Hyper-gravity effects on the Arabidopsis transcriptome

Callus cultures of Arabidopsis thaliana (cv. Columbia) in Petri dishes were exposed to altered g -forces by centrifugation (1-10 g). Using semi-quantitative RT-PCR transcripts of genes coding for metabolic key enzymes (ADP-glucose pyrophosphorylase, ADPG-PP; beta-amylase, fructose-1,6-bisphosphatase, FBPase; glyceraldehyde-P dehydrogenase, GAPDH; hydroxymethylglutaryl-CoA reductase, HMG; phenylalanine-ammonium-lyase, PAL; PEP carboxylase, PEPC) were used to monitor threshold conditions for g-number (all) and time of exposure (beta-amylase) which led to altered amounts of the gene product. Exposure to approximately 5 g and higher for 1 h resulted in altered transcript levels: transcripts of beta-amylase, PAL, and PEPC were increased, those of ADPG-PP decreased, while those of FBPase, GAPDH, and HMG were not affected. This probably indicates a shift from starch synthesis to starch degradation and increased rates of anaplerosis (PEPC: supply of ketoacids for amino acid synthesis). In order to get more information about g-related effects on gene expression, we used a 1-h exposure to 7 g for a microarray analysis, using a commercial A. thaliana chip with 4105 unique annotated clusters/genes (IncyteGenomics). Transcripts of more than 200 genes were significantly increased in amount (ratio 7 g /1 g control; 2(1.6) and larger). They fall into several categories. Transcripts coding for enzymes of major pathways form the largest group (25%), followed by gene products involved in cellular organization and cell wall formation/rearrangement (17%), signalling, phosphorylation/dephosphorylation (12%), proteolysis and transport (10% each), hormone synthesis plus related events (8%), defense (4%), stress-response (2%), and gravi-sensing (2%). Many of the alterations are part of a general stress response, but some changes related to the synthesis/rearrangement of cell wall components could be more hyper-g -specific. We only found few gene products, which were decreased in relation to 1 g controls, and these were less significant (ratio < 2(1.6)). We thus assume that g-forces above a threshold of about 5 g for 1 h are sensed by plant cells in general, causing distinct metabolic responses, which obviously in part, are regulated by gene expression.