Oxygen supply is a prerequisite for response to aluminum in cultured cells of tobacco (<i>Nicotiana tabacum</i>)

Responses to aluminum (Al) were investigated in tobacco cells (cell line SL) in a calcium-sucrose solution for up to 24 h under shaking (aerobic) condition. Microarray analysis of upregulated and downregulated genes under Al exposure and following Gene Ontology (GO) enrichment analysis of biological process category revealed only one GO term to be enriched for the upregulated genes, "response to chitin," annotated with genes encoding transcription factors (NtERF1 and NtMYB3) and MAP kinase (WIPK), and nine GO terms for the downregulated genes, including "cell wall loosening" and "lipid transport," annotated with genes encoding expansin (NtEXPA4) and lipid transfer protein (LTP)/LTP-like (NtLTP3 and NtEIG-C29), respectively. Al triggered the production of nitric oxide (NO) then reactive oxygen species (ROS). Addition of NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide decreased the levels of NO and a part of the transcriptional changes described above, but increased the levels of ROS and a loss of growth capacity, suggesting a role of the NO to induce the transcriptional changes partly and to repress these toxic responses under Al exposure. Under non-shaking (anaerobic) condition, the cells exhibited upregulation of several hypoxia-responsive genes. The cells exposed to Al exhibited the same level of Al accumulation but much lower levels of the Al responses including NO production, ROS production, a loss of growth capacity, citrate secretion, and a part of the transcriptional changes described above, compared with the cells under shaking condition. These results suggest that coexistence of oxygen with Al is necessary to trigger the Al responses related to toxicity and tolerance.