Harry E. Calvert

Energy transfer from phycobiliproteins to Photosystem I in vegetative cells and heterocysts of Anabaena Variabilis

The presence of phycobilins in heterocysts of Anabaena variabilis is established on the basis of absorption and fluorescence spectroscopy. At 77 K heterocysts exhibit fluorescence emission bands at 645 and 661 nm indicative of phycocyanin and allophycocyanin, respectively. Both allophycocyanin levels and fluorescence emission at 695 nm were low in heterocysts relative to whole filaments. In situ fluorescence microscopy confirmed the presence of phycobilins in individual heterocysts, but the pigment levels varied considerably among cells. Heterocysts exhibited Photosystem I activity, as evidenced by photooxidation of P-700, but no Photosystem II activity. The quantum efficiency of phycobilins in sensitizing P-700 photooxidation was 50-70% that of chlorophyll a. Phycoibins were also effective in promoting light-dependent reduction of acetylene to ethylene. The results are discussed in terms of the role of the heterocyst in nitrogen fixation and of the significance of energy transfer from phycobilins to Photosystem I in heterocysts.

RECOGNITION AND INFECTION BY SLOW-GROWING RHIZOBIA

A field isolate of R. japonicum has recently been isolated that may help to answer the question of whether or not rhizobia need to attach to the host root surface in some particular way in order to infect and nodulate. This field isolate, designated 1007, was obtained from a large nodule in the crown region of a soybean plant from a local field (S.J. Vesper, T.V. Bhuvaneswari, W.D. Bauer, in preparation).

Ultrastructural ontogeny of leaf cavity trichomes inAzolla implies a functional role in metabolite exchange

SummaryAnabaena azollae is associated with two types of multicellular epidermal trichomes inAzolla leaf cavities, the simple and branched hairs. The observation of transfer cell ultrastructure in some hair cells led to speculation that the cavity hairs might participate in metabolite exchange between the symbionts. The developmental ontogeny of cavity trichomes is described here, using transmission electron microscopy, with a goal of improving our understanding of possible functions of these structures in the symbiosis. The observations have established that all cells of simple and branched hairs develop the structural characteristics of transfer cells, but not simultaneously. Rather, there is an acropetal succession of transfer cell ultrastructure beginning in terminal cells, moving to body cells where present, and ending in stalk cells. The transfer cell stage is followed immediately by senescence in all hair cells. The timing of transfer cell differentiation, considered together with information from other studies, suggests that branched hairs may be involved in exchange of fixed nitrogen between the symbionts, while simple hairs may participate in exchange of fixed carbon fromAzolla toAnabaena.

Lichens to Gunnera — with emphasis on Azolla

N2-fixing cyanobacteria occur in symbiotic associations with fungi (ascomycetes) as lichens and with a few green plants. The associated cyanobacterium is always a species of Nostoc or Anabaena. Only a small number of plant genera are involved but there is a remarkable range of host diversity. Associations occur with several bryophytes (e.g. Anthoceros, Blasia, Cavicularia), a pteridophyte (Azolla), cycads (nine genera including Macrozamia and Encephalartos) and an angiosperm (Gunnera). Except for Gunnera, where the cyanobacterium penetrates the plant cells, the cyanobacteria are extracellular with specialized morphological modifications and/or structures of the host plant organs providing an environment which facilitates interaction with the prokaryote.

THE AZOLLA-ANABAENA SYMBIOSIS: MORPHOLOGY, PHYSIOLOGY AND USE

ABSTRACT Heterosporous ferns of the genus Azolla contain an N2-fixing cyanobacterium, designated Anabaena azollae, in specialized leaf chambers. Our studies of the symbiosis per se have been primarily restricted to A. caroliniana. However, comparative studies have encompassed four of the six known Azolla species: A flliculoides, A. mexicana and A. pinnata, in addition to A. caroliniana. The physiological, biochemical, morphological and ultrastructural studies which support a current hypothesis of host-symbiont interactions and structure-function relationships are described. The latter is largely based on recent studies of the ontogenetic sequence of leaf development and associated events in the symbiotic cyanobacterium. Under optimal growth conditions the four Azolla species double their biomass within two days and maintain an N content of 5–6% of the dry weight with N2 as the only N source. Results of subsequent comparative studies of photosynthesis, respiration, nitrogen fixation and associated processe...

Exposure of Cotyledons to Light Affects Rhizobium Infection and Nodule Development in Soybeans

In soybean seedlings the exposure of cotyledons/hypocotyls to light strongly affects the initiation and development of root nodules. Two day-old soybean seedlings in plastic growth pouches were inoculated with R. japonicum in very dim light. At the time of inoculation, the position of the root tip (RT) of each seedling was marked on the face of the pouch. Upper portion of seedlings (cotyledons/hypotocotyl) were exposed to light at various times before and after inoculation. The number and position of nodules on the primary roots relative to RT mark were scored 7–8 days after inoculation or first exposure to light whichever came last. A set of 100 seedlings were used for each treatment. For anatomical studies representative root samples fixed in paraffin were serially sectioned, stained and then examined under microscope to score the location and stage of infection development.