Unravelling response-specificity in Ca2+ signalling pathways in plant cells.

Considerable advances have been made, both in the technologies available to study changes in intracellular cytosolic free Ca2+ ([Ca2+ ]i ), and in our understanding of Ca2+ signalling cascades in plant cells, but how specificity can be generated from such a ubiquitous component as Ca2+ is questionable. Recently the concept of 'Ca2+ signatures' has been formulated; tight control of the temporal and spatial characteristics of alterations in [Ca2+ ]i signals is thought to be responsible, at least in part, for the specificity of the response. However, the way in which Ca2+ signatures are decoded, which depends on the nature and location of the targets of the Ca2+ signals, has received little attention. In a few key systems, progress is being made on how diverse Ca2+ signatures might be transduced within cells in response to specific signals. Valuable pieces of the signal-specificity puzzle are being put together and this is illustrated here using some key examples; these emphasize the global importance of Ca2+ -mediated signal-transduction cascades in the responses of plants to a wide diversity of extracellular signals. However, the way in which signal specificity is encoded and transduced is still far from clear. Contents Summary 7 I. Introduction: Ca2+ as a signal transducer 8 II. Alterations in intracellular [Ca2+ ] 8 1. Measuring alterations in [Ca2+ ] 8 Imaging [Ca2+ ]i using Ca2+ -sensitive dyes 8 Measuring [Ca2+ ]i using aequorin 9 Imaging [Ca2+ ]i using cameleon 10 2. The concept of the 'Ca2+ signature 10 3. How might specific Ca2+ signatures be generated? 11 Control of intracellular Ca2+ release 11 Control of influx of extracellular Ca2+ 12 4. Examples of Ca2+ signatures and cellular responses to increases in [Ca2+ ] 13 Ca2+ signatures in stomatal guard cells in response to abscisic acid signals 14 Ca2+ signals in response to abiotic stimuli1 8 Ca2+ signatures involved in plant-pathogen responses 19 Ca2+ signatures in control of plant reproduction 20 Ca2+ signatures in root hairs in response to nodulation signals 23 III. Decoding the [Ca2+ ]i signatures 24 1. Coupling Ca2+ signals to responses through CaM 26 2. Coupling Ca2+ signals to responses through CDPK 27 3. Novel Ca2+ binding proteins as primary Ca2+ sensors 28 Conclusions and Perspective 28 References 29.