The mucilage secreted by roots and its possible role in cell-cell recognition for the adhesion of fungal pathogens to root surfaces of Zea mays L.

The outer cells of the roots of plants secrete a mucilage which lubricates the root and keeps it moist. The mucilage is secreted from the Golgi apparatus in vesicles which fuse at the plasma membrane. In maize roots a complex of at least three polysaccharides and glycoproteins are formed, some of which have a large proportion of fucose in their composition. The synthesis of these compounds can be readily monitored because fucose can be easily identified, and especially because exogenous fucose is not catabolized but is incorporated intact into the polymers. The synthesis of the polymers seems to be initiated in the endoplasmic reticulum in conjunction with polyprenoid oligosaccharides that contain fucose. Lipid-oligosaccharides of nine sugar residues can be obtained from the membrane preparations of the root cells. These compounds are polyprenyl diphosphate derivatives. A GDP-fucose:polyprenyl phosphate transfucosylase occurs in the endoplasmic reticulum, whereas fucosyl transferase that transfers fucose to a polymer occurs mainly in the Golgi apparatus. The indirect evidence suggests that oligosaccharides of polyprenyl diphosphate compounds are transferred to proteins, elaborated in the Golgi apparatus, and large molecular weight polysaccharides are finally exported as the mucus. Part of the mucus is acidic and in some respects resembles pectin. The presence of fucose in such large quantities in maize root mucilage suggested that this might have some significance for the recognition of these plants by parasitic root fungi. The adsorption of mucilage by pathogenic fungi was investigated with two types of fungi, a highly specialized ectotrophic root-infecting fungus, e.g. Phialophora radicicola and a vascular wilt fungus capable of attacking a great variety of tissues, e.g. Fusarium moniliforme. The adsorption of radioactively labelled and fluorescently labelled polymers by the pathogenic fungi was investigated. The character and proportion of fungal surfaces present in vitro were standardised by the production and semi-synchronous germination of populations of conidia. Changes in appearance of fungal walls, present before and after germination, were examined ultrastructurally. There was polyanionic material on hyphal but less on conidial surfaces of the ectotrophic root-infecting fungi. In contrast this material was present to similar extents on both hyphal and conidial surfaces of F. moniliforme.(ABSTRACT TRUNCATED AT 400 WORDS)