Ocd And Medicinal Plants

Time (days)

?ig. 1.9.17. Induction of secondary metabolism by ozone stress. One-year-old Scots pine (Pinus sylvestris) plants were exposed for 40 days to various ozone concentrations (8 h daily) and afterwards grown under natural conditions until autumn of the following year. A The fla-vane catechin (see Box 1.9.6), an antioxidant secondary metabolite, accumulated after the termination of ozone treatment and remained high during the following year. The concentration required to induce flavane accumulation was above the naturally occurring concentration but less than double the natural concentration. B Stilbenes are inducible phytoalexins (see Chap. 1.10.2.1) in sap wood and needles, but are constitutive in the heart wood. Their accumulation depends on the time and ozone concentration; incorporation into the cell wall caused a reduction in the soluble stilbene. The memory effect can clearly be seen in the case of stilbenes. (After Langebartels et al. 1997)

symptoms, which are expressed upon Mg or N deficiency

In contrast to inhibition of primary metabolism, ozone causes a fast activation of secondary metabolism, particularly of the phenylpropane metabolism (Box 1.2.6). At severe stress when damage becomes visible the antioxidative ascor-bate/glutathione system is likewise stimulated. Activation of secondary metabolism always precedes the occurrence of visible damage. This also applies to the apoplastic ascorbate peroxidase, pointing to an apoplastic detoxification system for ozone (and other oxidants).

Activity of some enzymes of flavonoid and stilbene metabolism is increased after 03 treatment. These are possibly the rate-limiting en zymes, because phenolics, particularly stilbenes and flavanes, accumulate as a result of the increased enzyme activities (Fig. 1.9.17). Both are powerful radical scavengers which polymerise after formation of radicals and whose polymers associate with the cell wall.

The concentration of biogenic amines (see Box 1.5.6), e.g. tyramine and its conjugates (peptides), with cinnamic acid is also increased. They are particularly potent radical scavengers, almost as effective as the ascorbate system.

At higher doses of 03 these defence systems are not sufficiently strong, as is shown by increased ethylene formation and premature senescence and shedding of the leaves. Expression of several typical senescence-associated genes was shown in Arabidopsis plants after fumigation with ozone, including aminocyclopropane-1-carboxylic acid synthase which is responsible for the production of ethylene (Miller et al. 1999; see also Box 1.4.3). Critical 03 stress as indicated by the onset of measurable damage and leading to premature senescence is one and a half to two times ambient ozone for beech during the complete vegetative period.

Ozone and Pathogen-Related Proteins In addition to enzymes of phenylpropane, glutathione and ascorbate metabolism, and the processes of senescence, the activities of which are stimulated by 03, further enzymes are known which are not only induced by ozone, but also by pathogen attacks (Table 1,9.9). They belong to the group of PR proteins (pathogenesis-re-lated proteins), e.g. hydrolases which can break down fungal cell walls and usually are expressed in context with a hypersensitive reaction (see Chap, 1,10.2.1). Expression of some of the ozone-related proteins is also induced by elici-tors, i.e. by soluble fragments of plant or fungal cell walls.

An example for the latter is coniferyl alcohol dehydrogenase which is one of the major enzymes in the biosynthesis of lignin. Upon stress by ozone, enhanced lignin formation was observed that is probably similar to stress lignin which is also typical of pressure wood and has a different chemical composition to normal lignin.

In total there are specific as well as unspecific biochemical stress responses to ozone which are, of course, related to the oxidising effects of 03. They may damage the plant or even protect it (cross protection). In many conifers, particularly the ozone-tolerant species, ozone stress

Table 1.9.9. Overview of the induced genes or stimulation of enzyme activity upon ozone stress. (Langebartels et al.

1997)

Enzyme

Tree type

Site of action

Cinnamoyl alcohol dehydrogenase

Spruce, pine

Transcription, enzyme activity

Alcohol dehydrogenase

Spruce

Transcription

Stilbene synthase

Pine

Enzyme activity, transcription

Phenylalanine ammonium lyase

Pine

Enzyme activity

Chalcone synthase

Pine

Enzyme activity

3-Hydroxymethyl-glutaryl-CoA synthase

Pine

Transcription

Extensin

Pine

Transcription

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