AUTHOR=Chen Xi , Marszałkowska Marta , Reinhold-Hurek Barbara 

TITLE=Jasmonic Acid, Not Salicyclic Acid Restricts Endophytic Root Colonization of Rice

JOURNAL=Frontiers in Plant Science

VOLUME=10

YEAR=2020

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2019.01758

DOI=10.3389/fpls.2019.01758

ISSN=1664-462X

ABSTRACT=<p>Research on the interaction between the non-nodule-forming bacterial endophytes and their host plants is still in its infancy. Especially the understanding of plant control mechanisms which govern endophytic colonization is very limited. The current study sets out to determine which hormonal signaling pathway controls endophytic colonization in rice, and whether the mechanisms deviate for a pathogen. The endophyte <italic>Azoarcus olearius</italic> BH72—rice model was used to investigate root responses to endophytes in comparison to the recently established pathosystem of rice blight <italic>Xanthomonas oryzae</italic> pv. <italic>oryzae</italic> PXO99 (<italic>Xoo</italic>) in flooded roots. In the rice root transcriptome, 523 or 664 genes were found to be differentially expressed in response to <italic>Azoarcus</italic> or <italic>Xoo</italic> colonization, respectively; however, the response was drastically different, with only 6% of the differentially expressed genes (DEGs) overlapping. Overall, <italic>Xoo</italic> infection induced a much stronger defense reaction than <italic>Azoarcus</italic> colonization, with the latter leading to down-regulation of many defense related DEGs. Endophyte-induced DEGs encoded several enzymes involved in phytoalexin biosynthesis, ROS (reactive oxygen species) production, or pathogenesis-related (PR) proteins. Among putative plant markers related to signal transduction pathways modulated exclusively during <italic>Azoarcus</italic> colonization, none overlapped with previously published DEGs identified for another rice endophyte, <italic>Azospirillum</italic> sp. B510. This suggests a large variation in responses of individual genotypic combinations. Interestingly, the DEGs related to jasmonate (JA) signaling pathway were found to be consistently activated by both beneficial endophytes. In contrast, the salicylate (SA) pathway was activated only in roots infected by the pathogen. To determine the impact of SA and JA production on root colonization by the endophyte and the pathogen, rice mutants with altered hormonal responses were employed: mutant <italic>cpm2</italic> deficient in jasmonate synthesis, and RNA interference (RNAi) knockdown lines of <italic>NPR1</italic> decreased in salicylic acid-mediated defense responses (<italic>NPR1-kd</italic>). Only in <italic>cpm2</italic>, endophytic colonization of <italic>Azoarcus</italic> was significantly increased, while <italic>Xoo</italic> colonization was not affected. Surprisingly, <italic>NPR1-kd</italic> lines showed slightly decreased colonization by <italic>Xoo</italic>, contrary to published results for leaves. These outcomes suggest that JA but not SA signaling is involved in controlling the <italic>Azoarcus</italic> endophyte density in roots and can restrict internal root colonization, thereby shaping the beneficial root microbiome.</p>