Gene expression profiling of systemically wound-induced defenses in hybrid poplar

As part of an ongoing effort to identify genes involved in poplar defense responses, and to provide a resource for comparative analysis of woody and non-woody plant defense, we generated expressed sequence tags (ESTs) from a library constructed from systemically wounded leaves of hybrid poplar (Populus trichocarpa x P. deltoides). Partial sequences were obtained from the 5' ends of 928 individual cDNAs, which could be grouped into 565 non-overlapping sequences. Of these, 447 sequences were singletons, while the remainder fell into 118 clusters containing up to 17 partially overlapping ESTs. Approximately 81% of the EST sequences showed similarity to previously described sequences in public databases. Of these, the distribution of gene functions within the EST set indicated that approximately 11% of the ESTs encode proteins potentially involved in defense or secondary metabolism, while photosynthesis and primary metabolism accounted for 45% of the expressed genes. Two types of defense proteins, Kunitz trypsin inhibitors and chitinases, were found among the ten most abundant ESTs, indicating the significant impact of wounding on the leaf transcriptome and suggesting that these functions are important for hybrid poplar defense. In the course of this work, three new wound-inducible Kunitz trypsin inhibitor-like genes and two new chitinase-like genes were characterized. A suite of other systemically wound-induced genes were identified using northern and macroarray analysis, indicating diversity and multiplicity in the induced defense response. Overall, we demonstrate that defense-related genes of hybrid poplar have a variety of functions, and show remarkably diverse expression patterns upon wounding.