Analysis of companion cell and phloem metabolism using a transcriptome-guided model of Arabidopsis metabolism
Companion cells and sieve elements play an essential role in vascular plants and yet the details of the metabolism that underpins their function remain largely unknown. Here we construct a tissue-scale flux balance analysis (FBA) model to describe the metabolism of phloem loading in a mature Arabidopsis leaf. We explore the potential metabolic interactions between mesophyll cells, companion cells, and sieve elements based on current understanding of the physiology of phloem tissue and through the use of cell-type-specific transcriptome data as a weighting in our model. We find that companion cell chloroplasts likely play a very different role to mesophyll chloroplasts. Our model suggests that, rather than carbon capture, the most crucial function of companion cell chloroplasts is to provide photosynthetically-generated ATP to the cytosol. Additionally, our model predicts that the metabolites imported into the companion cell are not necessarily the same metabolites that are exported in phloem sap; phloem loading is more efficient if certain amino acids are synthesised in the phloem tissue. Surprisingly, in our model predictions the H+-PPiase is the more important contributor than the H+ ATPase to the energisation of the companion cell plasma membrane.