Polysaccharides with (3-1,4-mannan and (3-1,4-glucomannanbackbones are abundant con­stituents of the wood of gymnosperms (78, 79) the cell walls of certain algae (80), and are present in lower amounts in many other species (81). Mannans also serve as carbohydrate reserves in a variety of plant species (78,82). Several groups have biochemically characterized glucomannan synthase activities from a variety of plant species (78,79,82,83). The enzymes were shown to use GDP-mannose and GDP-glucose as substrates and to produce polymers with varying ratios of the two sugars, depending on the ratio of the sugar nucleotides (83) Recently, transglycosylase enzymes that modify the architecture of mannan polysaccharides in plant cell walls have also been discovered (84).

Mannan synthase is encoded by genes of the CSLA gene family (85). Expression of a cDNA from guar in soybean cells led to the synthesis of a (3-1,4-mannan. Similarly, expression of three Arabidopsis CSLA proteins in Drosophila cells resulted in proteins that catalyzed synthesis of mannan from GDP-mannose (86). Similarly, several poplar orthologs expressed in Drosophila cells exhibited glucomannan synthase activity (87). Functional analysis of

CSLA genes from diverse species is consistent with the hypothesis that the function of the CSLA genes is conserved in all plants (81).

As noted previously in studies of impure enzymes (83), when provided with both GDP — glucose and GDP-mannose, the enzymes produce mixed linkage mannans. One of the proteins produced glucan when provided only with GDP-glucose. These studies suggest that no primer is required to initiate mannan synthesis. The ability of the CSLA enzymes to accept GDP-glucose or GDP-mannose is compatible with earlier suggestions that the ratio of mannose:glucose in glucomannans may be controlled by regulating the availabil­ity of the two sugar nucleotides. The observation that GDP-glucose is a substrate also may explain old observations from in vitro polysaccharide synthesis experiments that had initially been interpreted as evidence that GDP-glucose was the substrate for cellulose synthesis.

Genes encoding the galactomannan galactosyltransferase responsible for attaching galac — tan side chains have also been identified following purification of the enzyme from fenugreek (88). The a-1,6GalT galactosyltransferase cDNA encodes a 51282 Da protein, with a single transmembrane alpha helix near the N terminus. The protein has been functionally ex­pressed in the yeast Pichia pastoris and is active when the membrane-spanning domain is removed. Thus, presumably the membrane-spanning domain is required only to localize the protein to the Golgi apparatus where mannan is thought to be synthesized. The degree of substitution when UDP-galactose is available is variable and appears to be a stochastic process controlled both by enzyme specificity and the levels of a1,6GalT activity (89). Eight Arabidopsis gene sequences are very similar to the a-GalT from fenugreek (88).

In Arabidopsis, mannans have been localized not only in thickened secondary cell walls of xylem elements, xylem parenchyma, and interfascicular fibers, but also in the thickened walls of the epidermal cell of leaves and stems and, to a lesser extent, in most other cell types (90). Analyses of Arabidopsis mutants containing a transposon insertion in exon seven of the CslA7 gene showed that disruption of this gene results in defective pollen tube growth and disruption of embryonic development (91). Mutants (called rat4) containing a T-DNA insertion in the 3′ untranslated region of the AtCslA9 gene display resistance to Agrobacterium tumefaciens transformation, apparently caused by decreased binding of bacterial cells to roots. AtCslA9 promotor-GUS fusions indicated that this gene is expressed in a variety of Arabidopsis tissues, including lateral roots and the elongation zone, where the root is most susceptible to Agrobacterium transformation (92). In both mutant studies, the authors suggested that the mutant phenotypes resulted from alterations in polysaccharide content; however, in neither case was such a defect demonstrated.

A GDP-mannose transporter that is localized to the Golgi has been characterized (93).