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The Interaction of Cytokinin and Strigolactone in Controlling Shoot Branching in Arabidopsis Thaliana

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The Interaction of Cytokinin and Strigolactone in controlling Shoot Branching in Arabidopsis thaliana By Neelam Chaudhary Imperial College London Department of Life Sciences November 2013 A thesis submitted for the degree of Doctor of Philosophy and the Diploma of Imperial College London 1 For my Father (Late) and Mother They struggled hard to strengthen my faith in Almighty Allah and supported me to fulfil my dreams and to achieve my goals through their constant unconditional love, encouragement and prayers. For Scientists especially Life Scientists They spare time learning new ways to understand and solve problems in hopes of paving a better future for newer generations. They are more dedicated to making solid achievements than in running after swift but synthetic happiness. 2 ABSTRACT Shoot branching is regulated by auxin, cytokinin (CK) and strigolactone (SL). Cytokinin, being the only promoter of shoot branching, is antagonistic in function to auxin and strigolactone, which inhibit shoot branching. There is a close relationship between auxin and strigolactone, mediating each other to suppress shoot branching. Strigolactone reduces auxin transport from the buds, thus arresting bud outgrowth. On the other hand, auxin increases strigolactone production to control apical dominance. Antagonistic interaction between auxin and cytokinin has been reported as auxin inhibits lateral bud outgrowth by limiting CK supply to axillary buds. Previously, it has been found that levels of tZ-type CKs are extremely low in xylem sap of strigolactone mutants of Arabidopsis and pea.The current research aimed to explore the interaction between cytokinin and strigolactone, especially the regulatory mechanisms behind these low cytokinin levels. It was hypothesized that xylem-CKs may be controlled by strigolactone-mediated regulation of AtIPT genes in Arabidopsis thaliana. For this investigation, atipt double (atipt5,7, atipt3,5 and atipt3,7) and triple (atipt3,5,7) knockout mutants in wild-type and max backgrounds were screened and characterized. The GUS promoter-reporter system was used to study regulation of AtIPT expression by strigolactone. For this purpose, AtIPTs::GUS lines were generated in max SL mutant backgrounds. Further, cytokinin levels were quantified in root and shoot tissues as well as in phloem and xylem sap of atipt mutants in wild-type and max background. Cytokinin biosynthetic genes (AtIPTs) were shown to be regulated by strigolactone and cytokinin synthesis played an important role in the phenotype of max mutants. Loss of AtIPT3 from the SL-deficient mutant, max4, resulted in reduced growth and suppression of shoot branching. However, it was not possible to knockout AtIPT3 from the SL-insensitive mutant, max2, because double mutation of AtIPT3 and MAX2 genes was unexpectedly found to be lethal. Both max2 and max4 showed upregulation of AtIPT3 in phloem and downregulation of AtIPT5 in root and shoot. Application of GR24 (SL synthetic analogue) and NAA (auxin) reversed the regulation in max4. Auxin- and strigolactone-mediated regulation of AtIPT3 and AtIPT5 in root and shoot were strictly MAX2-dependent, apart from auxin upregulation of ATIPT5 in roots, independent of strigolactone. The phloem sap showed elevated levels of the CK metabolite, iPRP, and this was correlated with high expression of AtIPT3 in phloem of max mutants. The increased transport of iPRP to roots did not increase the production of tZ- type CKs in roots of max mutants. Rather the levels of tZ-type CKs in xylem sap of max mutants were highly reduced. The results lead to the conclusionthat SL controls shoot branching partly by mediating CK biosynthesis and iPRP in phloem may function as a feedback signal to modulate translocation of tZ-type CKs through xylem. 3 ACKNOWLEDGMENT According to Goethe, “All the knowledge I possess everyone else can acquire, but my heart is all my own”. That’s why, from the core of my heart I acknowledge everyone mentioned here. I would like to express my sincere gratitude to my supervisor, Dr. Colin Turnbull, for his great assistance and ever-presencethroughout this project. His immense encouragement and help in every possible way is appreciated beyond words. I feel lucky to be supervised by such a nice and humble person, who provided not only for learning the science but also for the development of my personality. The last degree under the guidance of such a great mentor fulfils the purpose of being highly educated. I am also grateful for his constant patience with this manuscript. I would also appreciateall the support from my advisors, Dr. John