The bacterium-filled nodules found on legumes represent a mutually beneficial arrangement. But it is evidently one with sophisticated checks and balances to ensure a fair deal for both partners in the marriage.
Some soil bacteria live in apparent harmony with plant cells in a mutually beneficial arrangement. The bacteria can reduce nitrogen gas, 'fixing' it into forms that the plants can use; in return, the plant cells provide the bacteria with products of photosynthesis. On page 722 of this issue1, Lodwig and co-workers describe an exchange-control system that enables the two partners to share their resources without either one becoming dominant.
The enzyme complex involved in nitrogen fixation is nitrogenase, which is ancient and widespread among bacteria. Nitrogenase can use a variety of substrates, but its main role in today's world is the production of ammonia (NH3) from nitrogen gas (N2).
Whereas free-living nitrogen-fixing bacteria use ammonia for their own growth, those living in symbiosis with other organisms, such as in the nodules on roots of pea and bean plants, normally hand it over to their host in the form of ammonium ions, in exchange for products of photosynthesis that are used to provide the energy for nitrogen reduction2. Why should these bacteria behave so altruistically, when by so doing they lose their own source of amino acids?
Lodwig et al. propose an answer. Using plants of the garden pea, they induced the formation of root nodules containing either wild-type or mutant nitrogen-fixing bacteria (known collectively as rhizobia). Through analysis of these nodules, they could then separate and dissect the processes of nitrogen reduction, assimilation of ammonium into amino compounds, and transport between the two partners. To the authors' surprise, the host plant cells could not assimilate ammonium when they were nodulated by bacterial mutants in which amino-acid transport was blocked.
There were two aspects to this observation. First, mutants that could fix nitrogen at rates comparable to the wild-type bacteria could not pass the products on to the host cell unless they were supplied with an amino acid, probably glutamate, by the host cell. Second, the host cell could not assimilate ammonium from bacteria unless it was also supplied with another amino acid, aspartate. Lodwig et al. propose that these two transport systems may have distinct functions in symbiosis (see Fig. 4 on page 725). One serves to import glutamate from plant to bacteria, and the other to export aspartate from bacteria to plant. So each side can impose a sanction on the other, by withholding a vital amino acid. If this circuit is in place, bacteria can export ammonium and ensure both their own amino-acid supply and that of their host. Thus, both sides have a strong interest in maintaining the marriage.
Before a host plant accepts bacteria into this intimate association, an intricate dialogue occurs between the two partners, which tests their mutual compatibility. Events begin in the soil, when plants and rhizobia exchange signals. They proceed via 'infection pathways' and nodule development (Fig. 1 shows a variety of nodule types). And they culminate in the formation of symbiotic units such as those studied by Lodwig et al.1. But does this courtship always end in harmony? Unfortunately not.
Problems may occur at any stage, and two are illustrated by the work of Lodwig et al. First, with bacterial mutants that can induce nodulation but cannot allow ammonium assimilation, numerous small, 'ineffective' nodules result, typical of those sometimes found in nature. In this case, host control over the number of nodules produced3 is depressed. Second, mutants that cannot effectively use the host products of photosynthesis to fuel nitrogen fixation may store those products in the form of the polymer polyhydroxybutyrate (PHB). This polymer accumulates naturally in bacteria of certain nodules, most notably those of soybean, but much less so in their close relatives, such as Phaseolus vulgaris (French bean, navy bean) or species of Vigna (cowpea, green gram). Does this mean that soybean nodules and their bacteria are less well matched? Or does PHB have another function4? These are just two of the questions raised by the new results1.
More broadly, other issues arise when we look at the full landscape of nodulation processes. Our detailed knowledge of nodulation comes from just a few species of the more highly evolved legumes, mainly from temperate or sub-tropical regions. But legumes are the third largest family of flowering plants, and nodulation has arisen in them on several separate occasions during evolution; many woody species still lack this ability5. There are thus wide variations in the specificity and strength of the association with rhizobia, especially in the tropics (Fig. 2).
Many interactions lack the close co-evolution of host and bacteria that leads to the highly effective recognition and developmental processes evident in pea and most temperate species. A single host species may be nodulated by several different genera and species of bacteria5, with bacteria inside the nodules varying from the essentially parasitic to the highly effective in delivering ammonia6. Similarly, a single strain of bacterium may nodulate many genera and species of legume. Bacteria that induce nodules are now known to be far more heterogeneous than once thought, with many having close relationships with plant or animal pathogens — even to the extent of being members of the same genus, as occurs, for example, with species of Burkholderia and Ralstonia. Symbiotic and pathogenic relatives may have similar ways of avoiding their host's defence responses7. Genetic exchange between bacteria in soil may lead to some species losing the genes determining symbiosis and nitrogen fixation, and others gaining these genes8. We can expect many 'new' nodulating bacteria to be found in the future.
When coupled with the impressive range of techniques for studying whole genomes of legumes and other organisms9, and the detailed literature on the various steps in nodulation2, highly targeted work such as that of Lodwig et al. will deepen our understanding of how nitrogen-fixing symbioses function. If this is extended to other legumes and other nodulating bacteria, exciting prospects are raised for answering questions ranging from why some legumes cannot nodulate to what distinguishes a pathogen from a symbiont. Above all, perhaps, given their agricultural importance, a better understanding of tropical legumes will assist the management of nitrogen fixation in those areas of the world that need it most.
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