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by connecting the aerial shoots to the subterranean roots that take
up water and mineral salts [ 2 ]. The long-distance movement
throughout the entire organism of this aqueous fl uid, called xylem
sap, mainly results from root pressure and leaf water evapotranspi-
ration. The tracheary element differentiation is ending by cell
death, leaving a protoplast-free hollow tube surrounded by a ligni-
fi ed secondary cell wall [ 3 ]. Then, the xylem sap is considered as part
of the apoplast and the xylem vessels form a “super-apoplast” struc-
ture allowing supra-cellular transport processes. Some plant patho-
gens borrow the “super-apoplast” route to move from the primary
infection site and colonize distant tissues. Besides, xylem orches-
trates the allocation of nutrients by supplying amino acids, organic
acids, and carbohydrates within the organism [ 4 ]. Furthermore,
the xylem network participates in the regulation of plant develop-
ment by providing long distance transport of hormones like cyto-
kinin and abscissic acid.
The origin and the function of the xylem proteins became the
focus of several investigations using proteomic analyses. The xylem
sap proteome was investigated in different plant genera, including
Brassica [ 5 - 7 ], Cucurbita and Cucumis [ 5 , 8 ], Glycine [ 9 , 10 ],
Malus [ 11 ], Oryza [ 12 ], Populus [ 13 ], Vitis [ 14 , 15 ] and Zea [ 16 ].
Different strategies were used for protein separation and identifi ca-
tion by mass spectrometry (MS) (Table 1 ). In most cases, proteins
were separated by standard 1D- (1D-E) or 2D-electrophoresis (2D-E)
prior to tryptic digestion and MS or LC (liquid chromatography)-
MS analysis. In one case, peptides were directly analyzed by 2D-LC
MS/MS without any protein separation by electrophoresis [ 12 ].
In another case, a short run of 1D-E was performed to get only three
samples to analyze by LC-MS/MS [ 7 ]. LC-MS/MS was used in
most cases since the nucleotide sequence data (ESTs or genomic) are
scarce for most of the studied plants. The xylem sap proteomes of
only four plants were obtained with homologous sequences:
Brassica oleracea [ 7 ], Glycine max [ 9 , 10 , 22 ], Oryza sativa [ 12 ],
and Solanum lycopersicon [ 20 ]. In the other cases, heterologous
sequences were used for protein identifi cation. This strategy did
not allow the precise identifi cation of genes encoding the identi-
fi ed proteins especially in the case of multigene families.
Subtilisin Ser-proteases, oxido-reductases (among which
peroxidases), lipases, and enzymes involved in carbohydrate metab-
olism are the most represented protein families in analyzed xylem
saps. Many xylem sap proteins were found to be secreted [ 7 , 13 ].
The proportion of proteins predicted to be intracellular may reach
one third of the total xylem sap proteome in Populus [ 13 ], but is
less important (<15 %) in annual plants like Brassica [ 6 , 7 ] and Z.
mays [ 16 ]. Intracellular proteins identifi ed in xylem sap could be
released during the programmed cell death of precursor cells
involved in xylem differentiation. Several of the so-called
pathogenesis-related (PR) proteins (thaumatin-like, chitinases)
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