Unravelling response-specificity in Ca signalling pathways in plant cells.

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