miR159 Represses a Constitutive Pathogen Defense Response in Tobacco.

MicroR159 (miR159) regulation of expression is highly conserved in terrestrial plants; however, its functional role remains poorly understood. In Arabidopsis (), although genes are constitutively transcribed during vegetative growth, their effects are suppressed by strong and constitutive silencing by miR159. GAMYB expression occurs only if miR159 function is inhibited, which results in detrimental pleiotropic defects, questioning the purpose of the miR159- pathway. Here, miR159 function was inhibited in tobacco () and rice () using miRNA technology. Similar to observations in Arabidopsis, inhibition of miR159 in tobacco and rice resulted in pleiotropic defects including stunted growth, implying functional conservation of the miR159- pathway among angiosperms. In tobacco, transcriptome profiling revealed that genes associated with defense and programmed cell death were strongly activated, including a suite of 22 () genes that were 100- to 1,000-fold upregulated. Constitutive expression of a miR159-resistant transgene in tobacco resulted in phenotypes similar to that of tobacco and activated gene expression, verifying the dependence of the above-mentioned changes on expression. Consistent with the broad defense response, tobacco appeared immune to infection. These findings suggest that the tobacco miR159- pathway functions in the biotic defense response, which becomes activated upon miR159 inhibition. However, gene expression was not upregulated in Arabidopsis or rice when miR159 was inhibited, suggesting that miR159- pathway functional differences exist between species, or factors in addition to miR159 inhibition are required in Arabidopsis and rice to activate this broad defense response.