Abstract
Suppression of host innate immunity appears to be required for the establishment of symbiosis between rhizobia and host plants. In this study, we established a system that included a host plant, a bacterial pathogen and a symbiotic rhizobium to study the role of innate immunity during symbiotic interactions. A pathogenic bacterium, Pseudomonas syringae pv. tomato strain DC3000 (Pst DC3000), was shown to cause chlorosis in Medicago truncatula A17. Sinorhizobium meliloti strain Sm2011 (Sm2011) and Pst DC3000 strain alone induced similar defense responses in M. truncatula. However, when co-inoculated, Sm2011 specifically suppressed the defense responses induced by Pst DC3000, such as MAPK activation and ROS production. Inoculation with Sm2011 suppressed the transcription of defense-related genes triggered by Pst DC3000 infection, including the receptor of bacterial flagellin (FLS2), pathogenesis-related protein 10 (PR10), and the transcription factor WRKY33. Interestingly, inoculation with Pst DC3000 specifically inhibited the expression of the symbiosis marker genes nodule inception and nodulation pectate lyase and reduced the numbers of infection threads and nodules on M. truncatula A17 roots, indicating that Pst DC3000 inhibits the establishment of symbiosis in M. truncatula. In addition, defense-related genes, such as MAPK3/6, RbohC, and WRKY33, exhibited a transient increase in their expression in the early stage of symbiosis with Sm2011, but the expression dropped down to normal levels at later symbiotic stages. Our results suggest that plant innate immunity plays an antagonistic role in symbiosis by directly reducing the numbers of infection threads and nodules.
Highlights
Several genera of plants can benefit from atmospheric nitrogen fixation through symbiosis with particular microorganisms
M. truncatula could be infected by Pst DC3000 and shows disease symptoms, such as localized necrosis surrounded by diffuse chlorosis on the leaves (Supplementary Figure S1C)
We established a convenient system, to study the interplay between immunity and symbiosis, which consists of M. truncatula A17 plants, pathogenic bacterium Pst DC3000 and rhizobia
Summary
Several genera of plants can benefit from atmospheric nitrogen fixation through symbiosis with particular microorganisms. Symbiotic nitrogen fixation makes a major contribution to soil fertility and plays a critical role in sustainable agriculture It is significant, as well as challenging, to transfer the establishment of such symbiosis to non-legumes for future agriculture development (Gewin, 2010). NFs are required for early steps of legume infection and root nodule organogenesis (Denarie et al, 1996) They are recognized by the LysM domain receptor kinases NFR1 and NFR5 in Lotus japonicus and LYK3 and NFP in Medicago truncatula (Amor et al, 2003; Limpens et al, 2003; Madsen et al, 2003; Radutoiu et al, 2003) to elicit calcium spiking and to reprogram the expression of specific symbiosis genes (Oldroyd, 2013). M. truncatula NFP, initially described as a putative NF receptor, can play a role in defense against pathogens, resistance to both the oomycete Aphanomyces euteiches and the fungus Colletotrichum trifolii (Rey et al, 2013)
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