The ultrastructure of the mucilaginous layer on plant roots

Abstract

A mucilaginous external layer or mucigel was observed on the roots of all 16 species of common agricultural crop plants examined. On axenic roots, the mucigel appears in the electron microscope as an unevenly distributed layer of granular and fibrillar material covering the outer surface of the root and most root hairs. The greatest quantity of mucigel was found around the root cap. Apart from the presence of soil particles adhering to the external surface of the mucigel on soil-grown plants, structural differences were seldom observed between the mucigels on plants grown in soil, sand or nutrient solution. Some soilgrown axenic peas ( Pisum sativum L.) and mustard ( Sinapis alba L.) however, had particulate and amorphous soil material distributed throughout the mucigel. When plant roots were colonized by Pseudomonas sp., Cytophaga johnsonii Stanier or a mixture of several micro-organisms, larger quantities of mucigel developed than on axenic roots of the same plant species. The number of root-surface cells, which were either dead or showed changes in the structure of their walls, was also greater when micro-organisms were present. In the root-elongation zone and in other zones where microbial colonization was sparse, the mucigel retained its usual appearance near micro-organisms. In densely colonized regions of the mucigel, however, the region immediately around the micro-organisms could usually be differentiated from the bulk of the mucigel. When micro-organisms were present the mucigel often had a distinct outer boundary.