Distribution of cell-wall xylans in bryophytes and tracheophytes: new insights into basal interrelationships of land plants

The poplar glycosyltransferase GT47C is functionally conserved with Arabidopsis Fragile fiber8

Xylan is the major hemicellulose in dicot wood. Unraveling genes involved in the biosynthesis of xylan will be of importance in understanding the process of wood formation. In this report, we investigated the possible role of poplar GT47C, a glycosyltransferase belonging to family GT47, in the biosynthesis of xylan. PoGT47C from the hybrid poplar Populus alba x tremula exhibits 84% sequence similarity to Fragile fiber8 (FRA8), which is involved in the biosynthesis of glucuronoxylan in Arabidopsis. Phylogenetic analysis of glycosyltransferase family GT47 in the Populus trichocarpa genome revealed that GT47C is the only close homolog of FRA8. In situ hybridization showed that the PoGT47C gene was expressed in developing primary xylem, secondary xylem and phloem fibers of stems, and in developing secondary xylem of roots. Sequence analysis suggests that PoGT47C is a type II membrane protein, and study of the subcellular localization demonstrated that fluorescent protein-tagged PoGT47C was located in the Golgi. Immunolocalization with a xylan monoclonal antibody LM10 revealed a nearly complete loss of xylan signals in the secondary walls of fibers and vessels in the Arabidopsis fra8 mutant. Expression of PoGT47C in the fra8 mutant restored the secondary wall thickness and xylan content to the wild-type level. Together, these results suggest that PoGT47C is functionally conserved with FRA8 and it is probably involved in xylan synthesis during wood formation.

A comparative cytological analysis of fungal endophytes in the sporophyte rhizomes and vascularized gametophytes of Tmesipteris and Psilotum

This article describes the results of a light and electron microscopic study of the fungal endophytes and vascular anatomy in the rhizomes and gametophytes of Tmesipteris and Psilotum. The parenchymatous cortical cells of the rhizomes and subterranean gametophytes of Tmesipteris and Psilotum contain intracellular aseptate glomeromycotean fungi resembling the “Paris-type” of arbuscular mycorrhizas found in seed plants. The fungi differentiate into multinucleate vesicles and hyphal coils, both containing bacteria-like structures and accumulating lipid masses and crystals as they age. After several cycles of infection in the same cell, degenerate hyphae form amorphous masses encased by host wall material. Nearly identical host–fungus cytology between the autotrophic sporophytes and the heterotrophic gametophytes suggests that these psilophyte associations are exploitative of the fungus in both generations. Following the description of tracheids nearly 60 years ago in the gametophytes of Psilotum, vascular elements are described for the first time in the haploid generation of Tmesipteris. Close similarities between the water- and food-conducting elements in both generations, viz. vessel elements with scalariform perforation plates and sieve cells with refractive spherules and lacking callose at all stages in their develoment, add support to the homologous theory of the alternations of generations. Mitochondrial aggregations, cross-linked by small electron-opaque rods, are common in the stelar cells of both generations and appear to be a unique feature of the psilophyte clade.