ReviewB l a c k w e l l P u b l i s h i n g L t d O x f o r d , U K N P H N e w P h y t o l o g i s t 0 0 2 8 -6 4 6 X 1 4 6 9 -8 1 3 7 © T h e A u t h o r s ( 2 0 0 8 ) . J o u r n a l c o m p i l a t i o n © N e w P h y t o l o g i s t ( 2 0 0 8 ) SummaryDomination of the global biosphere by human beings is unprecedented in the history of the planet, and our impact is such that substantive changes in ecosystems, and the global environment as a whole, are now becoming apparent. Our activity drives the steady increase in global temperature observed in recent decades. The realization of the adverse effects of greenhouse gas emissions on the environment, together with declining petroleum reserves, has ensured that the quest for sustainable and environmentally benign sources of energy for our industrial economies and consumer societies has become urgent in recent years. Consequently, there is renewed interest in the production and use of fuels from plants. The 'first-generation' biofuels made from starch and sugar appear unsustainable because of the potential stress that their production places on food commodities. Second-generation biofuels, produced from cheap and abundant plant biomass, are seen as the most attractive solution to this problem, but a number of technical hurdles must be overcome before their potential is realized. This review will focus on the underpinning research necessary to enable the cost-effective production of liquid fuels from plant biomass, with a particular focus on aspects related to plant cell walls and their bioconversion.New Phytologist (2008) 178: [473][474][475][476][477][478][479][480][481][482][483][484][485]

SummaryThe occurrence and function of the side chains occurring in the rhamnogalacturonan I domain of pectic polysaccharides have been investigated during carrot cell development using monoclonal antibodies to de®ned epitopes of (1®4)-b-D-galactan and (1®5)-a-L-arabinan. Immunolocalization studies of carrot root apices indicated that cell walls in the central region of the meristem contained higher levels of (1®5)-a-arabinan than the cell walls of surrounding cells. In contrast (1®4)-b-galactan was absent from the cell walls of the central meristematic cells but appeared abundantly at a certain point during root cap cell differentiation and also appeared in cell walls of differentiating stele and cortical cells. This developmental pattern of epitope occurrence was also re¯ected in a suspension-cultured carrot cell line that can be induced to switch from proliferation to elongation by altered culture conditions. (1®4)-bgalactan occurred at a low level in cell walls of proliferating cells but accumulated rapidly in cell walls following induction, before any visible cell elongation, while (1®5)-a-arabinan was present in cell walls of proliferating cells but was absent from cell walls of elongated cells. Immunochemical assays of the cultured cells con®rmed the early appearance of (1®4)-b-galactan during the switch from cell proliferation to cell elongation. Anion-exchange chromatography con®rmed that (1®4)-b-galactan was attached to acidic pectic domains and also indicated that it was separate from a distinct homogalacturonan-rich component. These results indicate that the neutral components of pectic polysaccharides may have important roles in plant cell development.

SummaryHere we demonstrate that the pectic rhamnogalacturonan-I-associated LM5 (1!4)-b-D-galactan epitope occurs in a restricted manner at the root surface of intact Arabidopsis seedlings. The root surface occurrence of (1!4)-b-D-galactan marks the transition zone at or near the onset of rapid cell elongation and the epitope is similarly restricted in occurrence in epidermal, cortical and endodermal cell walls. The extent of surface (1!4)-b-D-galactan occurrence is reduced in response to genetic mutations (stp-1, ctr-1) and hormone applications that reduce root cell elongation. In contrast, the application of the arabinogalactanprotein (AGP) binding b-glucosyl Yariv reagent (bGlcY) that disrupts cell elongation results in the persistence of (1!4)-b-D-galactan at the root surface and in epidermal, cortical and endodermal cell walls. This latter observation indicates that modulation of pectic (1!4)-b-D-galactan may be an event downstream of AGP function during cell expansion in the Arabidopsis seedling root.

A monoclonal antibody (LM8) was generated with specificity for xyloglacturonan (XGA) isolated from pea (Pisum sativum L.) testae. Characterization of the LM8 epitope indicates that it is a region of XGA that is highly substituted with xylose. Immunocytochemical analysis indicates that this epitope is restricted to loosely attached inner parenchyma cells at the inner face of the pea testa and does not occur in other cells of the testa. Elsewhere in the pea seedling, the LM8 epitope was found only in association with root cap cell development at the root apex. Furthermore, the LM8 epitope is specifically associated with root cap cells in a range of angiosperm species. In embryogenic carrot suspension cell cultures the epitope is abundant at the surface of cell walls of loosely attached cells in both induced and non-induced cultures. The LM8 epitope is the first cell wall epitope to be identified that is specifically associated with a plant cell separation process that results in complete cell detachment.

Summary The Arabidopsis genome includes seven family 34 glycosyltransferase (GT34) encoding genes. XXT1 and XXT2 have previously been shown to encode XyG α‐1,6‐xylosyltransferases, while knockout mutants of a third, XXT5, exhibit decreased XyG content, suggesting a similar activity. Here, we extend the study to the rest of the Arabidopsis GT34 genes in terms of biochemical activity and their roles in XyG biosynthesis. The enzyme activity of XXTs was investigated using recombinant protein expressed in E. coli. XyG analysis of single and double T‐DNA insertion knockouts, together with overexpression of GT34s in selected mutant lines, provided detailed function of each gene. We reveal the activity of the third member of the GT34 gene family (XXT4) that exhibits xylosyltransferase activity. Double mutants for either xxt2 or xxt5 had a large impact on XyG content, structure and size distribution. Overexpression of the remaining member, XXT3, was able to restore XyG epitopes in xxt2, xxt5 and xxt2 xxt5 double knockouts, suggesting that it also encodes a protein with XXT activity. Our work demonstrates that five of the seven Arabidopsis GT34 genes encode XXT enzymes.