From Plant Cell Walls to Biofuels—Arabidopsis thaliana Model - Bioenergy Crop Plants - page 89

XTR or the XTH gene family comprises groups of genes encoding XG endotransglucosylases/
hydrolases, which are responsible for XG modifying/rearranging in plants. Xu et al. (1996) first
reported on the Arabidopsis XET - related ( XTR ) gene family and their potential roles in cell wall
modification. Their studies gave a detailed insight into the regulation of expression of the XTR
gene family by hormonal and environmental stimuli and demonstrated that distinct but related
wall-modifying enzymes (XTRs) might be recruited at various developmental stages and environ-
mental conditions (Xu et al. 1996). Yokoyama and Nishitani (2001), by examining XTH gene fam-
ily (all 33 genes) expression profiles using quantitative real-time reverse transcriptase-polymerase
chain reaction, were able further understand the XTR gene's tissue specificity and hormonal regu-
lation of expression. According to this work, most XTR genes are distinct in their expression pat-
terns, although with some showing resemblance, as relates to tissue specificity or in response to
a hormonal signal. Recent findings show that AtXTH members, AtXTH17 , -18, -19, and -20, are
homologous genes that could have resulted from a duplication events (Vissenberg et al. 2005). These
members have varied expression patterns in Arabidopsis roots emphasizing specific physiological
roles of each of these members in cell wall biosynthesis (Vissenberg et al. 2005). Osato et al. (2006)
functionally analyzed RNAi plants with downregulated expression of the AtXTH gene and subse-
quently reported the principal role of AtXTH18 in A. thaliana root growth. A comprehensive review
is available describing the biochemical and functional diversity of XTHs with an overview of the
structure and evolutionary organization of the Arabidopsis XTH gene family (Rose et al. 2002).
Previous reports by Mellerowicz and Sundberg (2008), Mellerowicz et al. (2008), and Bourquin
et al. (2002) explain the potential roles of XETs and XTHs in the formation of secondary cell wall
layers in trees, such as Populus , that are popular current targets for biofuel production.
As summarized in Table 5.3, XG synthesis has three main parts, XG backbone synthesis XG
side-chain synthesis, and XG modification. AtCSLC4 is the only Arabidopsis gene characterized
so far that plays a role in XG backbone synthesis by encoding a β-1,4 glucan synthase (Cocuron
et al. 2007). Most of the other XG biosynthetic enzymes characterized are those that synthesize
side chains. Reiter et al. (1993) reported on specific Arabidopsis plants, termed mur1 mutants, with
abnormal growth and altered cell walls due to a fucose-deficient mutation. These mur1 plants lack
a gene that encodes an enzyme, GDP-d-mannose-4,6-dehydratase 2, which catalyzes the initial
critical step in the de novo synthesis of GDP-l-fucose (Bonin et al. 1997). The MUR1 protein thus
may be important for xyloglucan fucosylation. The MUR2 protein is encoded by fucosyltrans-
ferase 1 ( FUT1 or AtFUT1 ). Plants deficient in the FUT1 protein are called mur2 mutants. These
plants also show altered cell wall structures and abnormal plant growth (Perrin et al. 1999; Reiter
2002). Recent studies by Cavalier et al. (2008) and Zabotina et al. (2008) conclude that A. thaliana
XXT1, XXT2, and XXT5 proteins encode xylosyltransferases that are necessary for XG biosyn-
thesis. Mutation of these genes resulted in plants with characteristic abnormal root hair phenotypes
(Zabotina et al. 2008; Cavalier et al. 2008). The lack of measurable XG in the xxt1 X xxt2 double
mutants results in diminished plant growth, emphasizing the critical role played by these genes in
XG biosynthesis (Cavalier et al. 2008).
5.2.6.3 cell-Wall-associated Proteins
Cell walls must be extremely flexible and elastic to cope with cell growth (Cosgrove 1999). This
necessitates many controlled, concerted, and coordinated actions between various cell-wall-asso-
ciated proteins and wall polysaccharides. Various models depicting these mechanisms have been
explained previously (Cosgrove 1999). It is known that a large proportion of cell-wall-associated
proteins studied thus far contribute to cell wall loosening and expansion. A broad classification
of the cell-wall-associated proteins divides them into families such as hydroxyproline-rich glyco-
proteins (HRGPs) that include extensins (Showalter 1993), proline-rich proteins (PRPs; Showalter
1993), glycine-rich proteins (GRPs; Keller 1993), arabinogalactan proteins (AGPs; Oxley and Bacic
1999), wall-associated kinases (WAKs; He et al. 1999), lectins (Herve et al. 1999), and expansins
(Cosgrove 1999). Because the major challenge in the biofuel industry is identification of the means