Rossiter and Dr. Alexander Grabov. I extend my heartiest gratitude to the members of my lab, especially Dr.Rosa Lopez-Cobollo, for their full support and company. I am also thankful toMark Bennett for his assistance with LC-MS/MS, Ian Morris for all his help in the lab and department, Dr.Martin Selby for taking care of plants in growth rooms and Fiona May for her patience with the ordering of materials required in the project. I wish to thank Higher Education Commission, Pakistanand International Student Housefor the scholarship which made this study possible. I am really indebted to my brothers and sisters, especially Saima Farnaz for her great help in learning English grammar during my school days as well as for her invaluable advices and encouragement throughout my studies, Rubina Ghazal for her assistance in resolving computer-related problems, Farah Zaib for her humorous attitude to make me relaxed, and Amjad Javaid for his special aid to accomplish all the requirements of scholarship and travel to UK. Special thanks are due to my friends who always stand with me in the time of trial.I am really obliged to Dr. Nazia Nisar. She is truly an example of “A friend in need is a friend indeed”. I am really thankful to Durr e Shahwar for her very good company when I was feeling lonely away from my family and husband. I have no words to thank my husband, Dr. Muhammad Umer. This PhD would not have been possible without his constant support, encouragement and patience. I can never forget his efforts to facilitate me in completing my PhD, especially during thesis writing. His calm acceptance and compassionate understanding kept me going at tough times and his PhD experience gave me an inspiration to finish this most difficult task of my life. I conclude my acknowledgement by saying that all was done only by the grace and mercy of Almighty Allah. 4 DECLARATION OF ORIGINALITY I hereby declare that this thesis is my own work. To the best of my knowledge, none of the work presented in this thesis has been previously submitted for a qualification either at Imperial College London or at any other institution. It contains no materials previously published, however, some parts of this work have been presented in a poster at the 2nd Cytokinin meeting (July 2012) held at Dahlem Centre of Plant Sciences, Free University of Berlin, Germany. I also declare that the intellectual content of this thesis is the product of my own work. Signature: Date: 19-11-2013 COPYRIGHT DECLARATION ‘The copyright of this thesis rests with the author and is made available under a Creative Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform or build upon it. For any reuse or redistribution, researchers must make clear to others the licence terms of this work.’ 5 ABBREVIATIONS AHK Arabidopsis histidine kinase AHP Arabidopsis histidine phosphotranspher AM Axillary meristem ARR Arabidopsis response regulator BAP Benzyl amino purine CaMV Cauliflower mosaic virus CCD Carotenoid cleavage dioxygenase CK Cytokinin CRF Cytokinin response factors cZ cis-Zeatin cZR cis-Zeatin riboside cZRP cis-Zeatin ribotide phosphate DMAPP Dimethylallyl diphosphate DZ Dihydrozeatin EMS Ethyl methanesulfonate GFP Green fluorescent protein GUS Beta Glucuronidase HMBDP Hydroxymethylbutenyl diphosphate iP Isopentenyladenine iPR Isopentenyladenine riboside iPRP Isopentenyladenine ribotide phosphate IPT Isopentenyltransferase MAX More axillary shoots NAA 1-Naphthaleneacetic acid NADPH Nicotinamide adenine dinucleotide phosphate NOA Naphthoxyacetic acid NPA 1-N-Naphthylphthalamic acid OE Overexpressor PAT Polar auxin transport SAM Shoot apical meristem SL Strigolactone TIBA 2,3,5-triiodobenzoic acid tZ trans-Zeatin tZR trans-Zeatin riboside tZRP trans-Zeatin ribotide phosphate WT Wild-type w.r.t with respect to 6 TABLE OF CONTENTS ABSTRACT .........................................................................................................................................3 ACKNOWLEDGEMENT ...................................................................................................................4 ABBREVIATIONS..............................................................................................................................6 LIST OF FIGURES ...........................................................................................................................14 LIST OF TABLES .............................................................................................................................17 CHAPTER 1 − INTRODUCTION .................................................................................................18 1.1 Importance of Studies on Shoot Branching..........................................................................18
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