Update on Plant Lipoxygenases Plant Lipoxygenases. Physiological and Molecular Features

Lipoxygenases (LOXs; EC 1.13.11.12) are nonheme iron-containing dioxygenases widely distributed in plants and animals. LOX catalyzes the addition of molecular oxygen to polyunsaturated fatty acids containing a (Z,Z)-1,4-pentadiene system to produce an unsaturated fatty acid hydroperoxide. LOX initiates the synthesis of a group of acyclic or cyclic compounds collectively called oxylipins, which are products of fatty acid oxidation, with diverse functions in the cell. In plants, linolenic and linoleic acids are the most common substrates for LOX (Siedow, 1991). Oxygen can be added to either end of the pentadiene system (regiospecificity). In the case of linoleic or linolenic acids, this leads to two possible products, the 9and 13-hydroperoxy fatty acids (Siedow, 1991). In vitro, most LOXs prefer free fatty acids, though it has been shown that sterified fatty acids are also substrates for LOX in vivo (Feussner et al., 2001; Stelmach et al., 2001), suggesting that membrane lipids could be substrates for oxylipin biosynthesis. The hydroperoxy fatty acid products of the LOX reaction can be further converted to different compounds through the action of enzymes participating in at least six pathways (Fig. 1). In plants, products of the LOX pathway have several diverse functions (Table I). In addition, LOX has been associated with some processes in a number of developmental stages (Siedow, 1991; Kolomiets et al., 2001), and with the mobilization of storage lipids during germination (Feussner et al., 2001). LOX is also used as a storage protein during vegetative growth (Fischer et al., 1999; Fig. 2). LOX gene expression is regulated by different effectors such as the source/sink status (Fischer et al., 1999), JA (Creelman and Mullet, 1997), abscisic acid (Melan et al., 1993), and also by different forms of stress, such as wounding (Porta et al., 1999), water deficiency (Porta et al., 1999), or pathogen attack (Melan et al., 1993). In addition, LOX genes isolated from different plant species show differential organspecific expression (Griffiths et al., 1999; Kolomiets et al., 2001; Table II). In recent years, our knowledge of the function of LOX and oxylipins in plants has increased with the contributions of many research groups. In addition, a number of plant LOX sequences are now available, making possible the analysis of their phylogenetic relationships and the elucidation of the connections between both LOX sequences and structures and their regiospecificity and activity. The objective of this review is to discuss recent advances on the role of LOXs in the physiology of plants.