JA phytohormone signaling. Two proteins which also showed increased expression in egg induced elms are patatin like protein and heat shock protein 81. Patatin proteins are related to the major storage protein known from potato tubers and have the enzymatic activity of phospholipases and re lease fatty acids from membrane lipids. These proteins have been identified in many plant species and were shown to be involved inter alia in pathogen triggered cell death and to be induced by wound stimuli. They might also be associated with the herbivore induced defense pathway via the mobilization of lino lenic acid from the cell membrane, which activates the octadecanoid pathway and finally leads to the synthesis Brefeldin_A of JA and other oxylipins.

HSPs meanwhile, are molecular chaperones which can modulate the folding of a variety of other specific target proteins involved, for in stance, in cell cycle control and signal transduction. HSP 81 belongs to the HSP 90 family of stress proteins, which are known to influence several resistance gene signaling pathways, the inhibition of which lead to decreased resistance to pathogens and increased resist ance to insect herbivores. Thus, a suite of defense response genes, that work together to protect the plant from insect attack appears to be coordinately activated by egg laying on elm. Transcripts of jasmonic acid biosynthesis genes are present in high abundance JA has been determined to be an integral part of the plant signal transduction pathway, which leads to the ac tivation of direct and indirect defenses against herbivor ous insects.

Decreased resistance to herbivores and enhanced egg laying activity has been observed in tomato mutants with impaired JA biosyn thesis. Moreover, transcriptome analyses using microarrays indicated that a large portion of herbivory induced responses are mediated through the JA pathway. In egg induced elms, we found high levels of tran scripts of genes encoding key enzymes involved in the biosynthesis of JA including lipoxygenase and allene oxide synthase. Our findings support the expected in volvement of the octadecanoid signal transduction path way in egg induced plant defense, as the treatment of elms with MeJA leads to the release of volatiles that are attractive to egg parasitoids. Genes involved in JA bio synthesis were also upregulated after pierid eggs laying on A. thaliana.

However, we also found enhanced transcript abundances after egg laying in comparison to the other treatments for jasmonate ZIM domain pro teins, which are known to repress JA responsive genes. Auxin might be another phytohormone involved in elm responses to eggs, and transcripts of both positive and negative regulators of auxin signal transduction, an auxin receptor and an auxin repressed protein, were also found. After JA treatment of poplar, down regulation of genes involved in auxin signaling was observed. Auxin interferes with JA and SA signaling, and the negative regulation of auxin is supposed to mediate