Wound-responsive gene expression in poplars

Plants have evolved elaborate biochemical defense mechanisms to protect themselves from pathogen invasion and insect herbivory. Often these biochemicals are synthesized only when the plant is under attack, and may accumulate systemically (throughout the plant) in response to a local injury. Some of the defensive molecules are low molecular weight phenolic or terpenoid compounds; others are proteins harmful to the invading pathogen or grazing herbivore. The de novo synthesis of defensive proteins is often preceded by a coordinate increase in transcription of the genes that code for these proteins. Hybrid poplars (Populus trichocarpa T. and G. × P. deltoides Marsh) respond to mechanical wounding (simulating pathogen invasion or insect chewing) by systemically accumulating specific transcripts in the unwounded upper leaves of trees whose lower leaves have been damaged. Some of the induced poplar leaf mRNAs code for chitinases, enzymes which may play a role in degrading the cell walls of pathogenic fungi and bacteria. Other wound-regulated transcripts encode proteins with amino acid sequence similarity to a family of trypsin inhibitors. Proteinase inhibitors protect plants by disrupting the digestive process in the gut of a grazing insect. By understanding the response of trees to wounding at the molecular level, it may be possible to improve disease and insect resistance by augmenting the tree's natural defenses through selective breeding and genetic engineering. Trees that possess increased levels of chitinases or proteinase inhibitors may be superior in detering pests. Genetic variability in the levels of these defensive proteins could be used to design breeding strategies for enhanced pest resistance. Additionally, pest control genes from other sources, such as the insecticidal toxin genes from Bacillus thuringiensis, could be introduced into poplar trees by genetic engineering. Transcription of the bacterial toxin genes could be regulated in an appropriately wound-inducible manner by fusing the bacterial genes to transcriptional promoters from wound-responsive poplar genes.