A native parasitic plant and soil microorganisms facilitate a native plant co-occurrence with an invasive plant

Iva xanthiifolia leaf extract reduced the diversity of indigenous plant rhizosphere bacteria

BACKGROUND

Iva xanthiifolia, native to North America, is now widely distributed in northeastern China and has become a vicious invasive plant. This article aims to probe the role of leaf extract in the invasion of I. xanthiifolia.

METHODS

We collected the rhizosphere soil of Amaranthus tricolor and Setaria viridis in the invasive zone, the noninvasive zone and the noninvasive zone treated with extract from I. xanthiifolia leaf, and obtained I. xanthiifolia rhizosphere soil in the invasive zone. All wild plants were identified by Xu Yongqing. I. xanthiifolia (collection number: RQSB04100), A. tricolor (collection number: 831,030) and S. viridis (collection number: CF-0002-034) are all included in Chinese Virtual Herbarium ( https://www.cvh.ac.cn/index.php ). The soil bacterial diversity was analyzed based on the Illumina HiSeq sequencing platform. Subsequently, taxonomic analysis and Faprotax functional prediction were performed.

RESULTS

The results showed that the leaf extract significantly reduced the diversity of indigenous plant rhizosphere bacteria. A. tricolor and S. viridis rhizobacterial phylum and genus abundances were significantly reduced under the influence of I. xanthiifolia or its leaf extract. The results of functional prediction showed that bacterial abundance changes induced by leaf extracts could potentially hinder nutrient cycling in native plants and increased bacterial abundance in the A. tricolor rhizosphere related to aromatic compound degradation. In addition, the greatest number of sensitive Operational Taxonomic Units (OTUs) appeared in the rhizosphere when S. viridis was in response to the invasion of I. xanthiifolia. It can be seen that A. tricolor and S. viridis have different mechanisms in response to the invasion of I. xanthiifolia.

CONCLUSION

I. xanthiifolia leaves material has potential role in invasion by altering indigenous plant rhizosphere bacteria.

Parasitism Shifts the Effects of Native Soil Microbes on the Growth of the Invasive Plant Alternanthera philoxeroides

Soil microbes play an important role in plant invasion, and parasitic plants regulate the growth of invasive plants. However, the mechanisms by which parasitic plants regulate the effects of soil microbes on invasive plants have not been investigated. Here, we used the invasive plant Alternanthera philoxeroides and the holoparasitic plant Cuscuta grovonii to test whether and how C. grovonii parasitism shifts the effect of native soil microbes on the growth of A. philoxeroides. In a factorial setup, A. philoxeroides was grown in pots with the presence versus absence of parasitism and the presence versus absence of native soil microbes. The findings showed that native soil microbes increased the biomass and clonal growth of A. philoxeroides only in the absence of a parasite, whereas parasitism decreased the biomass and clonal growth of A. philoxeroides only in the presence of soil microbes. In addition, the presence of soil microbes increased the deleterious effects of the parasite on A. philoxeroides. These results indicate that parasitism can shift the effects of native soil microbes on the growth of the invasive plant A. philoxeroides. Our results enrich the understanding of the mechanisms underlying the success of plant invasion.

Resetting our expectations for parasites and their effects on species interactions: a meta-analysis

Despite the ubiquitous nature of parasitism, how parasitism alters the outcome of host-species interactions such as competition, mutualism and predation remains unknown. Using a phylogenetically informed meta-analysis of 154 studies, we examined how the mean and variance in the outcomes of species interactions differed between parasitized and non-parasitized hosts. Overall, parasitism did not significantly affect the mean or variance of host-species interaction outcomes, nor did the shared evolutionary histories of hosts and parasites have an effect. Instead, there was considerable variation in outcomes, ranging from strongly detrimental to strongly beneficial for infected hosts. Trophically-transmitted parasites increased the negative effects of predation, parasites increased and decreased the negative effects of interspecific competition for parasitized and non-parasitized heterospecifics, respectively, and parasites had particularly strong negative effects on host species interactions in freshwater and marine habitats, yet were beneficial in terrestrial environments. Our results illuminate the diverse ways in which parasites modify critical linkages in ecological networks, implying that whether the cumulative effects of parasitism are considered detrimental depends not only on the interactions between hosts and their parasites but also on the many other interactions that hosts experience.

Older populations of the invader Solidago canadensis exhibit stronger positive plant-soil feedbacks and competitive ability in China

PREMISE

The enemy release hypothesis predicts that release from natural enemies, including soil-borne pathogens, liberates invasive plants from a negative regulating force. Nevertheless, invasive plants may acquire novel enemies and mutualists in the introduced range, which may cause variable effects on invader growth. However, how soil microorganisms may influence competitive ability of invasive plants along invasion chronosequences has been little explored.

METHODS

Using the invasive plant Solidago canadensis, we tested whether longer residence times are associated with stronger negative plant-soil feedbacks and thus weaker competitive abilities at the individual level. We grew S. canadensis individuals from 36 populations with different residence times across southeastern China in competition versus no competition and in three different types of soils: (1) conspecific rhizospheric soils; (2) soils from uninvaded patches; and (3) sterilized soil. For our competitor treatments, we constructed synthetic communities of four native species (Bidens parviflora, Solanum nigrum, Kalimeris indica, and Mosla scabra), which naturally co-occur with Solidago canadensis in the field.

