Rhizobium-stimulated Callose Formation in Clover Root Hairs and its Relation to Infection

Abstract

Callose was detected in the cell walls of the tips of growing root hairs of Trifolium species and the non-legume Phleum pratense using u.v. fluorescence of fresh material stained with 0·005% aniline blue. Inoculation of the roots with Rhizobium trifolii, R. leguminosarum, R. meliloti, and R. japonicum, or addition of 10−7 and 10−8 M indole-3-acetic acid (IAA) increased tip callose formation. Most tip callose was formed at 12 °C, and amounts declined progressively at 18, 24, and 30 °C, with very little formed at 36 °C. Tip calloso usually became less and disappeared in individual root hairs as they aged. Callose which appeared prominently in the host cell walls at the points of initiation of infection threads did not usually disappear as the hairs matured. There was little or no extension of callose along the infection thread and none in the thread tip or in the cell nucleus. Presumptive regions of callose had similar structure and electron density as root hair wall material and were sometimes related to arrays of vesicles in the host cytoplasm. The external surface of the hair wall bore small pegs or papillae (0·1–0·2 μm) continuous with the outer layer of the wall and possibly associated with attachment of bacteria. Bacteria were usually umboriate at the point of attachment and their polyphosphate granules were much larger near the root hair than at the distal end.