Background:The pathway to form green leaf volatiles is important for plant defense against pathogens and herbivores. Results: Galactolipids with truncated oxoacids are formed as counterparts of green leaf volatiles. Conclusion: Galactolipid-hydroperoxides are substrates for hydroperoxide lyase. Significance: The biosynthetic pathway partly proceeds directly with galactolipids, without the involvement of lipases, to form free fatty acids.

Oxylipins are formed from fatty acids in plants in response to various stresses, and exert their roles as signal molecules or direct defense molecules. Oxylipins are usually found in free forms; however, oxylipins esterified in galactolipids have been identified recently, which implicates an alternative biosynthetic pathway to form oxylipins without a lipid-hydrolyzing step. In order to get insight into the alternative pathway, we examined catalytic nature of soybean seed lipoxygenase-1 on galactolipids. The lipoxygenase showed no activity on the galactolipids; however, significant activity was observed when deoxycholic acid was added into the reaction mixture. HPLCmass spectrometry analyses showed that either or both the acyl moieties were oxygenated by the lipoxygenase in regio specific manner. Accordingly, we concluded that galactolipids could serve as substrates for lipoxygenases, from which direct oxygenation of galactolipids in an oxylipin metabolism under stress condition in plants was proposed.