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The Cycad Pages
CORALLOID ROOTS AND NITROGEN FIXATION
Juliana Medeiros and Dennis Stevenson

Future areas of study

The future of research in cycad/cyanobacteria symbiosis is wide open. Rai et al. (2000) cite several questions in this area still open to investigation, noting that even the basic physiology of the symbionts is at best poorly understood. Possibilities include: role of carbon nutrition in regulation of heterocyst differentiation; the dynamics and details of nutrient transfer including carbon, nitrogen, vitamins, minerals growth factors, hormones and trace elements; the way in which structural/functional interactions proceed toward maturity along the age gradient of coralloids; details of cycad/cyanobacterial sensing and signaling; effects of microaerobiosis on cyanobacterial cell division, growth, heterocyst differentiation, nitrogen fixing and nitrogen metabolism; elucidation of changes in the pathway of carbon fixation and assimilation in cyanobacteria; and investigation into weather cycad defenses are suppressed to allow entry into coralloids or, conversely, how cyanobacteria might evade defenses to gain entry. In addition, genetic research is needed in order to identify the cyanobacterial species involved in symbioses, the symbiotic genes and gene products, possible plasmids bearing symbiotic genes, and the potential involvement of gene rearrangement in cyanobacteria and cycads under symbiotic conditions. Developing a clear understanding of nitrogen fixing symbioses in nature is also the key to understanding why these types of associations are not universal.

Another possible area of future research is the possible presence of haemoglobin genes in plants. Legume nodules containing nitrogen fixing Rhizobium contain leghaemoglobin. Plants in symbiosis with the nitrogen fixing actinomycete Frankia have also been shown to contain haemoglobin (Tjepkema, 1983). More recently, Appleby et al. (1989) found evidence that haemoglobin is present in the root tissue of Trema and Parasponia (Ulmaceae), which do not form symbiotic relationships with any known nitrogen fixing bacteria. These haemoglobins are analogous to those found in animals. In legumes they are assumed to transport oxygen out of root nodules, providing a microaerobic site for nitrogenase. Actinorhizal plants nodulated by Frankia, however, have other structural features which protect nitrogenase from oxygen, and the role of haemoglobin in these roots is poorly understood. Similarly, the function of haemoglobin found in non-nitrogen fixing Ulmaceae is unknown. They suggest that haemoglobin genes may be present in all plants, where they serve an unidentified physiological function, and that haemoglobin genes might have been present in the ancestral organisms which gave rise to both plants and animals. Previously, it has been proposed that haemoglobin genes were transferred to plants such as legumes from animals. Appleby et al. (1989) note that investigations into the presence or absence of haemoglobin in evolutionarily primitive plants such as monocots, pteridophytes (ferns), bryophytes (mosses and liverworts) and gymnosperms (pines and cycads) will help to answer this question.



The Cycad Pages

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Written and maintained by Ken Hill 1998-2010
Maintained by Leonie Stanberg and Dennis Stevenson 2010-2012
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