Induction of Salt and Osmotic Stress Tolerance by Overexpression of an Intracellular Vesicle Trafficking Protein AtRab7 (AtRabG3e)

Abstract

Adaptation to stress requires removal of existing molecules from various cellular compartments and replacing them with new ones. The transport of materials to and from the specific compartments involved in the recycling and deposition of macromolecules is carried out by an intracellular vesicle trafficking system. Here, we report the isolation of a vesicle trafficking-regulating gene, AtRabG3e (formerly AtRab7), from Arabidopsis. The gene was induced during programmed cell death after treatment of intact leaves with superoxide and salicylic acid or infection with necrogenic pathogens. Transgenic plants that expressed the AtRabG3e gene under the constitutive 35S promoter from cauliflower mosaic virus exhibited accelerated endocytosis in roots, leaves, and protoplasts. The transgenic plants accumulated sodium in the vacuoles and had higher amounts of sodium in the shoots. The transgenic plants also showed increased tolerance to salt and osmotic stresses and reduced accumulation of reactive oxygen species during salt stress. These results imply that vesicle trafficking plays an important role in plant adaptation to stress, beyond the housekeeping function in intracellular vesicle trafficking.Plants are constantly exposed to changes in the environment that results in development of stress, compelling them to adjust to the new conditions. The perturbations in the surrounding environmental conditions often cause an oxidative stress. A mild oxidative stress usually induces antioxidant defenses, whereas a severe stress causes rapid necrosis. Intermediate levels of reactive oxygen species (ROS) often trigger a programmed cell death (PCD) cascade, which eliminates the compromised cells (Datt et al., 2003). The induction and execution of PCD are tightly controlled processes and can be modulated by signaling molecules, such as jasmonic acid, salicylic acid (SA), and ethylene (Lam et al., 1999).Little information exists on the role of intracellular vesicle trafficking in resistance to environmental stresses. Endocytosis has been viewed traditionally as a constitutive housekeeping function in both animal and plant cells. Recently, however, a syntaxinrelated protein, NtSyr1, which is one of the central components of the vesicle trafficking machinery in eukaryotes, was implicated in abscisic acid-mediated responses in tobacco (Nicotiana tabacum; Leyman et al., 1999). In yeast (Saccharomyces cerevisiae), we found that vesicle trafficking between cytosol and the plasma membrane was inhibited by oxidative stress. Overexpression of an Arabidopsis synaptobrevin homolog partially restored the traffic and provided tolerance to lethal concentrations of hydrogen peroxide (H 2 O 2 ; Levine et al., 2001). A link between regulation of endocytosis by GDI:Rab5 complex and the p38-dependent stress response was described recently in animal cells (Cavalli et al., 2001).We have shown previously that a low concentration of superoxide in the presence of nontoxic concentration of SA-induced PCD in intact Arabidopsis leaves (Mazel and Levine, 2001)...

Suppression of Arabidopsis vesicle-SNARE expression inhibited fusion of H 2 O 2 -containing vesicles with tonoplast and increased salt tolerance

“…We next analyzed the accumulation of ROS in Arabidopsis roots that has been shown previously to be induced by the NADPH oxidase during salt stress (8). The generation of ROS was observed several minutes after the salt treatment in wild-type and antisense plants (Fig.…”

“…On the contrary, the disruption of individual members of the v-SNARE family improved salt tolerance, as opposed to the t-SNAREs. Interestingly, the proteins that interact with the SNARE-pin complex include the GTP binding proteins of the AtRabG3 (AtRab7) class (44), which are homologous to yeast small GTPase Ypt7p that also were shown to function in salt-stress tolerance (8).…”

“…The major intracellular sources of ROS in plants are chloroplasts, mitochondria, and peroxisomes, although ROS also are produced by enzymatic reactions as byproducts or by the NADPH oxidase that is induced by many abiotic stresses, including salt (8,18). Recently, induction of endosome-associated ROS generation by NADPH oxidase was described in guard cells treated with abscisic acid (19).…”

“…A synaptobrevin-like v-SNARE, VAMP711C, was identified in Arabidopsis vegetative vacuoles in the complex that mediated the vesicle docking to the tonoplast (12). We have shown previously that vesicle trafficking to the vacuole plays an important role in salt tolerance (8). The vacuole plays a central role in salt tolerance by sequestering sodium.…”

“…Plants contain an especially large set of SNAREs (2) that corresponds to existence of multiple vacuoles with different functions (3,4). Vesicle trafficking has been traditionally viewed as a housekeeping process, but recent findings in plant, yeast, and animal cells show that it also can play an important role in stress responses (5)(6)(7)(8).…”