Compatible solutes may also function as oxygen radical scavengers. Evidence for such a function comes from studies on fungal pathogen interactions where the pathogens are protected by the synthesis and secretion of mannitol. Plants and animals produce oxygen radicals in response to pathogen attack. The rapid production and local accumulation of reactive oxygen species leads to localized cell death in the host, which then may limit the spread of the pathogen.56 In response, some pathogens seem to have evolved mechanisms which detoxify the reactive oxygen species produced by the host. Cryptococcus neoformans, a yeast which opportunistically infects humans with a compromised immune system, produces and secretes mannitol. A mutant strain that does not produce mannitol is less virulent.57 Similarly, during the infection process the tomato pathogen Cladosporium fulvum produces mannitol, which seems to protect the fungi from damage by reactive oxygen species produced by the plants.58
Mannitol has been shown in vitro to function as a scavenger of reactive oxygen species, ROS.59-60 ROS is a generic term which is used to include not only free radicals such as superoxide and hydroxyl radicals, but also singlet oxygen and H2O2. Smirnoff and Cumbes26 designed experiments that compared the radical scavenging capabilities of different compatible solutes. They reported that mannitol, sorbitol, glycerol, proline, ononitol and pinitol were active scavengers at different concentrations in vitro, while glycinebetaine was not.26,28 The relative radical scavenging efficiency of these compounds seemed dependent on their rate constants for reactions with hydroxyl radicals. For example, the rate constant of mannitol is four-fold higher than that of proline,61 and thus it was more effective than proline as a hydroxyl radical scavenger. Under water deficit conditions, radical production increases in plants,62 and it may be that the accumulation of polyols provides some protective effect against oxidative damage of proteins.
Recently, additional experiments shed light on the radical scavenging capacity of mannitol in in vivo experiments.6-7 Mannitol 1-phosphate dehydrogenase was modified such that the protein was imported into chloroplasts, and the gene construct was expressed in transgenic tobacco. We argued that a potential function in radical scavenging in vivo might best be demonstrated with chlo-roplasts which abundantly produce a variety of ROS when stressed by water deficit (for a recent review, see Noctor and Foyer63). Mannitol was present in concentrations of approximately 100 mM in the chloroplasts.6 Using different conditions, such as illumination with high light, paraquat treatment, enhanced H2O2 generation and DMSO infiltration, it could be shown that plants containing manni-tol in their chloroplasts were better able to maintain high carbon fixation rates and showed less chlorophyll bleaching.6 Further experiments indicated that mannitol was active specifically against hydroxyl radicals and not against hydrogen peroxide or radical oxygen. This is important information, considering that chloroplast detoxification systems exist that can deal with H2O2 and radical oxygen, while there is no enzyme system described that could deal with the extremely short lived and highly reactive hydroxyl radicals.
Further experiments7 indicated that even under high light conditions the major effect of increased hydroxyl radical production was on the dark reactions of photosynthesis, while the photosystems themselves functioned normally. It could be demonstrated that some enzymes of the Calvin cycle were predominantly affected by hydroxyl radicals. Phosphoribulo-kinase, PRK, and likely other SH enzymes of the Calvin cycle, showed sensitivity to hydroxyl radicals, and the activity of PRK was protected by the presence of mannitol.7 It appears that mannitol in the chloroplast interferes with the so-called Fenton reaction through which Fe2+ and H2O2 react, producing hydroxyl radicals and Fe3+.6-7 Iron increases have been found to induce the expression of ascorbate peroxidase genes, possibly also for the purpose of prevention the Fenton reaction.64
It would be overextending the compatible solute concept if we assigned a main function in hydroxyl radical scavenging to these solutes. Plants, especially in their chloroplasts and mitochondria, are endowed with an efficient non-enzymatic (a-tocopherol, carotenoids, flavonoids, etc.) and enzymatic (e.g., SOD, catalase, ascorbate/glutathione cycle enzymes) array of armaments to counteract radical species. The unexpected effect and likely function of mannitol, however, highlight the general importance of radical scavenging mechanisms in stress tolerance, especially in plants for which the generation of excited states of chromophores and electron transport is essential to photosynthesis. Several groups have now manipulated resident scavenging enzyme systems for reactive oxygen species (Table 19.2). Protective effects have been observed, for example, by altering catalase, SOD, or the glutathione cycle enzymes.5,65-67 How far engineering of these pathways can go is unclear, because it must be understood that such manipulations might interfere with signaling processes which include the production and recognition of reactive oxygen species (see Bohnert and Sheveleva).68
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