P-438 - Individual leaf cells respond at different speeds to pathogen signals

Plants possess a sophisticated immune system comprising pattern-triggered immunity (PTI) and effector-triggered immunity (ETI) to defend against pathogens. Recent studies describe the heterogeneous immune responses at individual cell resolution during pathogen infection; however, how individual cells reprogram their transcriptome during the early phase of ETI responses remains unclear. We have resolved the transcriptomic landscape of the early ETI response over time in Arabidopsis leaves at the single-cell level using an estradiol-inducible AvrRpt2 line. Our data revealed heterogeneous ETI responses with certain subpopulations of mesophyll and xylem parenchyma cells exhibiting accelerated immune activation. Intriguingly, in a subset of cells with accelerated response, some immune markers were already expressed. Whether this is a stochastic phenomenon or indicates pre-activation of immune genes is currently unclear. Genes encoding p</span>athogenesis-related proteins and receptor-like proteins were found to be up-regulated in these cells. Promoter-reporter lines suggested relatively broad expression of genes from this cluster, suggesting that cells with increased expression of immune markers before effector activation might be the result of stochastic activation at the level of individual genes. An important goal for future studies will be to determine whether certain cells can bias such a stochastic gene expression program.