Protein Sorting and Protein Modification Along the Secretory Pathway in BY-2 Cells. In: Nagata T, Hasezawa S, Inzé D, editors. Tobacco BY-2 Cells. Berlin: Springer Berlin Heidelberg; 2004. pp. 283-301. [DOI: 10.1007/978-3-662-10572-6_19]

Polarized localization and borate-dependent degradation of the Arabidopsis borate transporter BOR1 in tobacco BY-2 cells

In Arabidopsis the borate transporter BOR1, which is located in the plasma membrane, is degraded in the presence of excess boron by an endocytosis-mediated mechanism. A similar mechanism was suggested in rice as excess boron decreased rice borate transporter levels, although in this case whether the decrease was dependent on an increase in degradation or a decrease in protein synthesis was not elucidated. To address whether the borate-dependent degradation mechanism is conserved among plant cells, we analyzed the fate of GFP-tagged BOR1 (BOR1-GFP) in transformed tobacco BY-2 cells. Cells expressing BOR1-GFP displayed GFP fluorescence at the plasma membrane, especially at the membrane between two attached cells. The plasma membrane signal was abolished when cells were incubated in medium with a high concentration of borate (3 to 5 mM). This decrease in BOR1-GFP signal was mediated by a specific degradation of the protein after internalization by endocytosis from the plasma membrane. Pharmacological analysis indicated that the decrease in BOR1-GFP largely depends on the increase in degradation rate and that the degradation was mediated by a tyrosine-motif and the actin cytoskeleton. Tyr mutants of BOR1-GFP, which has been shown to inhibit borate-dependent degradation in Arabidopsis root cells, did not show borate-dependent endocytosis in tobacco BY-2 cells. These findings indicate that the borate-dependent degradation machinery of the borate transporter is conserved among plant species.

A comprehensive gene expression analysis toward the understanding of growth and differentiation of tobacco BY-2 cells

To understand how plant cell changes gene expression during cell division and after termination of cell division, we analyzed the change of gene expression during the growth of tobacco BY-2 cell lines using a cDNA microarray, which contained about 9,200 expression sequence tag fragments and corresponded to about 7,000 genes. We found that log phase cells predominantly expressed DNA/chromosome duplication gene homologs. In addition, many genes for basic transcription and translation machineries, as well as proteasomal genes, were up-regulated at the log phase. About half of the kinesin homolog genes, but not myosin homolog genes, were predominantly expressed at the dividing phase as well. In contrast, stationary phase cells expressed genes for many receptor kinases, signal transduction machineries and transcription factors. Several hundreds of genes showed differential expression after incubation of stationary phase cells with medium containing either salicylic acid or abscisic acid. These findings suggested that BY-2 cells at the stationary phase express genes for perceiving extracellular signals.