Decay of UPF Proteins is the Key Process for the NMD-Mediated Plant Immunity

Nonsense-mediated mRNA decay (NMD), an mRNA quality control process, has been implicated in plant immunity. A subet of transcripts of the Arabidopsis resistance (R) genes are reportedly upregulated during bacterial infection due to a decrease in NMD efficiency. Here, we report that 81.2% and 65.1% of fully spliced natural TIR-NBS-LRR (TNL) and CC-NBS-LRR (CNL) transcripts, respectively, retain characteristics of NMD regulation. The perception of bacteria by pattern recognition receptors (PRRs) initiates the destruction of UPF1, UPF2 and UPF3 within 30 minutes of inoculation via the independent ubiquitination of UPF1 and UPF3 and the 26S proteasome pathway, and subsequently, NMD-sensitive TNL and CNL transcript levels increase. Induction of UPF1 and UPF3 ubiquitinations was delayed specifically in mpk3 or mpk6 at an early immune response. Our findings demonstrate how NMD is the control node through which PRRs can fine-tune R transcript levels to reduce fitness costs and achieve effective immunity.