Applications of modified levels of plant PDglucans

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In addition to the biological importance of these polysaccharides, interest in the GT2 and GT48 enzymes that are believed to be involved in their biosynthesis has been stimulated by the commercial and nutritional properties of the polysaccharide products of the enzymes. Thus, these plant polysaccha-rides are of fundamental importance in plant growth and development, resistance to pathogen invasion, the quality of plant-based foods and the properties of plant fibres and fuels. Cellulose, for example, represents the world 's largest renewable carbon resource. Cellulose biogenesis by land plants and marine algae from photosynthetically-derived carbohydrate occurs at the prodigious rate of 8.5 x 1010 tonnes per year. One particularly promising agro-mdustrial application is in the replacement of fossil fuels with bioethanol. Currently, bioethanol production is increasing rapidly, and with recent advances in the catalytic efficiency of hydrolytic enzymes and non-enzymatic methods for depolymerizing wall polysaccharides, it is becoming apparent that lignocellulosic complexes and other crop residues that consist predominantly of cellulose and non-cellulosic polysaccharides of wall origin will become economical as a source of fermentable sugars for ethanol production. Attention so far has focused on the enzymic degradation of the cellulose and non-cellulosic polysaccharides, but increased knowledge of biosynthetic mechanisms will also provide opportunities to manipulate the fine structures of the polysaccharides, the interactions of cellulose and non-cellulosic polysaccharides within the wall and their relative abundance in walls during plant growth, such that major crop residues and other sources of plant biomass will be more amenable to rapid enzymic degradation and the products of hydrolysis might be modified to enhance the efficiency of the fermentation process. In addition, the cellulose content of cereal stems is related to strength (Appenzeller et al. 2004 ), and increased cellulose content would be expected to reduce the susceptibility of crops to losses that result from lodging prior to harvest.

Similarly, the (1,3;1,4)-P-d-glucans of cereals and grasses have a number of important nutritional and industrial applications that have led to interest in the enzymes involved in their biosynthesis. A high proportion of our daily caloric intake is obtained from rice (Oryza sativa), wheat (Triticum aestivum), sorghum (Sorghum bicolor), barley (Hordeum vulgare), the millets (Panicum miliaceum and Pennisetum americanum) and sugar cane (Saccharum officinarum), while numerous forage and fodder grass species support the production of sheep, cattle and other domesticated livestock. Maize (Zea mays) is also used widely for animal feed, while switchgrass (Panicum virga-tum) and other perennial grasses are showing considerable promise as future biomass energy crops in North America (McLaren 2005; Burton et al. 2006 ). In the areas of human health, the (1,3;1,4)-P -d- glucans are components of dietary fibre that are highly beneficial in the prevention and treatment of serious human health conditions, including colorectal cancer, high serum cholesterol and cardiovascular disease, obesity, and non-insulin-dependent (type 2) diabetes (Brennan & Cleary 2005). In contrast, (1,3;1,4)-P-d-glucans have antinutritive effects in monogastric animals such as pigs and poultry (Brennan & Cleary 2005), and are important in many cereal processing applications, including malting and brewing.

In most of these applications, the key property of the (1,3;1,4)-P-d-glucans is their molecular asymmetry and the resultant high viscosity in aqueous solutions. Thus, highly viscous contents of the small intestine in monogastric animals slow the diffusion both of hydrolytic enzymes to substrates such as starch, and of hydrolytic products back to the intestinal epithelial layer for absorption. This reduces the metabolizable energy of animal feeds and hence growth rate, but is beneficial in human health because it slows the release of glucose into the bloodstream and flattens the insulin response curve. In the brewing industry, excessive levels of undegraded (1,3;1,4)-P -d- glucans in malt extracts will increase their viscosity and hence slow filtration steps in the process. Undegraded (1,3;1,4)-P-d-glucans can also lead to the formation of undesirable hazes in the final product (Bamforth 1993). In summary, therefore, there are commercial incentives to both increase and decrease levels of (1,3;1,4)-P-d-glucans in common cereal grains and vegetative tissues.

The potential applications for (1,3)-P -d-glucan synthases in plants is likely to be related to their role in plant-pathogen interactions, during which plant host cells respond to microbial attack by rapidly synthesizing and depositing callose in close proximity to the invading pathogen (Ryals et al. 1996 ; Donofrio & Delaney 2001). These callosic deposits are commonly referred to as papillae and are thought to contain, in addition to (1,3)-P-d-glucan, minor amounts of other polysaccharides, phenolic compounds, reactive oxygen intermediates and proteins (Smart et al. 1986; Bestwick et al. 1997; Heath et al. 2002). It has been suggested that the papillae act as a physical barrier to microbial penetration, but no general agreement on the precise function of callosic papillae during microbial attack has been reached (Stone & Clarke 1992). If the deposited callose did indeed slow or immobilize the invading microorganisms, the host plant could focus upon them a number of antimicrobial compounds, such as wall-degrading enzymes, phytoalexins and active oxygen species, or initiate cascade responses involving race-specific resistance genes (Jacobs et al. 2003). In Arabidopsis T-DNA insertion lines in which wound callose formation is compromised, the absence of callose in papillae or haustorial complexes correlates with growth cessation of several normally virulent fungal pathogens (Jacobs et al. 2003; Nishimura et al. 2003). This observation therefore offers a potential opportunity to increase crop plant resistance to certain fungal pathogens through down-regulation of callose synthase activity. Manipulation of callose in plasmodes-mata might also find applications in plant protection against viral diseases, through the inhibition of viral movement within the plant.

As noted above, these commercial incentives, together with the broader importance of cellulose, (1,3;1,4)-p-d-glucans, (1,3)-P-d-glucans and related polysaccharides in biological terms, have resulted in an intense interest in the biosynthetic enzymes of the GT2 and GT48 families that are believed to mediate in the biosynthesis of these polysaccharides.

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