Nuclear protein kinases: still enigmatic components in plant cell signalling

Summary Plants constantly face changing conditions in their environment. Unravelling the transduction mechanisms from signal perception at the plasma membrane level down to gene expression in the nucleus is a fascinating challenge. Protein phosphorylation, catalysed by protein kinases, is one of the major posttranslational modifications involved in the specificity, kinetic(s) and intensity of a signal transduction pathway. Although commonly assumed, the involvement of nuclear protein kinases in signal transduction is often poorly characterized. In particular, both their regulation and mode of action remain to be elucidated and may lead to the unveiling of new original mechanisms. For example, unlike animal cells, plant cells contain only a few strictly nucleus‐localized protein kinases, which calls into question the role of this cellular distribution between the cytosol and the nucleus in their activation and functions. The control of their nucleocytoplasmic trafficking appears to play a major role in their regulation, probably through promoting interactions with their substrates under specific cellular conditions. However, recent findings showing that the nucleus can generate complex networks of second messengers (e.g. Ca 2+ or diacyglycerol) suggest that nuclear protein kinases could play an active role in the decoding of such signals. Contents Summary 355 I. Introduction 356 II. Features of plant protein kinases 356 III. Nuclear protein kinases 358 IV. Nuclear PKs in the control of the cell cycle and mitosis 361 V. Nuclear calcium‐dependent protein kinases 362 VI. Nucleus‐localized MAPKs 362 VII. Nuclear protein kinases: cytosolic translocation of an activated form or activation in the nucleus? 363 VIII. Concluding remarks 365 Acknowledgements 365 References 365