How does hornwort benefit from cyanobacteria?
By Chelsie Vandaveer
February 19, 2004
Suggested Reading: Click here.
Hornworts are members of the Anthocerophyta, plants that together with liverworts and mosses are classified as bryophytes. Hornworts are easy to miss; the leafy haploid gametophytes are only a couple of centimeters across, the spiky diploid sporophytes are only a centimeter or two tall. The sporophyte depends upon its parent, the gametophyte, for life. (See Plant of the Week, February 16, 2004)
The gametophyte is attached to the surface of the soil by rhizoids, tiny root-like structures. The rhizoids function mostly to hold the gametophyte to the soil and, unlike roots, have little to do with the uptake of water and nutrients. The hornwort depends upon a symbiont, Nostoc, a nitrogen-fixing cyanobacteria (blue-green algae).
According to David G. Adams (2002), the cyanobacteria inhabit a mucilaginous or slime cavity within the gametophyte. But Nostoc does not end up there by chance. When the gametophyte has insufficient combined nitrogen (nitrogen in nitrate form, NO3) to carry on
photosynthesis, the plant 'invites' the Nostoc to enter.
Nostoc has two forms: a long-filament, immobile, nitrogen-fixing heterocyst and a short-filament, mobile, infective hormogonium. When the hornwort needs nitrogen, it releases a chemical called the hormogonium-inducing factor (HIF) into its immediate environment. The stationary heterocysts in the vicinity begin producing hormogonia that glide toward the hornwort. The hormogonia slide under the hornwort and enter through narrow pores into the slime cavity.
But the hormogonia do not fix atmospheric nitrogen for use by the plants. The hornwort needs the heterocysts. Unless induced, the hormogonia continue reproducing as hormogonia prolonging the stage where the cyanobacteria are capable of traveling and colonizing. Once infected, the hornwort produces another chemical, hormogonium-repressing factor (HRF). The HRF is thought only released into the slime cavity; the hormogonia start producing heterocysts.
Plant of the Week 02/16/2004
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Hornwort (Anthoceros)
Hornworts have an alternation of generations; one generation, the gametophyte, is haploid with a single set of chromosomes. It appears as a leafy thallus lying close to the soil. The leafy gametophyte is the light green plant near the center of the photograph. The gametophyte gives rise to gametes, the sex cells (sperm and eggs). The sperm cells are produced in a structure called the antheridium. They are flagellated and must swim on a film of water to the archegonium where an egg is produced.
Plant of the Week 02/16/2004
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The symbiotic relationship changes both partners. The cells of the hornwort grow larger and round. The Nostoc heterocysts do not form a thickened cell wall and lose the ability to fix CO2 to make sugar and starch. The hornwort gets its nitrogen from the Nostoc and the Nostoc gets its carbon from the hornwort.
(Compiled from: "Chapter 15: Bryophytes", Biology of Plants, P. Raven, R. Evert, and S. Eichhorn, 5th Edition, Worth Publishers, 1992; "Introduction to Anthcerophyta, The Hornworts" University of California, Berkeley, Museum of Paleontology, 2004; and "The Liverwort-Cyanobacterial Symbiosis", David G. Adams, Biology and Environment: Proceedings of the Royal Irish Academy, Vol. 102B, No.1, 2002)
Suggested Reading:
Zamia herrerae Plant of the Week - August 13, 2001
What is a mermaid's wineglass? Weird Plants - May 30, 2002
How will you use red algae today? Plants that Changed History - July 16, 2002
What 'plant' is not a plant? Weird Plants - July 18, 2002
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