The role of 16C monounsaturated fatty acids in cis-3-hexenal production and plant response to stress

In plants, many fatty acids or fatty acid-derived molecules have been found to play important roles in the plant defense response. In this thesis, an insect acyl-CoA Δ11 desaturase gene and the yeast acyl-CoA Δ9 desaturase gene were introduced into tobacco (Nicotiana tabacum). The introduction of these genes in tobacco plants caused changes in fatty acid profiles. The fatty acids that increased the most in these transgenic plants are the products formed by the transgenic desaturases they expressed, viz. palmitoleic acid (16:1Δ9) and palmitvaccenic acid (16:1Δ11). In addition, transgenic tobacco plants displayed marked increases in cis-3-hexenal, a major leaf volatile compound derived from α-linolenic acid (18:3) that is found to have antimicrobial activity. However, levels of 18:3 lipid declined slightly and the pool of free 18:3 disappeared completely in the transgenic plants. On the other hand, both kinds of transgenic plants were found to have a two-fold increase in the activity of 13-lipoxygenase (13-LOX), which catalyzes the first of two steps leading to hexenal production from 18:3. Further study showed that although 16:1Δ9 and 16:1Δ11 had no direct effects on 13-LOX activity in vitro, cis-3-hexenal production increased in tobacco leaves treated with these fatty acids, suggesting that they may act in vivo by stimulating 13-LOX gene expression.; Transgenic plants were also found to senesce more rapidly than wild-type plants in response to the stress agent McJA. Fatty acid analysis showed there were significantly increased levels of 16:1 and 18:3, two fatty acids that were found to induce senescence in the detached leaf disc assay, in both wild-type and transgenic plants after McJA treatment. Transgenic plants have a much higher level of 16:1 than wild-type plants, and 16:1 was found to induce 13-LOX activity. The enhanced 13-LOX can then more efficiently convert the increased level of 18:3 in McJA-treated plants into hydroperoxide and hexenal. Hydroperoxide and hexenal are known to cause cell death and chlorophyll loss. Therefore, the transgenic plants exhibited an accelerated rate of senescence than wild-type plants after McJA treatment.