WATER-DEFICIT RAPIDLY STIMULATES THE ACTIVITY OF A PROTEIN-KINASE IN THE ELONGATION ZONE OF THE MAIZE PRIMARY ROOT

The mechanisms by which plants detect water deficit and transduce that signal into adaptive responses is unknown. In maize (Zea mays L.) see dlings, primary roots adapt to low water potentials such that substant ial rates of elongation continue when shoot growth is completely inhib ited. In this study, in-gel protein kinase assays were used to determi ne whether protein kinases in the elongation zone of the primary root undergo activation or inactivation in response to water deficit. Multi ple differences were detected in the phosphoprotein content of root ti ps of water-stressed compared with well-watered seedlings. Protein kin ase assays identified water-deficit-activated protein kinases, includi ng a 45-kD, Ca2+-independent serine/threonine protein kinase. Water-de ficit activation of this kinase occurred within 30 min after transplan ting seedlings to conditions of low water potential and was localized to the elongation zone, was independent of ABA accumulation, and was u naffected by cycloheximide-mediated inhibition of protein translation. These results provide evidence that the 45-kD protein kinase acts at an early step in the response of maize primary roots to water deficit and is possibly involved in regulating the adaptation of root growth t o low water potential.