Torin 1,TOR Inhibitor Enhances Cellular Proliferation in NT-1 Tobacco Suspension Cell Cultures

Torin 1 is an ATP-competitive TOR inhibitor which inhibits the signaling of TOR and S6K kinase in mammals and plants.The objective of this research is to determine the effect of Torin 1 in a relatively simple and homogeneous plant system such as the NT-1 tobacco suspension cell cultures.Cultures of NT-1 cells were tested with 5,50,150 and 250 nM of Torin 1.During kinetics growth of NT-1 tobacco suspension cell cultures,150 and 250 nM Torin 1 inhibits the early growth and later enhanced the cellular proliferation during exponential growth by means of an increased expression of E2F1 and cyclin B.Furthermore,Torin 1 stimulates the growth of NT-1 cells during log phase with small shaped cell,characteristic of tobacco suspension cell cultures with high mitotic activity.

Dihydrosphingosine-lnduced Programmed Cell Death in Tobacco BY-2 Cells Is Independent of H2O2 Production

Sphinganine or dihydrosphingosine （d18：0, DHS）, one of the most abundant free sphingoid Long Chain Base （LCB） in plants, has been recently shown to induce both cytosolic and nuclear calcium transient increases and a correlated Programmed Cell Death （PCD） in tobacco BY-2 cells. In this study, in order to get deeper insight into the LCB signaling pathway leading to cell death, the putative role of Reactive Oxygen Species （ROS） has been investigated. We show that DHS triggers a rapid dose-dependent production of H2O2 that is blocked by diphenyleniodonium （DPI）, indicating the involvement of NADPH oxidase（s） in the process. In addition, while DPI does not block DHS-induced calcium increases, the ROS production is inhibited by the broad spectrum calcium channel blocker lanthanum （La^3＋）. Therefore, ROS production occurs downstream of DHS-induced Ca^2＋ transients. Interestingly, DHS activates expression of defense-related genes that is inhibited by both La^3＋ and DPI. Since DPI does not prevent DHS-induced cell death, these results strongly indicate that DHS-induced H2O2 production is not implicated in PCD mechanisms but rather would be associated to basal cell defense mechanisms.

Live-Cell Imaging of Dual-Labeled Golgi Stacks in Tobacco BY-2 Cells Reveals Similar Behaviors for Different Cisternae during Movement and Brefeldin A Treatment

In plant cells, the Golgi apparatus consists of numerous stacks that, in turn, are composed of several flattened cisternae with a clear cis-to-trans polarity. During normal functioning within living cells, this unusual organelle displays a wide range of dynamic behaviors such as whole stack motility, constant membrane flux through the cisternae, and Golgi enzyme recycling through the ER. In order to further investigate various aspects of Golgi stack dynamics and integrity, we co-expressed pairs of established Golgi markers in tobacco BY-2 cells to distinguish sub-compartments of the Golgi during monensin treatments, movement, and brefeldin A （BFA）-induced disassembly. A combination of cis and trans markers revealed that Golgi stacks remain intact as they move through the cytoplasm. The Golgi stack orientation during these movements showed a slight preference for the cis side moving ahead, but trans cisternae were also found at the leading edge. During BFA treatments, the different sub-compartments of about half of the observed stacks fused with the ER sequentially; however, no consistent order could be detected. In contrast, the ionophore monensin resulted in swelling of trans cisternae while medial and particularly cis cisternae were mostly unaffected. Our results thus demonstrate a re- markable equivalence of the different cisternae with respect to movement and BFA-induced fusion with the ER. In addi- tion, we propose that a combination of dual-label fluorescence microscopy and drug treatments can provide a simple alternative approach to the determination of protein localization to specific Golgi sub-compartments.

Apoptosis Induced by High Concentrations of Nicotinamide in Tobacco Suspension Cells

As an inhibitor of poly(ADP-ribose) polymerase (PARP), nicotinamide has a restraining effect on apoptosis at certain low concentrations. In our present study, apoptosis induced by high concentrations of nicotinamide was observed in tobacco suspension cells. When cells were preincubated with 250 mmol/L nicotinamide for 24 h, the hallmarks of apoptosis were detected, including DNA fragments increasing in size by multiples of 180-200 bp, the condensation and peripheral distribution of nuclear chromatin, and a positive reaction to the TUNEL assay. At the same time, the degradation of PARP and the reduction in the potential of the inner membrane of mitochondria appeared in apoptotic cells induced by high concentrations of nicotinamide. This result indicates that apoptosis induced by high concentrations of nicotinamide is associated with caspase-3-like activity and with the opening of mitochondrial permeability pores. These results partially support the hypothesis that high concentrations of PARP inhibitor could force cells to enter an apoptotic pathway by delay of DNA repair in replicating cells.

