Oh

Sesquiterpenes

Tryptophan or non-tryptophan pathway

Ocimene Linalool

Hexenyl acetate

Homoterpenes

Dimethyl-nonatriene

Sesquiterpenes

Farnesene Caryophyllene

Homoterpenes

Dimethyl-nonatriene

Ocimene Linalool

Farnesene Caryophyllene

Fig. 1.10.9. Biosynthetic pathways of secondary plant metabolites leading to volatile compounds. Indole can be synthesised via two mechanisms: either as an intermediate during tryptophan synthesis or by a tryptophan-independent path (Frey et al. 1997). Terpenoids may be synthesised via the alt-IPP path [the alternative desoxy-xylulose-5-phosphate (DOXP) pathway] by which isopentenyl pyrophosphate is synthesised in the chloroplast. For further information on the lipoxygenase pathway, see Box 1.10.2. (After Paré and Tumlinson 1999)

nomical importance with regard to the production of secondary plant metabolites, because more carbohydrates (and nitrogen) are then available for growth. In an interesting experiment, the cost-benefit relation of jasmonate-in-duced defence was analysed (Baldwin 1998). Seed production of the annual Nicotiana attenu-ata was measured after some plants had been treated with methyl jasmonate to induce the defence reactions while others were left untreated. Both "populations" were then exposed to different intensities of herbivory by the natural spectrum of herbivores and phytophagous insects. Under herbivory pressure the (previously) induced plants produced significantly more seeds than the non-induced "unprotected" control plants. With little or no herbivory, however, the non-induced plants were superior. In nature, herbivory is the normal case, and therefore species capable of jasmonate-induced defence are favoured by selection.

Jasmonate plays the role of transmitter not only in the signal cascade after wounding, but also after infestation by pathogens. The reaction to both stimuli may be very different, not only regarding the metabolic end products: If the signal cascade is triggered by a fungal elicitor and if arachidonic acid instead of, or in addition to, linolenic acid is released, a different isozyme of the HMGR is induced, compared to the reaction in which jasmonate is the elicitor ("hmgl" vs. "hmg2"). The biological sense behind this differentiation is still unknown.

A jasmonate-independent signal pathway was identified, beside the jasmonate-dependent transduction of the wound signal (Titarenko et al. 1997). However, the jasmonate-independent signal pathway does not lead to a systemic induction, but only to a local induction of defensive genes as in the previously discussed system-in-independent signal pathway. This path is shown in Fig. 1.10.5 by the broken arrow.

It is interesting that the isoprenoids which are released function as kairomones1 and particularly attract parasitoids, which then parasitise phytophages. Ichneumon wasps are especially attracted by the volatile terpenoids (Paré and Tumlinson 1999; Kessler and Baldwin 2001). Isoprenoids thus serve as signals with very high specificity regarding the host plant of the phy-

1 Kairomones are (usually) volatile signal compounds, which are useful for the detecting organism: attractants, repellents (escape), stimulants, warning signals.

tophage, but the signal does not give much information about the type of phytophage. This is understandable, as the signal originates from the plant and not from the phytophage. It might be a kind of co-evolution that the inducible signals are produced almost exclusively during the day, when parasitoids are active. Induction of HMGR can thus be interpreted as an active defence strategy of plants against herbivory.

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