How plants learn

The identification of signal-transduction components in plant cells is proceeding on an almost daily basis. In this issue, Kudla et al. (1) continue this important process by describing the isolation and characterization of regulatory subunits of calcineurin, a Ca2+/calmodulin-dependent protein phosphatase. Since the pioneering work of Cohen (2) and collaborators, biologists have classified protein phosphatases into four types: PP1, PP2A, PP2B, and PP2C. Although this classification was sensibly designed to bring order out of chaos, it had one major drawback. Division into just four types implied a limited diversity and perhaps a lack of specificity in the mechanism of protein dephosphorylation. Consequently, investigators have tended to concentrate attention on protein kinases where, it was surmised, all the action was to be found. Indeed, several hundred protein kinases have now been cloned or purified from plant tissues (3). Estimates suggest there will be nearly a thousand identified when the Arabidopsis genome sequence is finally published. However, the observations of Kudla et al. (1) suggest that the regulation of dephosphorylation by calcineurin (classified as PP2B) might be more complex than is currently appreciated. Calcineurin is a heterodimeric phosphatase. A conserved “A” type catalytic subunit combines with a class of variable and regulatory “B” type subunits. It is the latter subunit class that is the subject of the paper by Kudla et al. (1). Originally, calmodulin was thought to be the only B class subunit. Most early studies in plant cells suggested calmodulin to be of very limited diversity—one or two sequences at the most. It was assumed, therefore, that specificity in dephosphorylation would have to reside in some unique characteristics of individual calcium signals. Indeed, a multiplicity of hormonal, physical, chemical, and stress signals induces transient elevations in cytosolic Ca2+ in plant cells, and each signal induces unique kinetics …