Biosynthesis of Callose and Cellulose by Detergent Extracts of Tobacco Cell Membranes and Quantification of the Polymers Synthesized in vitro

The conditions that favor the in vitro synthesis of cellulose from tobacco BY-2 cell extracts were determined. The procedure leading to the highest yield of cellulose consisted of incubating digitonin extracts of membranes from 11-day-old tobacco BY-2 cells in the presence of 1 mM UDP-glucose, 8 mM Ca^2＋ and 8 mM Mg^2＋. Under these conditions, up to nearly 40% of the polysaccharides synthesized in vitro corresponded to cellulose, the other polymer synthesized being callose. Transmission electron microscopy analysis revealed the occurrence of two types of structures in the synthetic reactions. The first type consisted of small aggregates with a diameter between 3 and 5 nm that associated to form fibrillar strings of a maximum length of 400 nm. These structures were sensitive to the acetic/nitric acid treatment of Updegraff and corresponded to callose. The second type of structures was resistant to the Updegraff reagent and corresponded to straight cellulose microfibrils of 2-3 nm in diameter and 200 nm to up to 5 μm in length. In vitro reactions performed on electron microscopy grids indicated that the minimal rate of microfibril elongation in vitro is 120 nm/min. Measurements of retardance by liquid crystal polarization microscopy as a function of time showed that small groups of microfibrils increased in retardance by up to 0.047 nm/min per pixel, confirming the formation of organized structures.

Ectopic Expression of a Bacterium NhaD-type Na^+/H^+ Antiporter Leads to Increased Tolerance to Combined Salt/Alkali Stresses

AaNhaD, a gene isolated from the soda lake alkaliphile Alkalimonas amylolytica, encodes a Na＋/H＋ antiporter crucial for the bacterium＇s resistance to salt/alkali stresses. However, it remains unknown whether this type of bacterial gene may be able to increase the tolerance of flowering plants to salt/alkali stresses. To investigate the use of extremophile genetic resources in higher plants, transgenic tobacco BY-2 cells and plants harboring AaNhaDwere generated and their stress tolerance was evaluated. Ectopic expression of AaNhaD enhanced the salt tolerance of the transgenic BY-2 cells in a pH-dependent manner. Compared to wild-type controls, the transgenic cells exhibited increased Na＋ concentrations and pH levels in the vacuoles. Subcellular localization analysis indicated that AaNhaD-GFP fusion proteins were primarily localized in the tonoplasts. Similar to the transgenic BY-2 cells, AaNhaD.overexpressing tobacco plants displayed enhanced stress tolerance when grown in saline-alkali soil. These results indicate that AaNhaD functions as a pH-dependent tonoplast Na＋/H＋ antiporter in plant cells, thus presenting a new avenue for the genetic improvement of salinity/alkalinity tolerance.

Simplification of vacuole structure during plant cell death triggered by culture filtrates of Erwinia carotovora

Vacuoles are suggested to play crucial roles in plant defense-related cell death. During programmed cell death, previous live cell imaging studies have observed vacuoles to become simpler in structure and have implicated this simplification as a prelude to the vacuole＇s rupture and consequent lysis of the plasma membrane. Here, we examined dynamics of the vacuole in cell cycle-synchronized tobacco BY-2 （Nicotiana tabacum L. cv. Bright Yellow 2） cells during ceil death induced by application of culture filtrates of Erwinia carotovora. The filtrate induced death in about 90% of the cells by 24 h. Prior to cell death, vacuole shape simplified and endoplasmic actin filaments disassembled; however, the vacuoles did not rupture until after plasma membrane integrity was lost. Instead of facilitating rupture, the simplification of vacuole structure might play a role in the retrieval of membrane components needed for defense-related cell death.