CONTENTS Preface ...........................................................................................ix Authors .........................................................................................xi Abbreviations .............................................................................xv Auxin signal transduction ......................................................1 1 Gretchen Hagen Abstract .............................................................................................................. 1 Introduction ........................................................................................................ 1 Major molecular components involved in auxin signalling ................................. 3 Current models of auxin signalling ..................................................................... 6 Research in progress on auxin signalling ........................................................... 7 Developing areas of research on auxin signalling .............................................. 8 Future challenges ............................................................................................... 9 Summary .......................................................................................................... 10 References ....................................................................................................... 10 Plant cytokinin signalling .....................................................13 2 Erika A. Keshishian and Aaron M. Rashotte Abstract ............................................................................................................ 13 Introduction ...................................................................................................... 13 The structures and activities of cytokinins ....................................................... 14 Cytokinin signalling: multistep phosphorelay ................................................... 16 The receptors: histidine kinases ....................................................................... 17 Relaying the signal: histidine phosphotransfer proteins .................................. 18 Response regulators ........................................................................................ 19 Type-B response regulators ............................................................................. 20 Type-A response regulators ............................................................................. 22 Cytokinin-response factors .............................................................................. 22 Conclusion ....................................................................................................... 23 Three immediate future goals and directions for cytokinin research ............... 23 Summary .......................................................................................................... 24 References ....................................................................................................... 25 Abscisic acid: biosynthesis, inactivation, 3 homoeostasis and signalling ...............................................29 Ting Dong, Youngmin Park and Inhwan Hwang Abstract ............................................................................................................ 29 Introduction ...................................................................................................... 30 Physiological roles of ABA in plants ................................................................. 30 v vi Essays in Biochemistry volume 58 2015 ABA biosynthesis and catabolism .................................................................... 31 Long-distance ABA transport ........................................................................... 34 Regulation of ABA homoeostasis ..................................................................... 36 ABA perception and initiation of ABA signalling .............................................. 36 Conclusions and future perspectives ............................................................... 41 Summary .......................................................................................................... 42 References ....................................................................................................... 42 GA signalling and cross-talk with other 4 signalling pathways ..............................................................49 Vai S. Lor and Neil E. Olszewski Abstract ............................................................................................................ 49 Introduction ...................................................................................................... 49 GA signalling involves the destruction of proteins that repress GA responses ............................................................................................... 50 DELLA proteins are co-activators of transcription ........................................... 52 Mutant DELLA proteins underlie the ‘Green Revolution’.................................. 52 DELLA proteins mediate cross-talk with other signalling pathways ................ 53 Homoeostatic regulation of GA signalling: GA signalling turns off GA signalling ............................................................................................ 56 What are the gaps in our knowledge of GA signalling? ................................... 56 Summary .......................................................................................................... 57 References ....................................................................................................... 57 Mechanisms of ethylene biosynthesis and 5 response in plants .................................................................61 Paul B. Larsen Abstract ............................................................................................................ 61 The role of ethylene in plant development ....................................................... 62 Ethylene biosynthesis ....................................................................................... 62 The canonical ethylene signalling pathway ...................................................... 63 The biochemical rheostat that controls the level of the ethylene response ..... 66 Conclusion: future challenges for understanding the role of ethylene in plant biology ............................................................................................. 67 Summary .......................................................................................................... 68 References ....................................................................................................... 68 The molecular circuit of steroid signalling in plants .........71 6 Thomas Hartwig and Zhi-Yong Wang Abstract ............................................................................................................ 71 Introduction ...................................................................................................... 71 Plant steroid signalling pathway ....................................................................... 72 The network of signal cross-talk and integration ............................................. 75 Contents vii Summary .......................................................................................................... 80 References ....................................................................................................... 81 Novel players fine-tune plant trade-offs..............................83 7 Selena Gimenez-Ibanez, Marta Boter and Roberto Solano Abstract ............................................................................................................ 83 Introduction ...................................................................................................... 84 JA-Ile perception and the nature of the bioactive hormone ............................. 85 The JAZ family: a multifunctional and versatile hub of repressors ................... 86 Direct targets of JAZ proteins: the array of TFs that control JA-Ile responses ........................................................................................... 88 JAZ proteins as key hubs in cross-talk with other hormone signalling cascades ...................................................................................... 92 Conclusions ...................................................................................................... 94 Summary .......................................................................................................... 95 References ....................................................................................................... 96 Salicylic acid signalling: new insights and 8 prospects at a quarter-century milestone .........................101 Xiaoyu Liu, Kristin S. Rockett, Camilla J. Kørner and Karolina M. Pajerowska-Mukhtar Abstract .......................................................................................................... 101 Introduction: SA as the key immune hormone in plants ................................ 102 Salicylic acid biosynthesis .............................................................................. 103 SAR mobile signals ........................................................................................ 104 SA receptor(s): the stories of NPR1, NPR3 and NPR4 ................................... 105 Novel SA-signalling components ..................................................................