Cell-type-specific calcium responses to drought, salt and cold in the Arabidopsis root

Little is known about the signalling processes involved in the response of roots to abiotic stresses. The Arabidopsis root is a model system of root a natomy with a simple architecture and is amenable to genetic manipulation. Although it is known that the root responds to cold, drought and salt stres s with increases in cytoplasmic free calcium, there is currently no informa tion about the role(s) of the functionally diverse cell types that comprise the root. Transgenic Arabidopsis with enhancer-trapped GAL4 expression in specific cell types was used to target the calcium reporting protein, aequo rin, fused to a modified yellow fluorescent protein (YFP). The luminescence output of targeted aequorin enabled in vivo measurement of changes in cyto solic free calcium concentrations ([Ca2+](cyt)) in specific cell types duri ng acute cold, osmotic and salt stresses. In response to an acute cold stre ss, all cell types tested as well as plants constitutively expressing aequo rin displayed rapid [Ca2+](cyt) peaks. However, there were significant quan titative differences between different cell types in terms of their respons e to cold stress, osmotic stress (440 mM mannitol) and salt stress (220 mM NaCl), implying specific roles for certain cell types in the detection and/ or response to these stimuli. In response to osmotic and salt stress, the e ndodermis and pericycle displayed prolonged oscillations in cytosolic calci um that were distinct from the responses of the other cell types tested. Ta rgeted expression of aequorin circumvented the technical difficulties invol ved in fluorescent dye injection as well as the lack of cell specificity of constitutively expressed aequorin, and revealed a new level of complexity in root calcium signalling.