Quantitative detection of changes in the leaf-mesophyll tonoplast proteome in dependency of a cadmium exposure of barley (Hordeum vulgare L.) plants.
Although the vacuole is the most important final store for toxic heavy metals like cadmium (Cd(2+)), our knowledge on how they are transported into the vacuole is still insufficient. It has been suggested that Cd(2+) can be transported as phytochelatin-Cd(2+) by an unknown ABC transporter or in exchange with protons by cation/proton exchanger (CAX) transporters. To unravel the contribution of vacuolar transporters to Cd(2+) detoxification, a quantitative proteomics approach was performed. Highly purified vacuoles were isolated from barley plants grown under minus, low (20 microM), and high (200 microM) Cd(2+ )conditions and protein levels of the obtained tonoplast samples were analyzed using isobaric tag for relative and absolute quantitation (iTRAQ). Although 56 vacuolar transporter proteins were identified, only a few were differentially expressed. Under low-Cd(2+) conditions, an inorganic pyrophosphatase and a gamma-tonoplast intrinsic protein (gamma-TIP) were up-regulated, indicating changes in energization and water fluxes. In addition, the protein ratio of a CAX1a and a natural resistance-associated macrophage protein (NRAMP), responsible for vacuolar Fe(2+) export was increased. CAX1a might play a role in vacuolar Cd(2+) transport. An increase in NRAMP activity leads to a higher cytosolic Fe(2+) concentration, which may prevent the exchange of Fe(2+) by toxic Cd(2+). Additionally, an ABC transporter homolog to AtMRP3 showed up-regulation. Under high Cd(2+) conditions, the plant response was more specific. Only a protein homologous to AtMRP3 that showed already a response under low Cd(2+) conditions, was up-regulated. Interestingly, AtMRP3 is able to partially rescue a Cd(2+)-sensitive yeast mutant. The identified transporters are good candidates for further investigation of their roles in Cd(2+) detoxification.
Schneider
Thomas
T
Institute of Plant Biology, University of Zurich, Zollikerstrasse 107, Zurich, Switzerland. Thomas.Schneider@botinst.uzh.ch
Schellenberg
Maja
M
Meyer
Stefan
S
Keller
Felix
F
Gehrig
Peter
P
Riedel
Kathrin
K
Lee
Youngsook
Y
Eberl
Leo
L
Martinoia
Enrico
E
D002104:Cadmium / Q000633:toxicity*; D002853:Chromatography, Liquid; D018506:Gene Expression Regulation, Plant / Q000187:drug effects*; D001467:Hordeum / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism; D008658:Inactivation, Metabolic; D026901:Membrane Transport Proteins / Q000235:genetics / Q000378:metabolism; D010641:Phenotype; D018515:Plant Leaves / Q000166:cytology / Q000378:metabolism*; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D018087:Plastids / Q000378:metabolism*; D020543:Proteome; D040901:Proteomics / Q000379:methods; D019032:Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization; D014617:Vacuoles / Q000378:metabolism
D026901:Membrane Transport Proteins; D010940:Plant Proteins; D020543:Proteome; D002104:Cadmium