Function of microbial effectors in the establishment of beneficial plant symbioses

Abstract

The endophytic root colonising fungus Serendipita indica induces growth, as well as biotic and abiotic stress resilience in a broad range of host plants. Similar to plant pathogens, the symbiont needs to overcome host immunity to colonise plants. Plant-colonising microbes use, among others, small, secreted proteins termed effectors to alter plant signalling, including hormone pathways, to facilitate host colonisation by pathogens, as well as symbionts. However, S. indica is known to alter hormone signalling toward the increase of host fitness. This project was aimed at analysing whether S. indicaeffectors have additional function in modulation of signalling pathways that confer host benefits. The results confirmed that identified S. indica effector candidates (SIECs) are indeed secreted proteins. SIEC-plant protein interaction analyses identified the plant proteins targeted by SIECs. Subsequent comparative interactome analyses revealed common targets with pathogen effectors and unique ones for SIECs. These evaluations already revealed a pronounced SIEC targeting of proteins in host hormone signalling and metabolism. Functional analyses of SIECs in protoplasts discovered highly specific effector alterations of different hormone pathways. These could be confirmed by phenotyping root traits of plants expressing SIECs. Further, it was shown that the effectors affecting hormone signalling could often increase plant growth. Therefore, effectors are probably part of previously unknown mechanisms for beneficial effects in connection with hormone signalling modulation by symbionts. To advance our understanding of symbiont increased host fitness under stress, protoplast assays revealed effectors that modulated stress signalling and suggest a connection between stress and hormone signalling function of SIECs. Moreover, protein network analyses uncovered SIEC targeting of plant proteins with known functions in biotic and abiotic stress response, which implies that effector function is carried out through specific target interaction. Consistent with this, 35S::SIEC plants were more tolerant to abiotic stresses and increased resistance to pathogen infection. The results indicate that effector proteins are part of molecular processes used by symbionts to increase plant fitness. Combining the SIEC-plant protein interactome and functional protoplast data, the project investigated to what extent SIECs can serve as tools to inform highly interconnected plant protein networks. Effector-informed functional analyses of SIEC target mutants proved to be highly successful in uncovering novel plant protein functions and proteins that act as connection points between hormone subnetworks. Taken together, this project uncovered the function of symbiont effectors in activating host benefits, which at least partially discloses the previously unknown molecular nature of underlying processes. In addition, the symbiont effector data represent a valuable resource for informing plant networks and signalling pathways that are essential for plants to adapt to environmental stresses.