Cell Host Microbe http://doi.org/xmm (2014)

Microorganisms are recognized by plants using membrane receptors sensitive to specific microbe-associated molecular patterns (MAMPs). This first line of defence produces multiple outputs, including the activation of a central MAP kinase cascade, and induces a massive transcriptional reprogramming.

Ping He, of Texas A&M University, and colleagues have found that during this response, two antagonistic pathways modulate RNA polymerase II (RNAPII), a fundamental component of eukaryotic transcription. A genetic screen in Arabidopsis identified the phosphatase CPL3 as a negative regulator of the immune signalling network. CPL3 dephosphorylates residues on the long C-terminal domain (CTD) of RNAPII's largest subunit, altering the post-translational ‘CTD code’, which controls the recruitment of accessory proteins to orchestrate transcription. Although RNAPII is involved in global transcription, mutations in CPL3 predominantly affect the expression of defence-related genes, making mutant plants more resistant to pathogens. The MAMP flg22 induces phosphorylation of CTD, suggesting the presence of an additional antagonistic pathway. Ping He and colleagues found two cyclin-dependent kinases that also counteract the effects of CPL3. These are substrates of MAP kinases previously implicated in plant immunity.

This work establishes RNAPII phosphorylation as a dynamic regulator of gene expression in the plant's response to microorganisms.