Similarities and Differences in Global Gene Expression Profiles between Herbicide and Pathogen-Induced PSII Inhibition

Abstract

Plant pathogens, and photosynthesis inhibiting herbicides, can both damage photosystem II (PSII), causing it to be highly sensitive to damage by light energy, and to release high levels of reactive oxygen species (ROS). This photoinhibition of PSII could possibly be the source of the strong oxidative burst associated with the pathogeninduced, hypersensitive defense response (HR). To examine a possible mechanism of how the HR-associated ROS burst could originate from PSII inhibition, we compared the transcriptome responses in soybean undergoing photoinhibition induced by HR, to soybean undergoing photoinhibition induced by the herbicide bentazon, which specially stops PSII electron flow by preventing QB from binding to D1. Most genes shared similar expression patterns between HR and bentazon treatments; however, interesting differences were also observed. The most striking differences were seen with genes related to photosynthesis, where these genes were uniformly down regulated in HR, but were mostly up in response to bentazon. Another interesting difference was seen in genes of the phenylpropanoid pathway. These defense-related genes were mostly down or non-responsive to bentazon, but were generally induced in response to pathogen-induced HR, showing that soybeans activate the phenylpropanoidbased phytoalexins independent of PSII inhibition. We conclude that the PSII inhibition occurring during the HR is not being triggered simply by the inhibition of electron flow through the photosystem centers. Instead, it is more likely that the initial triggers of the HR halt the repair of damaged PSII which leads to enhancing photoinhibition and contributing the rapid production of ROS, sealing the fate of cells undergoing HR; and the triggers independently induce specific aspects of defenses such as the phenylpropanoid pathway.