Native, TALE-independent transcriptional activation of the rice executor gene Xa23 is regulated via histone acetylation during zygote development

Transcription activator-like effectors (TALEs) of the bacterial pathogen Xanthomonas are known to activate the transcription of so-called “executor” (E) genes. The resulting E proteins induce host cell death and inhibit the proliferation of the biotrophic pathogen. Until recently, transcripts encoding E proteins were detected only upon extrinsic stimulus, i.e., TALE-dependent transcriptional activation in infected host tissues. Due to the absence of evidence regarding the roles of E genes outside the context of immunity, E genes were considered solely as resistance genes. Here, we report that the native, TALE-independent expression of the rice Xa23 — one of the most well-studied E genes — is restricted to a specific cell type and developmental stage, i.e., zygotes 4–6 hours after fusion of gametes in vitro. Additionally, we activated Xa23 transcription in non-fertilized rice egg cells by treating them with a histone deacetylase inhibitor, suggesting that epigenetic modifications inhibit the native Xa23 expression. We identified native Xa23 transcriptional start sites and potential cis-regulatory elements. Thus, our pioneering work at the intersection of plant-microbe interaction and developmental biology provides a foundation for elucidating the native function of Xa23 beyond its known role in immunity and for deciphering the regulatory mechanisms of cell death control.