Characterization of a calcium/calmodulin-dependent protein kinase homolog from maize roots showing light-regulated gravitropism

Roots of many species respond to gravity (gravitropism) and grow downward only if illuminated. This light-regulated root gravitropism is phytochrome-dependent, mediated by calcium, and inhibited by KN-93, a specific inhibitor of calcium/calmodulin-dependent protein kinase II (CaMK II). A cDNA encoding MCK1, a maize homolog of mammalian CaMK, has been isolated from roots of maize (Zea mars L). The MCK1 gene is expressed in root tips, the site of perception for both light and gravity. Using the [S-35]CaM gel-overlay assay we showed that calmodulin-binding activity of the MCK1 is abolished by 50 mu M KN-93, but binding is not affected by 5 mu M KN-93, paralleling physiological findings that light-regulated root gravitropism is inhibited by 50 mu M KN-93, but not by 5 mu M KN-93. KN-93 inhibits light-regulated gravitropism by interrupting transduction of the light signal, not light perception, suggesting that MCK1 may play a role in transducing light. This is the first report suggesting a physiological function for a CaMK homolog in light signal transduction.