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. 2007 Dec 22;274(1629):3119-26.
doi: 10.1098/rspb.2007.1187.

Human selection and the relaxation of legume defences against ineffective rhizobia

E Toby Kiers  1 , Mark G Hutton, R Ford Denison
Affiliations

Human selection and the relaxation of legume defences against ineffective rhizobia

E Toby Kiers et al. Proc Biol Sci. .

Abstract

Enforcement mechanisms are thought to be important in maintaining mutualistic cooperation between species. A clear example of an enforcement mechanism is how legumes impose sanctions on rhizobial symbionts that fail to provide sufficient fixed N2. However, with domestication and breeding in high-soil-N environments, humans may have altered these natural legume defences and reduced the agricultural benefits of the symbiosis. Using six genotypes of soya beans, representing 60 years of breeding, we show that, as a group, older cultivars were better able to maintain fitness than newer cultivars (seed production) when infected with a mixture of effective and ineffective rhizobial strains. Additionally, we found small differences among cultivars in the ratio of effective:ineffective rhizobia released from their nodules, an indicator of future rhizobial strain fitness. When infected by symbionts varying in quality, legume defences against poor-quality partners have apparently worsened under decades of artificial selection.

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Figures

Figure 1
Figure 1
Yield differences of six cultivars inoculated with either (a) effective inoculum or (b) 1 : 1 mixture of effective and ineffective inoculum (mixed inoculum). To compare the inoculum effects while standardizing for base yield differences, values were calculated as difference (increase or decrease) from replicated control plots in which cultivars were grown with no inoculum. Means within a graph followed by the same letter are not statistically different (Tukey HSD, p<0.05).
Figure 2
Figure 2
(a) Yield difference ratios of cultivars under a 1 : 1 mixture of effective : ineffective rhizobia (mixed inoculum). Ratios were calculated for each cultivars as yield difference (see figure 1) in mixed treatment over yield difference for effective treatment: (mixed yield-control)/(effective yield-control). Dotted line at 1.0 indicates a ratio in which symbiotic benefits are maximized under mixed treatment conditions. (b) Absolute field yields of cultivars under a 1 : 1 mixture of effective : ineffective rhizobium treatment (mixed inoculum). Legend same as figure 1.
Figure 3
Figure 3
(a) Proportion of effective rhizobia from nodules of four cultivars of pouch-grown plants inoculated with a 1 : 1 mixture of effective : ineffective rhizobia. Nodules were harvested and pooled from each root system, sterilized and crushed to determine the proportion of effective to ineffective rhizobia per plant and (b) total nitrogen (N) per plant at harvest in mixed-inoculum treatment. Means within a graph followed by the same letter are not statistically significant (Tukey HSD p<0.05). Legend key is the same as figure 1.
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