Biosynthesis of Diterpene Phytoalexin Precursors in Cell-free Extracts of Rice

Abstract

Rice leaves have been reported to accumulate two groups of diterpenoid phytoalexins, the momilactones and the oryzalexins, in response to infection with Pyricularia oryzae (Kono et al, 1985) or irradiation with UV-light (Cartwright et al, 1981). Scheme 1 illustrates proosed pathways and key intermediates in the biosynthesis of these phytoalexins bas’ed on known pathways of biosynthesis of polycyclic diterpenes such as ent-kaurene. Cyclization of geranylgeranyl-PP to two labadienyl pyrophosphates of appropriate stereochemistry followed by further cyclization of these intermediates could give rise to two tricyclic pimaradienes. A series of oxidative transformations of 9βH-pimara-7, 15-diene and sandaracopimaradiene would be responsible for the formation of the momilactones and oryzalexins, respectively. This paper reports evidence for the formation of pimaradienes and ent-kaurene. in cell-free extracts of UV-treated rice leaves.