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The Effect of Quorum Sensing Signals on Nodulation of Medicago Truncatula

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The Effect of Quorum Sensing Signals on Nodulation of Medicago Truncatula The effect of quorum sensing signals on nodulation of Medicago truncatula Thesis submitted for the degree of Doctor of Philosophy At the Australian National University Research School of Biology By Debora Fabiola Véliz Vallejos October, 2016 ii Declaration This thesis is my own work and does not contain any results that have been generated by people other than myself, except where reference and acknowledgment has been made. These results have not been previously submitted by me for the purpose of obtaining a degree or diploma at any university or tertiary education institution. Debora Fabiola Véliz Vallejos October, 2016 iii "Our task must be to free ourselves... by widening our circle of compassion to embrace all living creatures and the whole of nature and its beauty." Albert Einstein iv Acknowledgments First of all I would like to thank my supervisor, Uli for her support, comprehension and continual guidance throughout my PhD. Her expertise, enthusiasm and advice helped me to bring this project into completion, particularly at difficult times during my project. In addition, I would like to extent my gratitude to my advisors Susanne von Caemmerer, John Evans and Michelle Watt, who supported me, especially at the beginning of my project. This project would not have been possible without the financial support received by the Chilean government through Becas Chile. I am particularly thankful to Giel for all the technical help and input in co-supervising my PhD. And also for his unconditional friendship and encouragement which ‘saved my life’ many times. I also extent this gratitude to his family, as they have also been supportive during my PhD. Further, I am thankful to current and previous members of Uli’s lab, especially Jason, who helped with the metabolomics assay for flavonoid quantification. I also thank Thy Truong who offered support with the technical aspects of the metabolomics assay. I would like to thank Aki for his help characterizing the Medicago truncatula root plant-associated bacteria, and for his friendship and advice. I thank the summer students Yuan (Hope) and Afira for their help with the qRT-PCR, flavonoids quantification and nodulation phenotypic experiments. I would like to extent my gratitude to RSB friends for their encouragement and support including Nur, Viri, Ivonne, Jordi, Richa, Vinson and Claudia thanks for the laughter and encouragement and support. I would like to thank all my friends who have made my stay in Canberra enjoyable and easier in one way or another: Vani, Cecilia, Liv, Monica, David and Constance, thanks for being part of my life in the good and the bad moments. Thank you for your unconditional help, advice and support. I am grateful for my friend Lauren who has been with and for me at every time no matter what as well as I would like to thank Gavin, Markus and Marcela who have supported me with their friendship from overseas in different stages of my PhD. I would like to thank my parents Angelica and Javier who with patience, dedication and effort dreamed for me to finish this project. I also thank to my sister Aly and her family for being “my number one fan” and always cheering me up. v Finally, I thank to my best friend and husband Owen Ellis, who has engouraged and supported me with his unconditional love. Thank you my sunshine! Last but not least, I would like to thank God for His infinite love and kindness to me. Without Him and His ever help and encouragement, I would never have been able to neither start nor finish this work. vi Abstract N-acyl homoserine lactones (AHLs) act as quorum sensing signals that regulate cell- density dependent behaviours in many gram-negative bacteria, in particular those important for plant-microbe interactions. AHLs can also be recognised by plants, and this may influence their interactions with bacteria. This thesis tested whether the exposure to AHLs affects the nodule-forming, nitrogen-fixing symbiosis between legume hosts and rhizobia. It used the legume, Medicago truncatula, and its symbiont, Sinorhizobium meliloti, as this model symbiosis has been well characterised on a molecular and cellular basis. In addition, previous studies have characterised the identities and roles of AHLs from S. meliloti during nodulation. First, protocols were established to grow M. truncatula plants under conditions conducive to symbiosis and, at the same time, to minimise growth of bacteria growing in and on roots. This was important as bacteria can destroy AHLs and could thus interfere with externally applied AHLs. M. truncatula was found to harbour culturable bacteria that were derived from the inside of the seed coat and were recalcitrant to surface sterilisation. A protocol using an antibiotic treatment of the seeds to minimise bacterial growth, and an axenic system growing M. truncatula seedlings on large agar plates was chosen for subsequent experiment. M. truncatula seedlings were exposed to a range of synthetic AHLs derived either from its specific symbiont, S. meliloti, or from the potential pathogens, Pseudomonas aeruginosa and Agrobacterium vitis. Increased numbers of nodules formed on root systems treated with the S. meliloti-specific AHL, 3-oxo-C14-homoserine lactone (HSL), while the other AHLs did not result in significant changes to nodule numbers. The increase in nodule numbers was dependent on AHL concentrations and was repeatedly observed at concentrations of 1 M and above. No evidence for altered nodule invasion by the rhizobia was found. 3-oxo-C14-HSL ‘primed’ Medicago plants before inoculation with rhizobia, indicating that the increase in nodule numbers occurs at early stages and as a direct effect on the plant and not on the rhizobia. Increased nodule numbers following 3-oxo-C14-HSL lactone treatment were not under control of autoregulation of nodulation and were still observed in the autoregulation mutant, sunn4 (super numeric nodules4). However, increases in nodule numbers by 3-oxo-C14-HSL were not found in the ethylene-insensitive sickle mutant. Gene expression analysis further suggested that vii this AHL affects the expression of ethylene-related genes during nodulation. It was concluded that plant perception of the S. meliloti-specific 3-oxo-C14-HSL influences nodule numbers in M. truncatula via an ethylene-dependent, but autoregulation- independent mechanism. A comparison of M. truncatula with M. sativa (alfalfa), Trifolium repens (white clover) and Lotus japonicus (Lotus) showed that the observed effects of AHLs on nodule numbers, at least at the concentration chosen, were specific to M. truncatula, despite M. sativa nodulating with the same symbiont. In M. truncatula, the effects of AHLs were specific for an increase in nodule numbers, but not lateral root numbers or root length. This result suggests a very specific effect of AHLs on nodulation, possibly via modulation of ethylene-controlled infection, but not on general root developmental processes. During the investigation of protocols to eliminate bacterial contamination, it was discovered that nodulation phenotypes in response to AHL exposure strongly depended on the presence of plant-associated bacteria. Therefore, the composition and possible role of the M. truncatula-associated microbiome was further investigated. High throughput sequencing showed that the antibiotic treatment significantly reduced the presence and composition of the microbiome. Interestingly, application of 3-oxo-C14- HSL also significantly altered the microbiome, but this effect was very specific for bacteria belonging to the genus Pantoea. Only in the absence, but not in the presence of the majority of the plant microbiome, did M. truncatula show increased nodulation in response to 3-oxo-C14-HSL, and this was associated with increased expression of early nodulation genes. These results suggest that the bacterial community of Medicago affects nodulation responses towards AHLs, particularly towards to 3-oxo-C14-HSL, likely by interfering with AHL stability, perception or plant responses. In summary, this thesis showed that plant perception of AHLs alters symbiosis-specific phenotypes, suggesting that AHLs are not only important to regulate bacterial behaviours during nodulation, but also that the plant has evolved mechanisms to respond to specific AHLs from its symbiont, likely by reducing ethylene signalling or synthesis to enhance the number of nodules. viii Table of contents Table of Contents Declaration ....................................................................................................................... iii Acknowledgments ............................................................................................................. v Abstract ........................................................................................................................... vii Table of contents .............................................................................................................. ix List of Figures ................................................................................................................ xiii List of Tables................................................................................................................. xvii List of abbreviations ..................................................................................................... xviii Chapter 1 General Introduction ................................................................................... 1 1.1 Introduction .......................................................................................................
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