Resource and competitive dynamics shape the benefits of public goods cooperation in a plant pathogen

Abstract

Cooperative benefits depend on a variety of ecological factors. Many cooperative bacteria increase the population size of their groups by making a public good available. Increased local population size can alleviate the constraints of kin competition on the evolution of cooperation by enhancing the between-group fitness of cooperators. The cooperative pathogenesis of Agrobacterium tumefaciens causes infected plants to exude opines--resources that provide a nearly exclusive source of nutrient for the pathogen. We experimentally demonstrate that opines provide cooperative A. tumefaciens cells a within-group fitness advantage over saprophytic agrobacteria. Our results are congruent with a resource-consumer competition model, which predicts that cooperative, virulent agrobacteria are at a competitive disadvantage when opines are unavailable, but have an advantage when opines are available at sufficient levels. This model also predicts that freeloading agrobacteria that catabolize opines but cannot infect plants competitively displace the cooperative pathogen from all environments. However, we show that these cooperative public goods also promote increased local population size. A model built from the Price Equation shows that this effect on group size can contribute to the persistence of cooperative pathogenesis despite inherent kin competition for the benefits of pathogenesis.

Figure 1

Path diagram of factors influencing the fitness of cooperative trait that affects the supply of resources. Terms over straight arrows describe the partial regression coefficients of the variables connected by the corresponding arrow, while the term over the curved arrow represents the covariance between the source variables. Changes in the supply of resources can affect population size in elastic populations, which in turn can influence fitness.

Figure 2

Model predictions of resource competition for opines and a substitutable nitrogen resource between virulent pTi⁺ and either avirulent pTi⁻ (A and B) or cheater (C) strains. In (A) pTi⁺ experiences a low cost associated with bearing the plasmid; whereas in (B) this cost is higher. The solid lines represent the lowest amounts of resource supporting persistence of the indicated strains. Because, the pTi⁻ strain does not bear the cost of the plasmid (Appendix B, Platt et al. 2011) and therefore is able to persist at lower nitrogen concentrations, the pTi⁻ line crosses the x-axis at a lower value than that of the pTi⁺. Only pTi⁺ cells can persist on opine alone and so only its isocline intersects the opine axis. We assume that pTi⁺ cells utilize both opines and other nitrogen sources less efficiently than pTi⁻ cells utilize non-opine nitrogen sources ( $u_{{\mathit{pTi}}^{-}}^A>u_{{\mathit{pTi}}^{+}}^A=u_{{\mathit{pTi}}^{+}}^O$), reflecting the costs associated with harboring the Ti plasmid. (C) Agrobacterial cells bearing a cheater plasmid, which do not pay costs associated with virulence but can catabolize opines, are predicted to locally outcompete virulent agrobacteria regardless of the environment’s supply of opines and non-opine nitrogen resources. We assume that pTi⁺ cells utilize both resource types less efficiently than cheater cells ( $u_{\mathit{cheat}}^A>u_{{\mathit{pTi}}^{+}}^A$ and $u_{\mathit{cheat}}^O>u_{{\mathit{pTi}}^{+}}^O$), reflecting the costs associated with being able to infect host plants. The vectors represent the resource consumption of the indicated strains. Each point in the resource space represents environmentally determined supply rates of opine and other nitrogen sources. For simplicity, we assume that all strain types encounter with the same probability ( $e_{{\mathit{pTi}}^{+}}^A=e_{{\mathit{pTi}}^{-}}^A=e_{{\mathit{pTi}}^{+}}^O$) and have the same handling time for ( $h_{{\mathit{pTi}}^{+}}^A=h_{{\mathit{pTi}}^{-}}^A=h_{{\mathit{pTi}}^{+}}^O$) all resources that they catabolize. Similarly all strains experience the same mortality rate (m_pTi⁺ = m_pTi⁻), while both resources are assumed to be supplied at the same concentration (A₀ = O₀) and flow out of the competitive arena at the same rate (D_A = D_O).

Figure 3

Model predictions for relationship between non-opine nitrogen supply (A) or opine nitrogen supply (B) and equilibrium population size for pTi⁺ and pTi⁻ cells. Parameter values are the same as in Figure 2A.

Figure 4

Strain 15955 (pTi⁺) can utilize octopine as the sole source of carbon, nitrogen, or both carbon and nitrogen supporting population growth. However strain TGP101 (pTi⁻) cannot grow on octopine. As a control showing that this population growth depends on octopine supplementation, neither strain grew when octopine was not provided as either the sole carbon or sole nitrogen source. Values represent mean ± standard error of three replicates.

Figure 5

The fitness of cells harboring the Ti plasmid increased as the level of available octopine increased (t = 18.35, p < 0.0001). Utilizable nitrogen levels limited growth of the population and was available in the form of (NH₄)₂SO₄ and to varying levels octopine. Both pTi⁻ and pTi⁺ cells catabolized the (NH₄)₂SO₄, however only pTi⁺ cells could catabolize octopine. Values are LS mean ± standard error of six replicates.

Figure 6

Fitness benefits of opine catabolism. Cells bearing the Ti plasmid became increasingly more common in populations that achieved the highest population density (A) (t = 19.46, p < 0.0001). The competitive advantage of cells bearing the Ti plasmid likely stems from their ability to grow to higher population numbers in response to increasing octopine availability (B) (t = 22.37, p < 0.0001). In contrast, cells lacking the Ti plasmid achieved their lowest population numbers when octopine availability was highest (C) (t = −4.53, p < 0.001). This result likely reflects the high numbers of pTi⁺ cells present driving the ammonium levels down such that the pTi⁻ cells effectively experienced lower nitrogen availability. Note that because pTi⁺ cells attained much larger population sizes than did pTi⁻ cells, the scale of the y-axes differs between panels B and C.

Figure A1

A. tumefaciens R10 cells harboring pTiR10 had a competitive advantage over pTi cured derivatives when octopine was present, but were at a competitive disadvantage when octopine was absent. Values represent mean ± standard error of eight replicates.

Figure A2

The carrying capacity of the pTi⁺ (R10), but not the pTi⁻ (KYC55), population increased with opine availability. Values represent mean ± standard error of eight replicates.

Figure A3

A. tumefaciens 15955 population size increases with octopine availability. Prior to octopine supplementation nitrogen (A) or carbon (B) limits population growth.