SummaryPlant cell walls represent the most abundant renewable resource on this planet. Despite their great abundance, only 2% of this resource is currently used by humans. Hence, research into the feasibility of using plant cell walls in the production of cost-effective biofuels is desirable. The main bottleneck for using wall materials is the recalcitrance of walls to efficient degradation into fermentable sugars. Manipulation of the wall polysaccharide biosynthetic machinery or addition of wall structure-altering agents should make it possible to tailor wall composition and architecture to enhance sugar yields upon wall digestion for biofuel fermentation. Study of the biosynthetic machinery and its regulation is still in its infancy and represents a major scientific and technical research challenge. Of course, any change in wall structure to accommodate cost-efficient biofuel production may have detrimental effects on plant growth and development due to the diverse roles of walls in the life of a plant. However, the diversity and abundance of wall structures present in the plant kingdom gives hope that this challenge can be met.

94305-41 O1 (S.S., C.S.)High-throughput automated partial sequencing of anonymous cDNA clones provides a method to survey the repertoire of expressed genes from an organism. Comparison of the coding capacity of these expressed sequence tags (ESTs) with the sequences in the public data bases results in assignment of putative fundion to a significant proportion of the ESTs. Thus, the more than 13,400 plant ESTs that are currently available provide a new resource that will facilitate progress in many areas of plant biology. These opportunities are illustrated by a description of the results obtained from analysis of 1500 Arabidopsis ESTs from a cDNA library prepared from equal portions of poly(A+) mRNA from etiolated seedlings, roots, leaves, and flowering inflorescences. More than 900 different sequences were represented, 32% of which showed significant nucleotide or deduced amino acid sequence similarity to previously charaderized genes or proteins from a wide range of organisms. At least 165 of the clones had significant deduced amino acid sequence homology to proteins or gene products that have not been previously characterized from higher plants. A summary of methods for accessing the information and materials generated by the Arabidopsis cDNA sequencing projeds is provided.

Xyloglucans are the main hemicellulosic polysaccharides found in the primary cell walls of dicots and nongraminaceous monocots, where they are thought to interact with cellulose to form a three-dimensional network that functions as the principal load-bearing structure of the primary cell wall. To determine whether two Arabidopsis thaliana genes that encode xylosyltransferases, XXT1 and XXT2, are involved in xyloglucan biosynthesis in vivo and to determine how the plant cell wall is affected by the lack of expression of XXT1, XXT2, or both, we isolated and characterized xxt1 and xxt2 single and xxt1 xxt2 double T-DNA insertion mutants. Although the xxt1 and xxt2 mutants did not have a gross morphological phenotype, they did have a slight decrease in xyloglucan content and showed slightly altered distribution patterns for xyloglucan epitopes. More interestingly, the xxt1 xxt2 double mutant had aberrant root hairs and lacked detectable xyloglucan. The reduction of xyloglucan in the xxt2 mutant and the lack of detectable xyloglucan in the xxt1 xxt2 double mutant resulted in significant changes in the mechanical properties of these plants. We conclude that XXT1 and XXT2 encode xylosyltransferases that are required for xyloglucan biosynthesis. Moreover, the lack of detectable xyloglucan in the xxt1 xxt2 double mutant challenges conventional models of the plant primary cell wall.

Glucuronoarabinoxylan, xyloglucan, and galactomannan are noncellulosic polysaccharides found in plant cell walls. All consist of ␤-linked glycan backbones substituted with sugar side chains. Although considerable progress has been made in characterizing the structure of these polysaccharides, little is known about the biosynthetic enzymes that produce them.

NTPs , and results in the irreversible Jennifer Davila-Aponte and interaction of precursors with the chloroplastic envelopes. Kenneth Keegstra 1At this stage, the precursor remains susceptible to exo-MSU-DOE Plant Research Laboratory, Michigan State University, genous protease and the transit peptide is not cleaved East Lansing, MI 48824, USA by the stromal processing peptidase, indicating that the 1 Corresponding author precursor has not completely traversed the envelope membranes (Cline et al., 1985). Translocation of precursors Cytoplasmically synthesized precursors interact with across the envelope membranes can be initiated by raising translocation components in both the outer and inner stromal ATP concentrations (Pain and Blobel, 1987; Theg envelope membranes during transport into chloro et al., 1989). After a precursor has traversed the envelope plasts. Using co-immunoprecipitation techniques, with membranes, the transit peptide is proteolytically removed antibodies specific to known translocation components, by a stromal processing peptidase, producing a maturewe identified stable interactions between precursor sized protein in the stromal compartment (Reed et al., proteins and their associated membrane translocation 1990). components in detergent-solubilized chloroplasticTranslocation of precursors across the two chloroplastic membrane fractions. Antibodies specific to the outer envelope membranes is thought to occur simultaneously envelope translocation components OEP75 and OEP34, at 'contact sites' (Schnell and Blobel, 1993), a term given the inner envelope translocation component IEP110 to regions where both envelope membranes are found in and the stromal Hsp100, ClpC, specifically co-immunoclose physical proximity. By analogy with mitochondria, precipitated precursor proteins under limiting ATP where precursors must also cross two membranes, preconditions, a stage we have called docking. A portion cursors at contact sites are thought to interact with proteinof these same translocation components was coaceous complexes from both the inner and outer immunoprecipitated as a complex, and could also be membranes (for review, see Schatz and Dobberstein, detected by co-sedimentation through a sucrose density 1996). In mitochondria, translocation complexes from the gradient. ClpC was observed only in complexes with outer and inner membranes can act independently from those precursors utilizing the general import apparone another, forming contact sites only when precursors atus, and its interaction with precursor-containing associate with both complexes simultaneously (Segui-Real translocation complexes was destabilized by ATP. et al., 1993;Horst et al., 1995). Whether simultaneous Finally, ClpC was co-immunoprecipitated with a porengagement is required in chloroplasts is presently tion of the translocation components of both outer and unknown. inner envelope membranes, even in the absence of Recent work on the chloroplastic protein import apparadded precursors. We discuss possible roles for stromal atus...