrates [15-H-3]ent-copalyl and [15-H-3]syn-co
palyl diphosphates (CPPs, 5-OPP, and 6-OPP). ent-kaurene (7), ent-sand
aracopimaradiene (8), 9 beta H-pimara-7,15-diene (9), and stemar-13-en
e (10) were identified as major products by comparisons with authentic
standards. Marked increases in diterpene synthase activities were obs
erved with enzyme from chitin-treated cells: (E,E,E)-GGPP (similar to
100 fold), ent-CPP (similar to 3 fold), and syn-CPP (similar to 60 fol
d). The very low conversions of (E,Z,E)-GGPP to hydrocarbon products e
xcludes its role in the biosynthesis of 9,10-syn-diterpenes in rice ce
lls. ent-Kaurene was the major diterpene formed from ent-CPP with enzy
me from unelicited cells. In contrast the enzyme from chitin-treated c
ells converted ent-CPP to a mixture of ent-kaurene, ent-sandaracopimar
adiene, and a third unidentified diterpene. With syn-CPP as substrate
the induced synthases afforded a mixture of 9 beta-pimaradiene, stemar
ene, and a third, unidentified syn-diterpene. Overall the results are
consistent with the hypothesis that rice cells respond to treatment wi
th chitin fragments by producing new diterpene synthases not present i
n the untreated cells, These induced cyclases initiate phytoalexin bio
synthesis by diverting (E,E,E)-GGPP into new cyclization modes that pr
oduce ent-sandaracopimaradiene, stemarene, and 9 beta-pimaradiene, the
presumed precursors to oryzalexins A-F, oryzalexin S, and momilactone
s A-C, respectively. The intermediate role of 9,10-syn-CPP in syn dite
rpene biosynthesis is verified. (C) 1996 Academic Press, Inc.