RESULTS

Solidago canadensis populations with longer residence times experienced stronger positive plant-soil feedbacks and had greater competitive responses (i.e., produced greater above-ground biomass and grew taller) in conspecific rhizospheric soils than in sterilized or uninvaded soils. Moreover, S. canadensis from older populations significantly suppressed above-ground biomass of the native communities in rhizospheric and uninvaded soils but not in sterilized soil.

CONCLUSIONS

The present results suggest that older populations of S. canadensis experience stronger positive plant-soil feedbacks, which may enhance their competitive ability against native plant communities.

A parasite indirectly affects nutrient distribution by common mycorrhizal networks between host and neighboring plants

Cascading effects are ubiquitous in nature and can modify ecological processes. Most plants have mutualistic associations with mycorrhizal fungi, and can be connected to neighboring plants through common mycorrhizal networks (CMNs). However, little is known about how the distribution of nutrients by CMNs to the interconnected plants is affected by higher trophic levels, such as parasitic plants. We hypothesized that parasitism would indirectly drive CMNs to allocate more nutrients to the nonparasitized neighboring plants rather than to the parasitized host plants, and that this would result in a negative-feedback effect on the growth of the parasitic plant. To test this, we conducted a container experiment, where each container housed two in-growth cores that isolated the root system of a single Trifolium pratense seedling. The formation of CMNs was either prevented or permitted using nylon fabric with a mesh width of 0.5 or 25 μm, respectively. In each container, either both T. pratense plants were not parasitized or only one was parasitized by the holoparasite Cuscuta australis. To quantify the nutrient distribution by CMNs to the host and neighboring plants, we used ¹⁵ N labeling. Growth and ¹⁵ N concentrations of C. australis and T. pratense were measured, as well the arbuscular mycorrhizal fungi-colonization rates of T. pratense. We found that parasitism by C. australis reduced the biomass of T. pratense. In the absence of the parasite, CMNs increased the ¹⁵ N concentration of both T. pratense plants, but did not affect their biomass. However, with the parasite, the difference between host and neighboring T. pratense plants in ¹⁵ N concentration and biomass were amplified by CMNs. Furthermore, CMNs decreased the negative effect of C. australis on growth of the host T. pratense plants. Finally, although CMNs did not influence the ¹⁵ N concentration of C. australis, they reduced its biomass. Our results indicate that when T. pratense was parasitized by C. australis, CMNs preferentially distributed more mineral nutrients to the nonparasitized neighboring T. pratense plant, and that this had a negative feedback on the growth of the parasite.

Parasitism induces negative effects of physiological integration in a clonal plant

Clonal integration often increases fitness of clonal plants, but it may decrease it when some but not all connected plants (ramets) within a clone are parasitized. This hypothesis was synthesized in a conceptual model and tested by growing pairs of connected ramets of two congeneric clonal plants, Sphagneticola trilobata and Sphagneticola calendulacea, with and without parasitizing one ramet with Cuscuta australis and with and without severing the connection (allowing or preventing integration). Consistent with the model, integration in S. calendulacea did not affect biomass of the parasitized ramet, decreased biomass of its connected, unparasitized ramet by 60% and of the clone by 40%, and increased biomass of the parasite by 50%. By contrast, integration in S. trilobata did not affect biomass of the clone or the parasite. The parasite increased export of nitrogen-15 from the connected, unparasitized ramet seven-fold in S. calendulacea but did not affect export in S. trilobata. Parasitism can cause clonal integration to negatively affect fitness in clonal plants because parasites can import resources from connected, unparasitized ramets, possibly partly through signaling. This is the first experimental demonstration that clonal integration can decrease fitness in plants induced by parasitism and may help explain community-level effects of parasites.

The invasive plant Solidago canadensis exhibits partial local adaptation to low salinity at germination but not at later life-history stages

PREMISE

Evolutionary adaptation may enable plants to inhabit a broad range of environments. However, germination and early life-history stages have seldom been considered in estimates of evolutionary adaptation. Moreover, whether soil microbial communities can influence evolutionary adaptation in plants remains little explored.

METHODS

We used reciprocal transplant experiments to investigate whether two populations of an invasive plant Solidago canadensis that occur in contrasting habitats of low versus high salinity expressed adaptation to the respective salinity levels. We germinated S. canadensis seeds collected from low-and high-salinity habitats under low- and high-salt treatments. We also raised S. canadensis seedlings from the two salinity habitats under low- and high-salt treatments and in the presence versus absence of microbial communities from the two habitats.

RESULTS

Genotypes from a low-salinity habitat had higher germination rates under low-salt treatment than genotypes from a high-salinity habitat. However, both genotypes had similar germination rates under a high-salt treatment. The two genotypes also had similar seedling survival and biomass responses to low- and high-salt treatments. Nevertheless, seedling biomass was significantly higher under low salt treatment. Soil microbial communities did not influence biomass of S. canadensis under the two salt treatments.

CONCLUSIONS

The results on germination rates suggest partial local adaptation to low salinity. However, there was no evidence of local adaptation to salinity at the seedling survival and growth stages. The finding that germination and seedling biomass responded to different salt treatments suggests that the two traits are important for salt tolerance.