'Natural' nitrogen-fixing bacteria protect soybeans from aphids | Penn State University
Rhizobia are bacteria that fix nitrogen (diazotrophs) after becoming established inside root nodules of legumes (Fabaceae). To express genes for nitrogen fixation, rhizobia require a plant host; they These groups include a variety of non-symbiotic bacteria. 12–20 million hectares of soybeans are inoculated annually. To Conclude the realtionship between the Soybean Plant and the Rhizobium Bacteria s a mutualistic relationship. Both benefit from each other. A classic example of mutualism is the relationship between insects that pollinate .. Legumes have a symbiotic relationship with bacteria called rhizobia, which create Soy Beans: Soy beans are a type of legume crop that rely on rhizobia.
Strains that fix only a small amount of N2, as opposed to none, might be considered either mutualistic or parasitic, depending on the relative value of carbon and nitrogen to a plant in particular circumstances.
Rhizobium strains that follow a third rhizobial strategy, forgoing symbiosis altogether, sometimes outnumber related symbiotic strains [ 137 ].
Why has not one of these three different rhizobial strategies displaced the others? All rhizobia spend some time in the soil, where they presumably compete for various resources. Symbiotic rhizobia, both mutualistic N2-fixers and parasitic nonfixers, compete for host plants in addition to soil resources. We might expect those rhizobium strains genetically programmed to fix N2 in symbiosis to displace the nonfixing or nonsymbiotic strains. But N2 fixation is an energetically expensive process, consuming resources that rhizobia could otherwise use for their own growth and reproduction.
Perhaps, therefore, we should expect nonfixing or nonsymbiotic rhizobia to displace N2-fixing mutualistic strains.
Instead, all three strategies persist, although their relative abundance varies among locations [ 1578 ]. Or does the relative fitness survival and reproduction of rhizobia following these three different strategies vary over time, allowing all three strategies to coexist? Before attempting to answer these questions, we will consider the merits of all three strategies, from the viewpoint of a single rhizobium cell. A key question is whether a symbiotic, N2-fixing rhizobium that lost one or more symbiotic genes due to mutation or loss of a symbiotic plasmid would thereby increase or decrease the expected number of descendants it would have in the soil a year later.
A single rhizobium cell that infects a soybean root may produce up to descendants inside a large nodule [ 910 ]. Even when some rhizobia inside some nodules lose the ability to reproduce, especially in nodules with indeterminate growth Fig.Isolating rhizobia from root nodules
If even a fraction of the rhizobia inside a nodule successfully escape into the soil, a rhizobium could produce many more descendants in the soil by founding a nodule than by remaining in the soil. Rhizobium numbers in nodules are much greater than shown, whereas their size is much less. Attempting to infect a legume root may carry significant risks for rhizobia, however.
First, the chances of success may be quite low. A soil in which a compatible host was last grown five years before can still contain 2. Furthermore, we suggest that aggregations of rhizobia around a root might attract high populations of predatory protozoa, which are a significant threat to rhizobia even in bulk soil [ 13 ].
'Natural' nitrogen-fixing bacteria protect soybeans from aphids
In addition, many rhizobia produce various antibiotics active against other rhizobia [ 14—16 ] and exposure to these antibiotics could be greater for rhizobia in the throng attempting to infect a legume root than it would be for those that remain in the bulk soil.
So is infecting plants a good strategy for rhizobia?
Spawning salmon would jump at those odds. But not all rhizobia inside a nodule escape alive into the soil. In nodules with indeterminate growth, as in alfalfa, reproductive viability is apparently limited mainly to those rhizobia that have not differentiated into the bacteroid form [ 11 ]. Some rhizobia may be digested by the plant during nodule senescence [ 17 ]. Although there is very little field-credible data on the microbiology of nodule senescence [ 18 ], dense populations of rhizobia escaping from senescing nodules might also be especially attractive to predators.
