Monogalactosyldiacylglycerol deficiency affects jasmonic acid biosynthesis and defense responses to insect herbivores in Nicotiana tobacum

Jasmonic acid (JA) plays critical roles in plant development and defense. Linolenic acid (18:3) and hexadecatrienoic acid (16:3) released from chloroplast lipids are now known to be precursors for JA biosynthesis, but the relationship between chloroplast lipids, especially galactolipids and JA biosynthesis still remains unclear. Here, this question was addressed by characterizing the transgenic tobacco plants, which had reduction in monogalactosyldiacylglycerol (MGDG) and trienoic fatty acids owing to the fact that MGDG synthase activity was down-regulated by using RNA interference technology. In response to wounding, the transgenic plants produced lower levels of JA than wild-type plants. Moreover, the expression of genes encoding lipoxygenase (LOX1), allene oxide cyclase (AOC), hydroperoxide lyase (HPL) and proteinase inhibitor (PI-I and PI-II) was strongly activated by mechanical wounding in wild-type plants but was diminished in transgenic plants. In addition, the transgenic plants were shown to be more susceptible to attack by Helicoverpa armigera larvae. Treatment of transgenic plants with methyl jasmonate restored resistance to H. armigera and expression of HPL, PI-I and PI-II. Our results suggest that MGDG plays important roles as source of 18:3 and 16:3 in JA biosynthesis and JA-mediated defense responses to insect herbivores in tobacco.