Ethylene negatively regulates local expression of plant defense lectin genes

Griffonia simplicifolia leaf lectin II (GS-II) is a N-acetylglucosamine (Gl cNAc) binding protein, consisting of large (rGS-II) and small (sGS-II) subu nits, that is presumed to function in plant defense because both the holopr otein and the rGS-II inhibited insect growth and development in Feeding bio assays. A cDNA encoding sGS-II has been isolated and the deduced peptide ha s sequence similarity to class III chitinases. However, neither the GS-II h oloprotein nor bacterially expressed recombinant sGS-II proteins had detect able chitinase or lysozyme activities. Both rGS-II and sGS-II mRNAs accumul ated in response to methyl jasmonate (Me-JA) treatment but not after woundi ng in local leaves unless injury was followed by treatment with the ethylen e action inhibitor norbornadiene (NBD). Salicylic acid (SA) suppressed woun ding/NBD induction of GS-II transcript accumulation and ethylene inhibited Me-JA-induced expression. Apparently. defensive gene expression induced by signal transduction through the octadecanoid pathway in local leaves is sup pressed by stress induced ethylene that is produced as a consequence of wou nding. However, in leaves systemic to the wound site, rGS-II mRNA levels in creased substantially indicating that physiological levels of ethylene are insufficient to down regulate defensive gene expression away from the site of injury. It seems that G. simplicifolia has evolved a rather specialized response to herbivore attack whereby local activation of defensive gene exp ression is attenuated in order to mount a more substantial defense distal t o the site of invasion.