Cell-specific RNA profiling reveals host genes expressed in Arabidopsis cells haustoriated by downy mildew

The downy mildew oomycete Hyaloperonospora arabidopsidis, an obligate filamentous pathogen, infects Arabidopsis (Arabidopsis thaliana) by forming structures called haustoria inside host cells. Previous transcriptome analyses have revealed host genes are specifically induced during infection; however, RNA profiling from whole infected tissues may fail to capture key transcriptional events occurring exclusively in haustoriated host cells where the pathogen injects virulence effectors to modulate host immunity. To determine interactions between Arabidopsis and H. arabidopsidis at the cellular level, we devised a translating ribosome affinity purification (TRAP) system using two high-affinity binding proteins, colicin E9 and Im9 (immunity protein of colicin E9), applicable to pathogen-responsive promoters, thus enabling haustoriated cell-specific RNA profiling. Among the host genes specifically expressed in H. arabidopsidis-haustoriated cells, we found genes that promote either susceptibility or resistance to the pathogen, providing insights into the Arabidopsis–downy mildew interaction. We propose that our protocol for profiling cell-specific transcripts will apply to several stimulus-specific contexts and other plant–pathogen interactions.