Considering all the risks and potential benefits of symbiosis for rhizobia, the number of descendants produced by the average symbiotic rhizobium cell over the course of a year could be less than that of a nonsymbiotic rhizobium, at least when populations of competing symbiotic rhizobia are high. For simplicity, assume an annual legume, with one opportunity per year for rhizobia to nodulate. The number of descendants produced by a symbiotic rhizobium averaging those near and far from the root would be the chance that it will succeed in founding a nodule, times the number of rhizobia released from an average nodule, times the fraction of those rhizobia that become established in the soil after nodule senescence Table 1.
If this product is greater than 1. Depending on conditions, population size might be more likely to overshoot and oscillate than to approach the equilibrium value of 1. Meanwhile, a nonsymbiotic rhizobium that divided every two months would produce 64 descendants in a year, assuming none died.
Actual generation times and survival rates for nonsymbiotic rhizobia in soil are not known.
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Root exudates would increase opportunities to reproduce in the rhizosphere, perhaps increasing the relative fitness of nonsymbiotic rhizobia. However, we assume that nonsymbiotic rhizobia would also be subject to some sort of density dependent population regulation. Symbiotic rhizobia have greater fitness than those that never infect legumes if: Symbiotic rhizobia are rare, there are more host plants, or less soil nitrogen Fewer symbiotic rhizobia and less soil N may favor nodule growth Rhizobia hoard PHB in nodule — but nodule may attract predators Among symbiotic rhizobia, those that fix N2 have greater fitness if: Some nonfixing rhizobia may be recognized and excluded Host sanctions favor N2-fixers, but maybe less if mixed nodules common Lingering effects of sanctions exceed benefit from maybe saving more PHB The table summarizes factors affecting these three parameters and how they, in turn, affect the relative fitness of symbiotic vs.
View Large Under what conditions would losing symbiotic genes decrease the expected reproductive success of a rhizobium?
Rhizobia - Wikipedia
As they introduce new crops into their soils, these inoculants may foster legume growth and success in the area, therefore giving farmers more options for planting. Using these inoculants provide many other benefits as well such as not having to use nitrogen fertilizers.
It has also been stated that "cereals were healthier and higher yielding when grown after a legume". Common crop and forage legumes are peas, beans, clover, and soy. Infection and signal exchange[ edit ] The formation of the symbiotic relationship involves a signal exchange between both partners that leads to mutual recognition and development of symbiotic structures.
The most well understood mechanism for the establishment of this symbiosis is through intracellular infection.
Rhizobia are free living in the soil until they are able to sense flavonoidsderivatives of 2-phenyl This is followed by continuous cell proliferation resulting in the formation of the root nodule. In this case, no root hair deformation is observed. Instead the bacteria penetrate between cells, through cracks produced by lateral root emergence. Ammonium is then converted into amino acids like glutamine and asparagine before it is exported to the plant.
This process keeps the nodule oxygen poor in order to prevent the inhibition of nitrogenase activity. Nature of the mutualism[ edit ] The legume—rhizobium symbiosis is a classic example of mutualism —rhizobia supply ammonia or amino acids to the plant and in return receive organic acids principally as the dicarboxylic acids malate and succinate as a carbon and energy source.
However, because several unrelated strains infect each individual plant, a classic tragedy of the commons scenario presents itself. Cheater strains may hoard plant resources such as polyhydroxybutyrate for the benefit of their own reproduction without fixing an appreciable amount of nitrogen. The sanctions hypothesis[ edit ] There are two main hypotheses for the mechanism that maintains legume-rhizobium symbiosis though both may occur in nature.
The sanctions hypothesis theorizes that legumes cannot recognize the more parasitic or less nitrogen fixing rhizobia, and must counter the parasitism by post-infection legume sanctions. In response to underperforming rhizobia, legume hosts can respond by imposing sanctions of varying severity to their nodules. Within a nodule, some of the bacteria differentiate into nitrogen fixing bacteroids, which have been found to be unable to reproduce.