Exploring Endophytic Colonization Mechanisms of the Nitrogen-fixing Diazotroph Azoarcus sp. BH72 in Rice
X. CHEN (1) (1) University of Bremen, Germany

Azoarcus sp. BH72 (Azo) is a N2-fixing endophyte isolated from Kallar grass. It can also colonize rice roots and promote rice growth. Here the Azo-rice model was used to study the unknown mechanisms of establishing an endophytic interaction. Additionally, we set up a new pathosystem for flooded rice roots with rice blast pathogen Xanthomonas oryzae pv. oryzae (Xoo) for comparative studies. Using the 44K Agilent microarray system, rice root gene expression profiles responding to Azo and Xoo infection were analyzed, and genes with over 1.5-fold expression change were further analyzed. In case of Azo, 260 genes were up- and 263 down-regulated. Noticeably, in Xoo infection, there are over twice as many of up-regulated genes (543) and less than half of down-regulated ones as in Azo case (116). Moreover, the up-regulated genes upon Xoo infection showed much higher overall fold change, and the down-regulated genes in Azo case were more strongly down-regulated. Functional analysis of the differently regulated genes revealed that Azo was able to induce a complete defense machinery even as an endophyte. Genes coding for PRRs, MAPKs, R proteins, WRKY proteins, phytoalexins, etc. were induced. In all these categories, Xoo induced a much stronger reaction. Further related to plant defense hormone regulations, JA synthesis and downstream genes were both induced in Xoo and Azo infected rice roots; however, SA synthesis and downstream genes were up-regulated upon Xoo infection and those of ethylene were down-regulated. The induced JA pathway against Azo colonization was further confirmed with a JA synthesis deficient mutant cpm2 and Q-PCR of JA marker genes in a timeline experiment. Interestingly, rice mutants deficient in SA pathway, down-regulated in npr1 expression by RNAi, were more susceptible to Xoo infection in the leaves in a previous study. However, in our root infection experiment, the Xoo infection rate was not different to the wild type rice.

Abstract Number: P2-17
Session Type: Poster