The Gunnera-Nostoc Nitrogen-fixing symbiotic consortium
Chapman, M.J., Dornhaus, A., Margulis, L.
Department of Biology, University of Massachusetts, Amherst, MA, USA
We present an 8-minute videotape review of current knowledge on
Gunnera (monogeneric family Gunneraceae), the only
angiosperm known to form symbioses with cyanobacteria. Habit,
varying according to species, may be succulent creeping ground
cover (e.g., G. monoica) or polystelous giant herb (e.g.,
G. manicata or "poor man's umbrella"). High water demands
restrict Gunnera to boggy, acidic habitats which are often
low in combined nitrogen. Nitrogen-depleted environments selected
symbiotic associations of Gunnera with Nostoc
cyanobacteria. Coevolution has led to symbiotic organs (glands),
where 6-8 large specialized parenchyma cells, first visible in
cotyledon stage seedlings, occur in hemispherical concentric
arrangements paired at each node. Infection by motile
Nostoc (hormogonia) occurs via copious carbohydrate.rich
mucus produced my the glands. Hormogonia penetrate the cell
membranes, within which Nostoc differentiate to form
nonmotile photosynthetic cells. The Gunnera gland cells
are soon packed with Nostoc akinetes; the proportion of
nitrogen-fixing heterocysts relative to dividing photosynthetic
cells increases. In G. manicata, we have observed young
seedlings well populated with Nostoc while still attached
to the viviparous compound (400-500 fruits) spike. Hormogonia
from the soil may be transmitted to the spike via specialized
involucral bracts, present in the giant herbs but not in the
ground cover, and often coated with mucus. These bracts also
subtend petioles in the adult plant. Petiolar cross-sections
reveal polystelous anatomy with each vascular trace heavily
infected. Bracts may also play a role in
transmission of Nostoc to newly formed steles.
| LOCATION |
DATE |
TIME |
| Lecture Hall II |
Tursday, April 9 |
05:50 pm |