Localization and control of expression of Nt-Syr1, a tobacco snare protein

Syntaxins and other SNARE proteins are crucial for intracellular vesicle tr afficking, fusion and secretion. Previously, we isolated the syntaxin-relat ed protein Nt-Syr1 from Nicotiana in a screen for ABA-related signalling el ements, and demonstrated its role in determining the ABA sensitivity of sto matal guard cells. Because the location and expression of SNAREs are often important clues to their functioning, we have examined the distribution and stimulus-dependent expression of Nt-Syr1 between tissues, as well as its l ocation within the cell, using antisera raised against purified recombinant peptides corresponding to overlapping cytosolic domains of Nt-Syr1. The Nt -Syr1 epitope was strongly represented in roots and to lesser extents in st ems, leaves and flowers of well-watered plants. Biochemical analysis and ex amination of immunogold labelling under the electron microscope indicated N t-Syr1 to be located primarily at the plasma membrane. Expression of the pr otein in leaves and to a lesser extent in flowers and stems was transiently enhanced by ABA, but not bit auxin, kinetin or gibberellic acid. Expressio n in leaves was promoted by salt stress and wounding, but not by cold. By c ontrast, Nt-Syr1 levels in the root were unaffected by ABA. In the leaves, enhanced expression of Nt-Syr1 by salt stress was not observed in abal muta nt Nicotiana, which is deficient in ABA synthesis, and in plants carrying t he Arabidopsis abi1 transgene that suppresses a number of ABA-evoked respon ses in these plants. However, an enhanced expression in response to woundin g was observed, even in the mutant backgrounds. We conclude that Nt-Syr1 ex pression at the plasma membrane is important for its function and is subjec t to control by parallel, stress-related signalling pathways, both dependen t on and independent of ABA. Nt-Syr1 may be associated with additional func tions, especially in the roots, that are unrelated to ABA or stress respons es in the plant.