| BEN |
| BOTANICAL ELECTRONIC NEWS |
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| ISSN 1188-603X |
| No. 223 May 4, 1999 | aceska@victoria.tc.ca | Victoria, B.C. |
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Haemoglobin is most commonly known as a protein that binds oxygen in the blood of animals and transports and releases the oxygen to respiring tissues. However, haemoglobins are in fact ubiquitous in most organisms, including not only animals, but also bacteria, protozoans, fungi and plants, where often their functions have not been clarified.
Only in recent years has it become apparent that haemoglobin proteins probably exist in all plants. These can be broadly grouped into symbiotic and nonsymbiotic haemoglobins. The first group has been characterised since the 1930s and include the well studied `leghaemoglobins' found in the root nodules of nitrogen-fixing plants. Nonsymbiotic haemoglobins, however, were only identified during the late 1980s in the tree family, Ulmaceae. They have since been studied in a range of dicots including the legumes; monocot species from barley and wheat to maize and rice and in the chloroplasts of algae.
All haemoglobins found across the spectrum of living organisms can reversibly bind oxygen. Symbiotic haemoglobins are synthesised in proximity to the plant's symbiotic partners and in large amounts. The protein readily gives up oxygen and acts very well in transporting and regulating oxygen supply to the plant's microorganism associates. Nonsymbiotic haemoglobins differ markedly in gene homology from the symbiotic and have been detected in a range of tissues, often only under stress situations and in much lower amounts than found for symbiotic haemoglobin. Also, they generally show remarkably high affinity for oxygen in that they will bind the molecule and keep it even at very low cellular levels of oxygen. These characteristics discount a role in facilitative oxygen transport and supply but leaves the possibility of haemoglobin acting as an oxygenase in a biochemical reaction. The question which remains is "What is their exact function?"
To this end the work on the barley haemoglobin provides some insight. It is synthesised in flooded roots and can be induced in isolated aleurone layers when atmospheric oxygen is reduced to 5%. It is also induced in the normally germinating caryopsis, in both the embryo and aleurone, and in the root and shoots of the young seedlings. In aleurones it has been shown that this induction is a consequence of lowered energy (i.e. ATP) levels. It appears therefore that at least barley haemoglobin is required during times when energy demands of the cell can not be met by the availability of oxygen for normal respiration. This hypothesis is further evident with the observation that maize cells, genetically engineered to overproduce barley haemoglobin, better maintain their energy levels when oxygen availability is compromised. If the cells are engineered to eliminate haemoglobin, then they are unable to maintain energy levels under the same conditions. This data provides valuable clues to the function and mode of action of nonsymbiotic haemoglobins in general.
The ubiquitous occurrence of nonsymbiotic haemoglobins in plants indicates some fundamental roles in plant survival. From the data for barley haemoglobin a role during transient waterlogging or ice encasement can be envisaged, where oxygen availability is low and there is a need to maintain energy levels. The plant can resort to fermentation, or anaerobic ATP production, but in doing so produces toxic compounds such as ethanol. Haemoglobin may provide an alternative or complimentary route. This would provide a plant able to synthesise haemoglobin with a distinct selective advantage. As more data accumulates on other nonsymbiotic haemoglobins, other roles may become apparent.
For more comprehensive reviews see:
I was wrong to cite the ICBN [International Code of Botanical Nomenclature], Article 57 of the Tokyo Code from 1993, when I suggested that the name "Scirpus americanus" should be rejected. At the time when Schueler made the typification of Scirpus americanus, the ICBN did not have this article. The Seattle Code (1969), effective at the time said: "A name is to be rejected if it is used in different senses and so has become a long-persistent source of error." The example included in Article 69 implied that you needed the name to be "applied almost equally" to two different species before it was to be rejected.
The next version of the ICBN, the Leningrad Code (1975), was already phrased differently. Again, Leningrad Code Article 69: "A name must be rejected if it has been widely and persistently used for a taxon not including its type. Names thus rejected shall be placed on a list of nomina rejicienda." Here the list of Nomina Rejicienda for species names was first established. Had the Intermountain Flora (published in 1977 and probably the first major Flora that accepted the application of the name of Scirpus americanus for "S. olneyi") followed that rule, it would have used Scirpus pungens and Scirpus olneyi instead.
Retaining the name "Scirpus americanus" will lead to further confusion, and the only logical way to minimize the damage is to propose that name for rejection. This action, however, requires a formal proposal to the Nomenclatural Committee of the International Association of Plant Taxonomists. If we reject "Scirpus americanus" we will use S. olneyi and S. pungens without any confusion. At the same time, all other combinations based on "Scirpus americanus" will be rejected too. When we treat this group of Scirpus s.lato as Schoenoplectus, we will use Schoenoplectus olneyi (A. Gray) Palla and Schoenoplectus pungens (Vahl) Palla.
Two major international databases of biological and agricultural literature are now available for free on WWW:
AGRICOLA, including both the journal article database from 1979 to present, and the catalog of the USDA National Agricultural Library:
AGRIS, the FAO agricultural journal article database: