Development of Stem Nodules in a Tropical Forage Legume,Aeschynomene afraspera

Abstract

Rhizobial infection occurred on the stem of Aeschynomene afraspera at the site of emergence of adventitious root primordia. Rhizobia invaded cortical cells at the base of the root primordium. The first infected cell enlarged and collapsed after rhizobia had multiplied in large numbers. At this time, a meristematic zone was initiated some distance beneath the first infected cell. Rhizobial penetration into the deeper cortex was by progressive collapse of infected cells towards the meristematic zone; rhizobia entering the cortical cells by invagination of the host cell wall. At the entry point, rhizobia were embedded in digitate cell wall material. These infection structures were restricted, always originating from the cell wall of an adjacent infected cell. Soon after infection, the cell collapsed progressively forming infection strand-like structures which developed up to the meristematic zone. When infection had reached the meristematic zone, invaded host cells ceased to collapse but divided repeatedly to form the nodule.