Transduction of Blue-Light Signals

BUT IS IT AN ELEPHANT? In the old tale, several blind men are taken to an elephant. In turn, each is asked to examine and identify the mystery object. One blind man examines a leg and claims it to be a tree, another, examining the trunk, claims it to be a snake, and yet another, feeling a tusk, claims it to be a smooth rock. The pattern continues: each describes a component, none identifies the gestalt. There are a wealth of blue-light responses in higher plants. It is clear that many are biochemically, physiologically, or genetically independent, and others are linked. The mechanism by which a photon of blue light is converted into a biochemical signal and transduced into a biological response is under investigation for several of these responses. In many cases, investigators have successfully defined one or more components of the signaling mechanism. Taken together, the identified components have the potentia1 to define an entire signal transduction mechanism; receptor, G-protein, diacylglycerol, inositol triphosphate, calcium, calmodulin, several kinases, ion channels, and so forth. However, direct demonstration of a11 the components has not been achieved in any one system. Indeed, in some cases specific carriers are known to be absent. Further, the locations of the respective carriers differ in accordance with the location of the specific response (Fig. l). Perhaps the mechanisms responsible for transducing the blue-light signals are best represented by a herd of elephants. Signal transduction paradigms are well established. However, recent data suggest that plants may have several nove1 signal carriers (Deng et al., 1992), and it is not certain that the paradigms will be transferable to plants. The majority of studies of signal carriers for blue-light responses in higher plants make use of standard biochemical tools: direct identification based on biochemical properties or antibody crossreactivity, or indirect identification based on inhibitor/activator studies. These biochemical activities are correlated with the respective blue-light response by the photobiological activity (fluence response and/or time course), physical location, or involvement with the response as demonstrated by activator/inhibitor studies. The biochemical approach has recently been joined by a vigorous genetic approach. There are four blue-light responses in higher plants cur' Supported in part by the National Science Foundation and the