Lipids plant-oxidation mechanisms

The toxic effects of ozone in plant systems have been studied for some time, yet the actual mechanisms of injury are not fully understood. In addition to visible necrosis which appears largely on upper leaf surfaces, many other physiological and biochemical effects have been recorded ( ). One of the first easily measurable effects is a stimulation of respiration. Frequently, however, respiration may not increase without concomitant visible injury. Furthermore, photosynthesis in green leaves as measured by CO2 assimilation, may decrease. It is well known that ozone exposure is accompanied by a dramatic increase in free pool amino acids ( ). Ordin and his co-workers ( ) have clearly shown the effect of ozone on cell wall biosynthesis. In addition, ozone is known to oxidize certain lipid components of the cell ( ), to affect ribosomal RNA (16) and to alter the fine structure of chloroplasts (7 ). 

In the ripening process of certain plants, e.g. vegetables, ethylene is produced to increase the respiration rate. The mechanism is believed to be a nonspecific membrane lipid effect. Ethylene has an anaesthetic effect comparable to nitrous oxide, and it has even bem used as a general anaesthetic agent. It is therefore natural to assume that the corresponding membrane bilayer... 

The citric acid cycle, also known as the tricarboxylic acid cycle or the Krebs cycle, is the final oxidative pathway for carbohydrates, lipids, and amino acids. It is also a source of precursors for biosynthesis. The authors begin Chapter 17 with a detailed discussion of the reaction mechanisms of the pyruvate dehydrogenase complex, followed by a description of the reactions of the citric acid cycle. This description includes details of mechanism and stereospecificity of some of the reactions, and homologies of the enzymes to other proteins. In the following sections, they describe the stoichiometry of the pathway including the energy yield (ATP and GTP) and then describe control mechanisms. They conclude the chapter with a summary of the biosynthetic roles of the citric acid cycle and its relationship to the glyoxylate cycle found in bacteria and plants. 

The principal mechanism by which fatty acids are broken down in plants is P-oxidation (Fig. 2.11) (Kindi, 1987). On germination, seeds with high oil content form glyoxysomes that contain the enzymes of P-oxidation. In this process, much of the energy stored in the lipids is converted to acetyl-CoA and is trapped in the thioester bond. Acetyl-CoA then enters the tricarboxylic acid cycle (TCA). P-Oxidation in plants appears to be identical to that of animals. 

The localization of enzymes of the oxylipin pathway has yet to be unequivocally elucidated. Nonetheless, LOX has been localized in plastids, vacuoles and the cytoplasm, e.g. [1], and in lipid bodies [36,37]. Since enzymes of the jasmonic acid biosynthetic pathway are thought to be localized mainly in plastids, mechanisms must exist to shuttle fatty acids released from the plasma membrane to the plastids. Furthermore, since p-oxidation of fatty acids normally occurs in peroxisomes, transport vesicles that carry the cargo between organelles may exist in plant cells. It is tempting to speculate that there could be fusion or mixing of compartments that contain either enzymes, fatty acids, and/or intermediate products, thus resulting in oxylipin biosynthesis. Intensive research is needed to address these questions as well as the cell-specific and the subcellular localization of oxylipin synthesis and the mechanisms that regulate this process. 

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