EXPRESSION OF A FLAX ALLENE OXIDE SYNTHASE CDNA LEADS TO INCREASED ENDOGENOUS JASMONIC ACID (JA) LEVELS IN TRANSGENIC POTATO PLANTS BUT NOTTO A CORRESPONDING ACTIVATION OF JA-RESPONDING GENES

Both jasmonic acid (JA) and its methyl ester, methyl jasmonate (MeJA), are thought to be significant components of the signaling pathway reg ulating the expression of plant defense genes in response to various s tresses, JA and MeJA are plant lipid derivatives synthesized from alph a-linolenic acid by a lipoxygenase-mediated oxygenation leading to 13- hydroperoxylinolenic acid, which is subsequently transformed by the ac tion of allene oxide synthase (AOS) and additional modification steps. AOS converts lipoxygenase-derived fatty acid hydroperoxide to allene epoxide, which is the precursor for JA formation. Overexpression of fl ax AOS cDNA under the regulation of the cauliflower mosaic virus 35S p romoter in transgenic potato plants led to an increase in the endogeno us level of JA. Transgenic plants had six- to 12-fold higher levels of JA than the nontransformed plants. Increased levels of JA have been o bserved when potato and tomato plants are mechanically wounded, Under these conditions, the proteinase inhibitor II (pin2) genes are express ed in the leaves. Despite the fact that the transgenic plants had leve ls of JA similar to those found in nontransgenic wounded plants, pin2 genes were not constitutively expressed in the leaves of these plants. Transgenic plants with increased levels of JA did not show changes in water state or in the expression of water stress-responsive genes. Fu rthermore, the transgenic plants overexpressing the flax AOS gene, and containing elevated levels of JA, responded to wounding or water stre ss by a further increase in JA and by activating the expression of eit her wound- or water stress-inducible genes. Protein gel blot analysis demonstrated that the flax-derived AOS protein accumulated in the chlo roplasts of the transgenic plants.