Detection of Sugar-Regulated Gene Expression and Signaling in Suspension-Cultured Rice Cells

To better understand the mechanism of sugar signaling in rice cell, the suspension-cultured rice cells were transferred from sucrose-containing (+S) to sucrose-free (-S) of MS culture medium, we found that ribosomal RNAs (rRNAs) were degraded progressively. This suggests that carbon, nitrogen, and phosphate were recycled in this process and the reduction in cellular rRNAs might lead to decreased translation to save energy in response to sugar starvation. Differential screening revealed that two groups of genes, sugar-starvation-repressed (SSR) and sugar-starvation-activated (SSA) genes, were regulated by sugar in an opposing manner. Northern-blot analysis showed that two major hybridization signals of 0.8 and 1.9 kb were induced strongly under sugar starvation. The two populations of genes corresponded with homologs of α-amylases (1.9 kb) and the glycine-rich proteins (GRPs) gene family (0.8 kb), and all were SSA genes. Expression of GRP genes was strongly induced in sugar-starved cells, which suggests that GRPs may help to protect cells against nutritional stress. Treatment of +S and -S cells with the protein kinase (PK) inhibitor staurosporine (St) and the serine/theronine phosphoprotein phosphatases 1 (PP1) and 2A (PP2A) inhibitor okadaic acid (OA) revealed that PP1 and PP2A (PPs) might be involved in increasing SSR gene expression in +S cells, and that activation of the majority of the SSA genes in -S cells might be due to PKs activity. These results suggested that PKs and PPs might be involved in the sugar regulation of SSR and SSA gene expression. An in-gel PK activity assay demonstrated that the activity of two classes of PKs (50 and 66 kDa) may be induced rapidly after transfer of +S cells to -S medium. Following transfer of -S cells to +S medium, a novel class of 38 kDa PK was induced rapidly and showed high activity. The 38 kDa PK might play a role in sugar sensing, and the 50 and 66 kDa PKs might play roles in signal sensing under sugar starvation in rice cells. These results provide valuable information on three classes of protein kinases that might play key roles in sugar sensing and signaling in rice.

Acceleration of Cell Growth by Xyloglucan Oligosaccharides in Suspension-Cultured Tobacco Cells

The incorporation of xyloglucan oligosaccharide （XXXG） into the walls of suspension-cultured tobacco cells accelerated cell expansion followed by cell division, changed cell shape from cylindrical to spherical, decreased cell size, and caused cell aggregation. Fluorescent XXXG added to the culture medium was found to be incorporated into the surface of the entire wall, where strong incorporation occurred not only on the surface, but also in the interface walls between cells during cell division. Cell expansion was always greater in the transverse direction than in the longitudinal direction and then, immediately, expansion led to cell division in the presence of XXXG; this process might result in the high level of cell aggregation seen in cultured tobacco cells. We concluded that the integration of this oligosaccharide into the walls could accelerate not only cell expansion, but also cell division in cultured cells.

Ultrastructural and Extracellular Protein Changes in Cell Suspension Cultures of Populus euphratica Associated with Low Temperature-induced Cold Acclimation

Populus euphratica Olive is the only tree species that can grow in the saline land and also survive cold winters in northwest China, and it plays a very important role in stabilizing the vulnerable ecosystem there. A cell suspension culture was initiated from callus derived from plantlets of Populus euphratica. Cold acclimation was induced (LT50 of 17.5 ℃) in cell suspension at 45 ℃ in the dark for 30 days and the freezing tolerance increased from LT50 of 12.5 ℃ in nonacclimated cells to LT50 of 17.5 ℃ in cold-acclimated cells. Microvacuolation, cytoplasmic augmentation and accumulation of starch granules were observed in cells that were cold-acclimated by exposure to low temperatures. Several qualitative and quantitative changes in proteins were noted during cold acclimation. Antibodies to carrot extracellular (apoplastic) 36 kD antifreeze protein did not cross react on immunoelectroblots with extracellular proteins in cell suspension culture medium of Populus euphratica, indicating no common epitopes in the carrot 36 kD antifreeze protein and P. euphratica extracellular proteins. The relationship of these changes to cold acclimation in Populus euphratica cell cultures was discussed.