Comparative Study of the Mitochondrial Proteome From Mesophyll, Vascular, and Guard Cells in Response to Carbon Starvation

A leaf is an organ composed of different tissues that fulfill specific functions. We hypothesized that since cells in vascular or mesophyll tissues as well as in stoma are developmentally tuned to operate their functions, mitochondria from these cells could exhibit significant metabolic differences. Using the IMTACT method, mitochondria were isolated from these three specific cell types, and the subsequent proteomes were analyzed. At steady state, mitochondria from vascular and guard cells had a significantly higher abundance of proteins associated with the mtETC, the TCA cycle, and the metabolic use of amino acids (glutamate, proline, isoleucine, leucine, and valine) as alternative substrates. Intriguingly, the mitochondria from guard cells also had a much lower abundance of proteins involved in the translation machinery, thus raising questions about the efficiency of the mitochondrial protein turnover in these cells. In a second step, we carried out the same comparative analysis, but with plants that were subjected to carbon starvation by placing them in prolonged darkness for three or 6 days. For all cell types studied, an increased abundance of proteins involved in branched-chain amino acid metabolism was detected. However, while guard cell mitochondria underwent a drastic reduction in proteins involved in respiration, translation, and RNA editing, suggesting a sharp downregulation of mitochondrial functions, mitochondrial proteomes from mesophyll and vascular cells did not show many differences, except for an increased arginine/proline/glutamate metabolism. Together, the results reported here support a differential regulation of the mitochondrial metabolism among the cell types constituting a leaf, a difference that is exacerbated upon stress.