CONTENTS

Preface...... ix Authors...... xi Abbreviations...... xv

Auxin signal transduction...... 1 1 Gretchen Hagen Abstract...... 1 Introduction...... 1 Major molecular components involved in 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 signalling...... 13 2 Erika A. Keshishian and Aaron M. Rashotte Abstract...... 13 Introduction...... 13 The structures and activities of ...... 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 biosynthesis...... 103 SAR mobile signals...... 104 SA receptor(s): the stories of NPR1, NPR3 and NPR4...... 105 Novel SA-signalling components...... 106 SA-mediated ER stress...... 107 SAR-mediated transgenerational immune memory...... 108 Closing remarks and future directions...... 109 Summary...... 110 References...... 110

Plant peptide hormone signalling...... 115 9 Ayane Motomitsu, Shinichiro Sawa and Takashi Ishida Abstract...... 115 Introduction...... 116 Class 1: small peptide hormones...... 116 Class 2: peptide hormones with disulfide bonds...... 124 Class 3: processed peptide hormones with disulfide bonds...... 125 Conclusion and perspective...... 126 Summary...... 127 References...... 128 viii Essays in Biochemistry volume 58 2015

Florigens and antiflorigens: a molecular 10 genetic understanding...... 133 Ianis G. Matsoukas Abstract...... 133 Introduction: the mystery...... 134 Floral signal transduction...... 135 The photoperiodic pathway: a model for the mode of action of FT...... 136 The role of FT homologous, orthologous and paralogous genes in other species...... 137 Florigen: beyond floral signal transduction...... 138 Antiflorigenic signal transduction...... 139 Floral induction may involve multiple florigens and antiflorigens...... 141 Conclusions...... 145 Summary...... 146 References...... 146

Hormone cross-talk during seed germination...... 151 11 Sonia Gazzarrini and Allen Yi-Lun Tsai Abstract...... 151 Introduction...... 152 ABA biosynthesis and signalling...... 153 GA biosynthesis and signalling...... 155 ABA–GA cross-talk...... 155 Ethylene biosynthesis and signalling...... 158 ABA–ethylene cross-talk...... 159 Conclusion...... 161 Summary...... 161 References...... 162

Comparison of signalling systems...... 165 12 Antoine Larrieu and Teva Vernoux Abstract...... 165 Introduction...... 166 Auxin, (JAs), (GAs), (SLs), (ABA) and salicylic acid (SA): signalling through cytoplasmic and/or nuclear receptors...... 166 Cytokinins (CKs), (BRs) and ethylene: signalling through transmembrane receptors...... 173 Discussion...... 177 Conclusions...... 178 Summary...... 178 References...... 179

Index...... 183