The Azolla–Anabaena symbiosis: endophyte continuity in the Azolla life‐cycle is facilitated by epidermal trichomes †

Abstract

SUMMARYLight, scanning electron, and transmission electron microscopy were used to follow sporocarp development in Azolla mexicana and the mechanism by which sporocarps become inoculated with symbiotic Anabaena. Packaging of the Anabaena into sporocarps is facilitated by branched epidermal trichomes, termed sporangial pair hairs. Sporocarps arc borne in pairs and arise from the ventral lobe initial of the first leaf of a lateral branch. Four sporangial fours ore associated with each sporangial pair from the onset of their development. These hairs differentiate rapidly and exhibit transfer cull ultrastructure. They protrude into the mass of undifferentiated Anabaena filaments which comprise the apical colony. Subsequently, the sporangial primordia and the sporangial hairs with in ten wined Anabaena filaments are displaced ventrally and covered by the developing involucre. A reservoir of Anabaena filaments is thus entrapped between the developing sporocarp pair and the lower surface of the involucre, A cup‐shaped indusium forms around the base of each sporangium, separating it from the sporangial hairs while entrapping the Anabaena filaments. As development continues, the indusia completely encapsulate the individual sporangia and engulf the Anabaena which is differentiating into akinetes. In mature megasporocarps the Anabaena cells are packaged into a chamber near the indusial pore and a cluster of simple trichomes occur external to the indusium around their base.The partner tabes of the developing sporangia form modified leaf cavities. These cavities are smaller than those in dorsal lobes not partnered with sporangia and they have a diminished complement of berth simple cavity trichomes and Anabaena. The Anabaena filaments in these modified cavities rapidly begin to differentiate akinetes rather than heterocysts. Factors which may contribute to akinete formation in the sporocarps and modified leaf cavities are discussed.