Anticipating endoplasmic reticulum stress: A novel early response before pathogenesis-related gene induction

When it is attacked by a pathogen, a plant produces a range of defense-rela ted proteins. Many of these are synthesized by the rough endoplasmic reticu lum (RER) to be secreted from the cell or deposited in vacuoles. Genes enco ding endoplasmic reticulum (ER)-resident chaperones, such as the lumenal bi nding protein (BiP), are also induced under these conditions. Here, we show that BiP induction occurs systemically throughout the plant. Furthermore, this induction occurs rapidly and precedes expression of genes encoding pat hogenesis-related (PR) proteins. The underlying signal transduction pathway was shown to be independent of the signaling molecule salicylic acid and t he unfolded protein response pathway. In addition, BiP induction was indepe ndent of PR gene induction. Overproduction of BiP alone was not sufficient to cause induction of PR gene expression; however, limiting the amount of B iP in the ER lumen via superimposed ER stress inhibited the induction of PR gene expression. We propose that the induction of BiP expression during pl ant-pathogen interactions is required as an early response to support PR pr otein synthesis on the RER and that a novel signal transduction pathway exi sts to trigger this rapid response.