Ca^(2+) acts as a second messenger in many of the diverse range of signal transduction pathways of plants. This raises fundermental questions regarding the mechanism(s) by which these pathways can be specific and how Ca^(2+)-based signalling systems c...
Ca^(2+) acts as a second messenger in many of the diverse range of signal transduction pathways of plants. This raises fundermental questions regarding the mechanism(s) by which these pathways can be specific and how Ca^(2+)-based signalling systems can be used to produce the graded physiological responses that are typical of many extracellular stimuli. Like animal cells, evidences have been accumulated to support that different stimuli generate different amplitude, duration, frequency and location of Ca^(2+)-signals in plant cells. Now question goes to how the variety of stimulus-specific Ca^(2+)-signals are decoded and which compound(s) play a role as primary Ca^(2+)-signal sensor(s)? Recent works in our laboratory suggested that calmodulin isoforms may play a role in the control of the specificity. Calmodulin isoforms are differentially expressed by specific stimuli and exhibit differential activation/inhibition of specific target enzymes with different Ca^(2+) sensitivities. Recently Arabidopsis genome sequence revealed that it contains over 35 calmodulin isoforms. Thus calmodulin isoforms may play a pivotal role in modulating a cell's Ca^(2+) signal transduction pathway. Calcium dependent protein kinase(CDPK) isoforms have been proposed as another candidate for primary sensors for stimuli-specific Ca^(2+) transit. These two candidate for primary Ca^(2+) signal sensors will be discussed.