Thioredoxins are believed to mediate starch and protein mobilization in germinating cereals and dicotyledons. Nothing is known about redox regulation of lipid mobilization in plants. The possible redox regulation by thioredoxin h (Trx h) of a thiol–protease which degrades the oleosin coat of the oil body and its impacts on lipid mobilization was investigated in sunflower (Helianthus annuus L.) seedlings. An alkaline proteolytic activity stimulated by light was detected in seedlings. In vitro, the activity of this alkaline protease increased after reduction by NADPH-thiordoxin reductase system (NTS). The expression pattern of an alkaline 65 kDa thiol protease detected by gelatin SDS–PAGE technique, corresponded to the activity profile of the NTS-activated protease. The thiol-specific fuorochrome monobromobimane (mBBr) showed that a 65 kDa protein was also in a reduced state in vivo and becomes reduced in vitro by NTS. Except for 17–20 kDa oleosins, other oil body associated mBBr-labeled proteins were disappeared within three days following germination. Treatments of sunflower oil bodies by the NTS-activated alkaline protease made them more susceptible to maize lipase action. Ascorbate application enhanced lipid mobilization of seedlings. A model for seedling oil body mobilization was proposed according to which Trx h or other Trx isoforms, reductively activates an oleosin degrading thiol–protease and some oil body proteins, thus renders the organelle more susceptible to subsequent lipolytic actions. For the first time the potential role of Trx in the mobilization of lipid reserves in plants has been shown.
