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The Role of XET and Galactose in Plant Growth

Galactosyl Residues Necessary For Tensile Strength

Table 2: Tensile Strength of wild-type, mur2, and mur3 plants

Plant Group	Tensile Strength
wild-type	0.9 MPa +/- 0.3 MPa
mur2	0.7 MPa +/- 0.3 MPa
mur3	0.4 MPa +/- 0.1 MPa

The mur2 knockout plants had only slightly reduced tensile strength compared to the wild-type. The mur3 knockout, however, had a large decrease in tensile strength. This suggests that it is galactose rather than fucose residues that play the most important role in providing tensile strength to the cell wall. [Back to Top]

No Difference In Xyloglucan Binding to Cellulose Between Mutants and Wild Type

Table 3: Cellulose binding tenacity of wild-type, mur2, and mur3 plants in vivo and in vitro

Plant group	[NaOH] where binding failed
wild-type (in vivo)	4 mMol
mur2 (in vivo)	4 mMol
mur3 (in vivo)	4 mMol
wild-type (in vitro)	0.6 mMol
mur2 (in vitro)	0.6 mMol
mur2 (in vitro)	0.6 mMol

The cellulose binding tenacity was the same in all three groups in vivo and in vitro. This suggests that, contrary to previous predictions (Levy, et al 1997), Gal and Fuc do not play any significant role in binding of XyG to cellulose. The reduced NaOH concentration in the in vitro studies illustrate simply that binding in vivo is stronger than in vitro, perhaps due to an assembly mechanism in vivo that is not present in vitro. [Back to Top]

XET Activity Decreased With Loss of Galactosylation

Figure 2: XET Activity with different substrates

The graphs (figure 2) depict the activity of XET with different substrates. "A" shows the reaction rate as a function of substrate concentration, while "B" depicts the overall incorporation of radioactive substrate as a function of time. Table 4 shows the primary form of XyG oligomer present for different sample types.

Table 4: Predominant XyG forms for different samples

Sample	Predominant XyG's
tamarind	XXXG, XXLG, and XLLG
mur3 leaves	XXXG and XLXG
wild-type leaves	XXXG, XXFG, and XLFG
mur2 cells	XXXG and XXLG
mur3 cells	Only XXXG

The mur3 leaves (not affected by the mutation) and tamarind contain a high level of galactosylated XyGs and are the best substrates for XET. Mur3 cells, where only XXXG is present (no galactosylation), show the lowest XET activity. Additionally, there is a large difference in XET activity between mur3 leaves, where galactosylation is present on the middle xylosyl residue(XLXG), and mur2 cells, where galactosylation is present on the final xylosyl residue(XXLG). These data illustrate that XyG lacking galactose are extremely poor substrates for XET and that XET prefers galactosylation on the middle xylosyl residue.

Since galactosylation is necessary for proper XET functionality, it correlates XET with tensile strength. If galactosylation is necessary for tensile strength and for XET activity, then XET activity seems to be correlated with maintaining proper tensile strength during growth. [Back to Top]