Submitted to: Biomed Central (BMC) Plant Biology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/17/2008
Publication Date: 4/17/2008
Citation: Dupont, F.M. 2008. Metabolic Pathways of the Wheat (Triticum aestivum)Endosperm Amyloplast Revealed by Proteomics. Biomed Central (BMC) Plant Biology. 8(39):1471-2229. Available: http://www.biomedcentralcom/1471-2229/8/39. Interpretive Summary: Amyloplasts plant organelles that are greatly underestimated. Amyloplasts are related to chloroplasts, and are generally understood to be specialized for starch production in plant storage tissues. Their role in many other metabolic processes has not been recognized, however. A recent proteomic study revealed that the amyloplasts in wheat grains have the enzymes necessary for a wide range of biochemical processes. The present study expands on the previous one to demonstrate that amyloplasts may play a major role in producing most of the molecules that the wheat grain needs as it fills with starch and protein. The metabolic pathways of the amyloplast are outlined in detail, along with the supporting evidence. It is suggested that in order to understand the trade-off between producing wheat grains with high starch content or high protein content it will be important to understand the regulation of the interconnected metabolic pathways within the amyloplast.
Technical Abstract: By definition, amyloplasts are plastids specialized for starch production. However, amyloplasts have a broader range of functions that are not widely recognized. A recent proteomic study of amyloplasts isolated from wheat (Triticum aestivum Butte 86) endosperm suggested that they share many of the metabolic and biosynthetic pathways found in chloroplasts (Balmer et al 2006b). In order to develop a detailed enzymatic map for the endosperm amyloplast, the proteins identified in that study were further evaluated and arranged into complete metabolic pathways. Of 284 proteins that were identified in the earlier study, 179 were classified as amyloplast proteins. Criteria for inclusion of a protein in an amyloplast pathway included three or more of the following: presence in the amyloplast fraction, homology with known chloroplast or other plastid proteins, prediction of a plastid transit peptide for a protein homolog for which the N-terminal sequence is known, and predicted plastid location for other members of the same pathway. Amyloplast proteins were grouped into the following pathways: glucose metabolism, glycolysis, pentose phosphate, malate and citrate metabolism, starch biosynthesis, folate one-carbon metabolism, aspartate family, branched chain amino acid family, aromatic amino acid family, synthesis of cysteine and other sulfur compounds, synthesis of purines, pyrimidines, isoprenoids, porphyrins, vitamins and fatty acids, and enzymes involved in ion transport, electron transport, energy metabolism, free radical scavenger systems, and the ferredoxin/thioredoxin system. In addition, there were proteins involved in transcription, translation and protein processing, as well as proteins of unknown function. Nine proteins of unknown function might be amyloplast proteins but there was insufficient information for verification. The remaining 96 proteins were classified as endomembrane, mitochondrial, vacuolar or nuclear components. Most of these additional proteins were associated with organelles or cellular structures that might easily be included in an amyloplast preparation or even be physically associated with amyloplasts. Because of the evidence for a wide range of metabolic pathways in the amyloplasts, it is proposed that they play a major role in endosperm metabolism, at least during the early stages of grain fill.