Arabidopsis thaliana cell suspension culture as a model system to understand plant responses under phosphate stress

The requirement of phosphate (Pi) in many biochemical processes means that a plant experiences stress when available Pi is limiting. The Phosphate Starvation Response (PSR) in plants helps avoid the stress brought on by Pi limitation. Molecular processes in plants are controlled by three different genomes. Sub-sets of genes in the nuclear, mitochondria and plastid genomes are coordinately expressed to sustain plant growth and development either in normal or stress conditions. In mitochondria, oxidative phosphorylation makes an essential contribution to plant energy status and is a Pi-requiring process that is limited by low Pi availability. The impact of the PSR on the expression of genes encoded in the mtDNA, especially those involved in oxidative phosphorylation, is unknown. A proteomics approach was undertaken to search for nucleic acid-binding proteins from Arabidopsis thaliana cells grown in culture to evaluate this system as a model to understand plant mitochondrial responses to Pi limitation. As a result, glycine rich RNA binding protein (GR-RBP5), endoribonuclease L-PSP, chaperonin 20, and malate dehydrogenase were identified in Pi-depleted cells as proteins that co-purify with mitochondrial nucleic acids during CsCl gradient centrifugation, an assay that was developed to identify mitochondrial nucleic acid-binding proteins. Based on available microarray data, the abundance of transcripts encoding these proteins does not respond to Pi availability. In the cell cultures, as Pi became depleted, members of three Pi transporter (PHT) gene families were upregulated, including PHT1;1, PHT1;2, PHT1;4, PHT1;7,and PHT3;2, as occurs in plant tissues. In addition, the accumulation of lactate indicated that Pi deficient cells might switch toward fermentative pathways of metabolism instead of aerobic respiration.