<i>Frankia</i> Root Nodulation Alters the Molecular Defence Responses Against <i>Phytophthora alni</i> Infection in <i>Alnus glutinosa</i>

ABSTRACT The expression of mechanisms involved in plant stress responses has recently been described during the interactions between Frankia and Alnus glutinosa . In the present study, we sought to investigate the ability of A. glutinosa to activate its defence mechanisms when infected by the pathogen Phytophthora alni , specifically in relation to the presence of Frankia in the root tissues. We report that nodulated plants with Frankia exhibit distinct immune responses when exposed to P. alni infection. This study is based on a comprehensive multi‐omics approach, incorporating transcriptomic, proteomic and hormone analyses to better understand the molecular dynamics involved. Our findings reveal that A. glutinosa plants nodulated with Frankia trigger specific defence mechanisms within their roots in response to P. alni infection, compared to non‐nodulated plants. Additionally, these plants also initiate a more pronounced widespread defence reaction, highlighting the intricate interactions between the host plant, its symbiotic partner and the pathogen. Interestingly, the results suggest that the presence of Frankia in the root tissues influences how A. glutinosa regulates its immune system, especially in the context of P. alni infection. Furthermore, we propose that A. glutinosa plants may employ defence mechanisms not only to fight the pathogen but also to control the growth of their symbiont.