ScienceWeek

PLANT BIOLOGY: ON RHIZOBIUM SYMBIOSIS

Rhizobium is a bacterial genus of motile rods that infect the roots of leguminous plants, the infection leading to the formation of root nodules in which gaseous nitrogen is converted to combined nitrogen. Initial penetration of the bacteria is by the root hairs, but Nod factors excreted by the bacteria induce curling of the root hairs and formation of cellulose tubes ("infection threads") by which the bacteria gain access to other root cells. Plant cell division is promoted by the Nod factors generating the nodule.

The following points are made by Clare Gough (Current Biology 2003 13:R973):

1) The symbiotic association between Rhizobia and legume plants leads to the formation of nitrogen-fixing nodules, conferring on host plants the ability to be grown without the addition of nitrogen fertilizer. This symbiosis is characterized by a high level of host specificity, mediated by specific recognition of rhizobial molecules called "Nod factors" [1]. These are lipochito-oligosaccharides, based on a backbone of generally four or five N-acetyl glucosamine residues which are N-acylated at the non-reducing end and carry various substitutions. The nature of the N-acyl chains and the other substitutions are important determinants of rhizobial host specificity.

2) The infection process that leads to the Rhizobium-legume symbiosis is controlled by the Nod factors; Rhizobia need them to be able to induce specialized structures called "infection threads", through which they penetrate into the cortex of host roots. Moreover, purified Nod factors are active in roots of host plants and at pico/nanomolar concentrations they can induce many of the responses characteristic of the bacteria themselves. These include both rapid, localized responses, such as calcium fluxes in root hair cells, and slower ones that encompass several cell layers of the root, for example the induction of gene expression and cell division that leads to the formation of nodule primordia. Nod factors thus act as symbiotic signalling molecules for conferring host specificity, for infection, and for nodule development, and are likely to be perceived by high affinity receptors [2]. The recent cloning of legume genes has revealed a new class of receptors containing LysM domains which function in Nod factor perception [3-5].

3) Genetic dissection of the mechanisms by which the Nod factor signal is perceived and transduced by host plants led to the identification of genes that control different steps of a Nod factor-activated signalling pathway. Some of these genes control both early steps of Nod factor signalling and the establishment of another, more ancient type of root endosymbiosis -- the arbuscular mycorrhizal symbiosis. Such "common symbiotic" genes define what is probably an ancient signalling pathway that has been recruited by legumes and Rhizobia and might be involved in transducing both fungal and rhizobial signals.

References (abridged):

1 Denarie, J., Debelle, F., and Prome, J.C. (1996). Rhizobium lipo-chitooligosaccharide nodulation factors: Signaling molecules mediating recognition and morphogenesis. Annu. Rev. Biochem. 65, 503-535

2 Cullimore, J.V., Ranjeva, R., and Bono, J.J. (2001). Perception of lipo-chitooligosaccharidic Nod factors in legumes. Trends Plant Sci. 6, 24-30

3 Madsen, E.B., Madsen, L.H., Radutiu, S., Olbryt, M., Rakwalska, M., Szczyglowski, K., Sato, S., Kaneko, T., Tabata, S., Sandal, N., and Stougaard, J. (2003). A receptor-kinase gene of the LysM type is involved in perception of rhizobial signals. Nature 425, 637-638

4 Radutoiu, S., Madsen, L.H., Madsen, E.B., Felle, H.H., Umehara, Y., Gruenlund, M., Sato, S., Nakamura, Y., Tabata, S., Sandal, N., and Stougaard, J. (2003). Plant recognition of symbiotic bacteria requires two LysM receptor-like kinases. Nature 425, 585-592

5 Limpens, E., Franken, C., Smit, P., Willemse, J., Bisseling, T., and Geurts, R. (2003). LysM domain receptor kinase regulating rhizobial Nod factor induced infection. Science 302, 630-633

Current Biology http://www.current-biology.com

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ORIGIN OF BIOLOGICAL NITROGEN FIXATION

The following points are made by R. Navarro-Gonzalez et al (Nature 2001 412:61):

1) Nitrogen is an essential element for life and is often the limiting nutrient for terrestrial ecosystems. Since most nitrogen is locked in the kinetically stable form N(sub2) in the Earth's atmosphere, processes that can fix nitrogen into biologically available forms -- such as nitrate and ammonia -- control the supply of nitrogen for organisms.

2) On the early Earth, nitrogen is believed to have been fixed abiotically as nitric oxide formed during lightning discharge. The advent of biological nitrogen fixation suggests that at some point the demand for fixed nitrogen exceeded the supply from abiotic sources, but the timing and causes of the onset of biological nitrogen fixation remains unclear.

3) The authors report an experimental simulation of nitrogen fixation by lightning over a range of Hadean (4.5 to 3.8 billion years ago) and Archaean (3.8 to 2.5 billion years ago) atmospheric compositions, from predominantly carbon dioxide to predominantly dinitrogen (but always without oxygen).

4) The results suggest that as atmospheric carbon dioxide decreased over the Archaean period, the production of nitric oxide from lightning discharge decreased by two orders of magnitude until about 2.2 billion years ago. After this time, the rise in oxygen (or methane) concentrations probably initiated other abiotic sources of nitrogen.

5) The authors suggest that although the temporary reduction in nitric oxide production may have lasted only 100 million years or less, this was potentially long enough to cause an ecological crisis that triggered the development of biological nitrogen fixation.

Nature http://www.nature.com/nature

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