ELECTRON MICROSCOPIC STUDIES ON THE INFECTION THREAD DEVELOPING IN THE ROOT HAIR OF TRIFOLIUM REPENS L. INFECTED WITH RHIZOBIUM TRIFOLII

Abstract

The development of the infection thread formed in the root hair of white clover (Trifolium repens L.) infected with Rhizobium trifolii was observed by electron microscopy on ultrathin sections of the infected root hairs.
Development of the infection thread originates from a curled tip of root hair. The thread appears to be formed by the invagination of the surface structure of root hair into the cytoplasm, resulting in the formation of a tubular structure characterized by a wall (sheath) in the inside and cytoplasmic organelles on the outside. In this case major part of the root hair is vacuolized.
A number of bacteroids are also seen in the sheath. When they are present in root hair, any observable membrane structure cannot be recognized. As the infection thread extends into the epidermal tissue of the root, the bacteroids are demarcated with irregularly shaped membrane, and in a rudimentary root nodule they become ovoid being demarcated with cell membrane but not with cell wall.