Bacterial Endophytes Enhance Competition by Invasive Plants

Premise of the study: Invasive plants can alter soil microbial communities and profoundly alter ecosystem processes. In the invasive grass Sorghum halepense, these disruptions are consequences of rhizome-associated bacterial endophytes. We describe the effects of N2-fixing bacterial strains from S. halepense (Rout and Chrzanowski, 2009) on plant growth and show that bacteria interact with the plant to alter soil nutrient cycles, enabling persistence of the invasive. Methods: We assessed fluxes in soil nutrients for ~4 yr across a site invaded by S. halepense. We assayed the N2-fixing bacteria in vitro for phosphate solubilization, iron chelation, and production of the plant-growth hormone indole-3-acetic acid (IAA). We assessed the plant’s ability to recruit bacterial partners from substrates and vertically transmit endophytes to seeds and used an antibiotic approach to inhibit bacterial activity in planta and assess microbial contributions to plant growth. Key results: We found persistent alterations to eight biogeochemical cycles (including nitrogen, phosphorus, and iron) in soils invaded by S. halepense. In this context, three bacterial isolates solubilized phosphate, and all produced iron siderophores and IAA in vitro. In growth chamber experiments, bacteria were transmitted vertically, and molecular analysis of bacterial community fingerprints from rhizomes indicated that endophytes are also horizontally recruited. Inhibiting bacterial activity with antibiotics resulted in significant declines in plant growth rate and biomass, with pronounced rhizome reductions. Conclusions: This work suggests a major role of endophytes on growth and resource allocation of an invasive plant. Indeed, bacterial isolate physiology is correlated with invader effects on biogeochemical cycles of nitrogen, phosphate, and iron

Community Impacts of Prosopis Juliflora Invasion: Biogeographic and Congeneric Comparisons

We coordinated biogeographical comparisons of the impacts of an exotic invasive tree in its native and non-native ranges with a congeneric comparison in the non-native range. Prosopis juliflora is taxonomically complicated and with P. pallida forms the P. juliflora complex. Thus we sampled P. juliflora in its native Venezuela, and also located two field sites in Peru, the native range of Prosopis pallida. Canopies of Prosopis juliflora, a native of the New World but an invader in many other regions, had facilitative effects on the diversity of other species in its native Venezuela, and P. pallida had both negative and positive effects depending on the year, (overall neutral effects) in its native Peru. However, in India and Hawaii, USA, where P. juliflora is an aggressive invader, canopy effects were consistently and strongly negative on species richness. Prosopis cineraria, a native to India, had much weaker effects on species richness in India than P. juliflora. We carried out multiple congeneric comparisons between P. juliflora and P. cineraria, and found that soil from the rhizosphere of P. juliflora had higher extractable phosphorus, soluble salts and total phenolics than P. cineraria rhizosphere soils. Experimentally applied P. juliflora litter caused far greater mortality of native Indian species than litter from P. cineraria. Prosopis juliflora leaf leachate had neutral to negative effects on root growth of three common crop species of north-west India whereas P. cineraria leaf leachate had positive effects. Prosopis juliflora leaf leachate also had higher concentrations of total phenolics and L-tryptophan than P. cineraria, suggesting a potential allelopathic mechanism for the congeneric differences. Our results also suggest the possibility of regional evolutionary trajectories among competitors and that recent mixing of species from different trajectories has the potential to disrupt evolved interactions among native species

Community Impacts of <em>Prosopis juliflora</em> Invasion: Biogeographic and Congeneric Comparisons

