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XANTHOMONAS LIVING THE SWEET LIFE: CHARACTERIZATION OF A PLANT SUGAR TRANSPORTER FAMILY AND ITS ROLE IN BACTERIAL BLIGHT OF COTTON SUSCEPTIBILITY A Dissertation by KEVIN LAMARR COX JR. Submitted to the Office of Graduate and Professional Studies of Texas A&M University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Chair of Committee, Libo Shan Committee Members, Michael Kolomiets Ping He Tom Isakeit Head of Department, Leland S. Pierson December 2018 Major Subject: Plant Pathology Copyright 2018 Kevin Lamarr Cox Jr. ABSTRACT Upland cotton (Gossypium hirsutum) is an important cash crop that is constantly pressured by various biotic stresses. Bacterial blight of cotton (BBC) caused by Xanthomonas citri subsp. malvacearum (Xcm) is a destructive disease that limits cotton production worldwide. Xcm causes disease by injecting transcription activator-like (TAL) effectors in plant cell to directly induce the expression of host genes. Despite having at least 10 TAL effectors cloned from Xcm, there have yet to be any susceptibility or resistance proteins identified and characterized. The rapid re-emergence of this disease in the last few years in the US calls for pressing needs to decipher the molecular mechanisms of BBC in order to provide resources to combat this pathogen. In this dissertation study, I identified a plant sugar transporter (GhSWEET) gene family in cotton that plays an important role in BBC susceptibility. To bypass the traditional map-based cloning to identify susceptibility targets, a combination of genome-wide transcriptome profiling and computational prediction of TAL effector binding sites, coupled with functional characterization of candidate genes was used to identify GhSWEET10 as a key susceptibility target of Avrb6, a major TAL effector in the strain of XcmH1005. This was the first reported susceptibility gene in cotton. Additionally, my data suggest that activating different GhSWEETs by a repertoire of TAL effectors is a common, but important, strategy for Xcm to cause infection, as current Xcm field isolates activated two different GhSWEETs, GhSWEET14a and GhSWEET14b. Further functional analysis on these two SWEETs revealed that activating ii expression of GhSWEET14a and GhSWEET14b was correlated with disease susceptibility and genetically required for Xcm to cause infection on cotton. Finally, by using genome editing and artificial microRNAs, I was able to develop strategies that could potentially be used to biologically control of BBC. In conclusion, my dissertation revealed the molecular mechanism of BBC and provided a set of toolkits that could be used in the future to develop BBC-tolerant cotton cultivars. iii DEDICATION I dedicate this dissertation to my lovely wife, Charneva Cox, for motivating me to go to graduate school, showing patience, and being my number one supporter, and my daughter, Kyra Cox, for allowing me to interrupt our fun activities at times in order to work in the lab. I also dedicate this dissertation to my father and mother, Kevin Sr. and Gloria Cox, and to my brother, Christopher Cox, for their encouragement and support. Finally, I dedicate this dissertation to my extended family and friends that gave me their support throughout my degree program. iv ACKNOWLEDGEMENTS I would like to sincerely thank my committee chair, Dr. Libo Shan, and her collaborator, Dr. Ping He, for allowing me to conduct my Ph.D. studies in their labs. I’m deeply grateful for their patience, constructive criticisms, and guidance during my time as a graduate student. Their mentoring had a dramatic impact on my development as a scientist. The experiences and opportunities I have encountered would not have been possible without the two of them. I deeply appreciate for all they have done for me and I look forward to applying the mentoring I’ve received from them to build my career. I would like to thank my committee members, Dr. Mike Kolomiets and Dr. Tom Isakeit, for their guidance and support throughout the course of this research. I appreciated the interactions and scientific discussions with them, as they provided valuable inputs to significantly improve my research. I greatly appreciate the collaboration of my work with Dr. Adam Bogdanove and members of his lab. This collaboration has been on-going for nearly my entire tenure as a graduate student. It has been nothing but fruitful and rewarding, as he and his lab conducted key experiments and provided useful inputs to help my projects. I couldn’t have some experiments in a timely manner without them. Many thanks to the current and previous lab members of Dr. Shan’s and Dr. He’s lab that I’ve worked with during my graduate studies. They provided great mentoring and scientific discussions during my time in the lab. In addition, they provided fun memories that I will treasure forever. I would like to give a special thanks to Dr. v Fanhong Meng, who collaborated with me on the work in Chapter 2 and provided valuable mentoring that allowed me to significantly improve my lab techniques. Additional thanks to Dr. Fangjun Li, Dr. Ping Wang, and Dr. Xiyu Ma for their help on various experiments presented in this work. Thanks to the students in the PLPM department for making my graduate studies here enjoyable. I will never forget the times we had together and there are bonds that I have made with some of you that I will cherish forever. I would also like to thank the faculty and staff in the PLPM department, as well as those members of the Institute for Plant Genomics and Biotechnology Center for their helpful interactions. Many thanks to my friends that I made outside the PLPM department for their fun times and memories. Finally, thanks to my mom, dad, and brother for their continued encouragement and love, my extended family for their support, and to my wife and daughter for their patience and their love. I’m deeply grateful for all of them. vi CONTRIBUTORS AND FUNDING SOURCES Contributors This work was supervised by a dissertation committee consisting of Professor Libo Shan [advisor], Professors Michael Kolomiets and Tom Isakeit of the Department of Plant Pathology & Microbiology, and Professor Ping He of the Department of Biochemistry & Biophysics. The work in Chapter 2 was performed in collaboration with Dr. Adam Bogdanove’s lab of Cornell University and contributed by the authors listed in the publication. The XcmH strains and plasmids were contributed by Dr. Dean Gabriel of the University of Florida. The crop transformation in Chapter 3 was performed by Mr. Marco and Mrs. Mayra Molina of the Laboratory of Crop Transformation located in the Institute of Plant Genomics and Biotechnology Center. All other work conducted for the dissertation was completed by the student independently. Funding Sources Graduate study was supported by a Graduate Diversity Fellowship from Texas A&M University and by a USDA-NIFA AFRI Pre-doctoral Fellowship under Grant Number 2017-67011-26060. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of USDA-NIFA. vii TABLE OF CONTENTS Page ABSTRACT .......................................................................................................................ii DEDICATION .................................................................................................................. iv ACKNOWLEDGEMENTS ............................................................................................... v CONTRIBUTORS AND FUNDING SOURCES ............................................................vii TABLE OF CONTENTS ............................................................................................... viii LIST OF FIGURES ........................................................................................................... xi LIST OF TABLES ...........................................................................................................xii CHAPTER I INTRODUCTION AND LITERATURE REVIEW: RETURN OF AN OLD FOE: RE-EMERGENCE OF BACTERIAL BLIGHT OF COTTON ...................... 1 Overview ........................................................................................................................ 1 Introduction .................................................................................................................... 1 Bacterial blight of cotton ................................................................................................ 2 Targeting veins to hijack the water source of plants ...................................................... 7 Increase in humidity and temperature plays a role in disease severity .......................... 9 Molecular strategies to breed resistant cultivars .......................................................... 10 Conclusions .................................................................................................................. 11 CHAPTER II TAL EFFECTOR DRIVEN INDUCTION OF A SWEET GENE CONFERS SUSCEPTIBILITY TO BACTERIAL BLIGHT OF COTTON ................... 13 Overview ...................................................................................................................... 13 Introduction .................................................................................................................. 14 Results .......................................................................................................................... 17 Genome and TAL effector sequences of Xcm H1005 and N1003 ........................... 17 Candidate