<div><p>We coordinated biogeographical comparisons of the impacts of an exotic invasive tree in its native and non-native ranges with a congeneric comparison in the non-native range. <em>Prosopis juliflora</em> is taxonomically complicated and with <em>P. pallida</em> forms the <em>P. juliflora</em> complex. Thus we sampled <em>P. juliflora</em> in its native Venezuela, and also located two field sites in Peru, the native range of <em>Prosopis pallida.</em> Canopies of <em>Prosopis juliflora</em>, a native of the New World but an invader in many other regions, had facilitative effects on the diversity of other species in its native Venezuela, and <em>P. pallida</em> had both negative and positive effects depending on the year, (overall neutral effects) in its native Peru. However, in India and Hawaii, USA, where <em>P. juliflora</em> is an aggressive invader, canopy effects were consistently and strongly negative on species richness. <em>Prosopis cineraria</em>, a native to India, had much weaker effects on species richness in India than <em>P. juliflora</em>. We carried out multiple congeneric comparisons between <em>P. juliflora</em> and <em>P. cineraria</em>, and found that soil from the rhizosphere of <em>P. juliflora</em> had higher extractable phosphorus, soluble salts and total phenolics than <em>P. cineraria</em> rhizosphere soils. Experimentally applied <em>P. juliflora</em> litter caused far greater mortality of native Indian species than litter from <em>P. cineraria</em>. <em>Prosopis juliflora</em> leaf leachate had neutral to negative effects on root growth of three common crop species of north-west India whereas <em>P. cineraria</em> leaf leachate had positive effects. <em>Prosopis juliflora</em> leaf leachate also had higher concentrations of total phenolics and L-tryptophan than <em>P. cineraria,</em> suggesting a potential allelopathic mechanism for the congeneric differences. Our results also suggest the possibility of regional evolutionary trajectories among competitors and that recent mixing of species from different trajectories has the potential to disrupt evolved interactions among native species.</p> </div

Total phenolic content (mean + SE) of soil amended with <i>Prosopis cineraria</i> (gray bar) and <i>P. juliflora</i> (black bar) leaf leachates and unamended soil (control, white bar). (A)

<p>Different letters above bars indicate significant differences (ANOVA, post-ANOVA Tukey’s test; p<0.05). <b>(B)</b> Total phenolic content (mean ± SE) of soil treated with no litter (white circles), <i>P. cineraria</i> (gray circles) or <i>P. juliflora</i> leaf litter (black circles) at rate of 12 mg/g soil, and incubated at 30–34°C under 12 h/12 h light/dark cycle for 0, 1, 2, 3, 4, 6, 8, 10 and14 days.</p

<i>Prosopis juliflora</i> in its native range of Venezuela (A); the invaded range of Haryana, India (B), along the National Highway to Rajasthan (C), at the boundaries of an agricultural field in India (D), and in Hawaii, USA (E); <i>Prosopis cineraria</i> in its native range, Rajasthan, India (F).

<p>Photo credits: Pascual J. Soriano (A); Inderjit (B, C, D and F) and Timothy J. Gallaher (E).</p

Root length (proportion of control, %) of <i>Brassica campestris</i> (upper panel), <i>B. juncea</i> (middle panel) and <i>Sorghum bicolor</i> (lower panel) seedlings grown in soil treated with different amounts (60, 150 and 300 µL/g soil) of <i>Prosopis cineraria</i> (gray bars) or <i>Prosopis juliflora</i> (black bars) leaf leachate.

<p>Soil treated with distilled water served as untreated control (white bars). Error bars represent +1SE of mean. F and P values are shown for two way ANOVAs.</p

Plant species richness (mean + SE) beneath canopies of <i>P. juliflora,</i> its native congener <i>P. cineraria</i>, and from open areas.

<p>Differences in plant species richness beneath <i>P. cineraria</i> and <i>P. juliflora</i> canopies and open areas were tested using one way-ANOVA and post-hoc Tukey’s test (p<0.05).</p

Chemical properties – pH (A), electrical conductivity (B), organic carbon (C), PO₄^3–Phosphorus (D), total organic nitrogen (E) and total phenolic content (F) of soil from the rhizosphere of <i>Prosopis juliflora</i>, <i>Prosopis cineraria</i> and soil from adjacent open area in Punjab, India.

<p>Error bars represent +1SE of mean. Different letters above bars indicate significant differences (one way ANOVA, post-ANOVA Tukey’s test; p<0.05).</p