Effects of coastal dynamics on colonization of Louisiana wetland plants by fungal endophytes

This dissertation explores the effects of stress and disturbance on fungal endophyte diversity and composition in obligate wetland plants in southeast Louisiana. I explored two types of endophytes: root and foliar. Each of these groups is known to live within apparently healthy host plants without causing symptoms of disease; therefore, plants are thought to benefit from this association. Few studies, however, have shown that plants in wetlands associate with fungi. In this dissertation, I show that both root and foliar endophytes not only exist in wetland plants living in flooded and saline environments, but also are abundant within these plants. In the chapter two I explore the distribution of arbuscular mycorrhizal fungi (AMF) and dark septate endophytes (DSE) in the roots of 18 plant species. Monocots were mostly colonized by DSE, and dicots, including woody species were mostly colonized by AMF. In the third chapter, I used a mesocosm approach, to explore the effects of soil conditions on foliar endophyte diversity and composition in two wetland plant species – Sagittaria lancifolia and Taxodium distichum. Endophytes were abundant in both hosts, but S. lancifolia contained higher diversity. In addition, endophyte composition was shaped by different factors in each host: water quality shaped communities in T. distichum, and hydrology in S. lancifolia. In the fourth chapter I expand on the third chapter by including four more plant species that range from highly salt tolerant to intolerant and simulated a category 3 hurricane to explore its effects on endophytes within each plant species. I found that endophytes of grasses, irrespective of salt tolerance, did not change in diversity or composition following the simulated hurricane. Endophyte diversity T. distichum did not change, but composition was dependent on habitat type. Sagittaria lancifolia’s endophyte diversity decreased with hurricane conditions, but overall composition did not change. The results of these studies not only demonstrate that endophytes are present in wetland plants, but that they are abundant and common. Further, I show that host identity interacts with environmental conditions to influence diversity and composition and that habitat type is important in determining the outcome of endophyte composition after hurricanes

Shifts in Symbiotic Endophyte Communities of a Foundational Salt Marsh Grass Following Oil Exposure From the Deepwater Horizon Oil Spill

<p>Dataset for PLOSone paper</p

How abundant are root-colonizing fungi in southeastern Louisiana’s degraded marshes?Wetlands. 2010; 30(2):189–199. Endophyte Responses to Oil Exposure

Abstract Despite earlier notions that fungi are not important in wetlands, it is becoming clear that root endophytes are abundant in wetlands and potentially can influence plant community dynamics. Little is known about the effects of wetland degradation on these fungi. We assessed two groups of root endophytes in a degrading marsh in southeast Louisiana that historically was a swamp forest dominated by Taxodium distichum (baldcypress) and Nyssa aquatica (water tupelo). We determined percent root colonization by arbuscular mycorrhizal fungi (AMF) and dark septate endophytes (DSE) in each of 18 vascular plant species. Fungi were present in all species that were assessed. In general, monocots were primarily colonized by DSE, whereas dicots were primarily colonized by AMF. Taxodium distichum was heavily colonized by AMF, as was the non-native, invasive Chinese tallow (Triadica sebifera). This study is the first to show that wetland plants in a degraded marsh harbor abundant and diverse root endophytes. These fungi and their interactions with stressed plants may be important in effective management of degrading wetlands

Shifts in Symbiotic Endophyte Communities of a Foundational Salt Marsh Grass following Oil Exposure from the Deepwater Horizon Oil Spill

<div><p>Symbiotic associations can be disrupted by disturbance or by changing environmental conditions. Endophytes are fungal and bacterial symbionts of plants that can affect performance. As in more widely known symbioses, acute or chronic stressor exposure might trigger disassociation of endophytes from host plants. We tested this hypothesis by examining the effects of oil exposure following the Deepwater Horizon (DWH) oil spill on endophyte diversity and abundance in <i>Spartina alterniflora</i> – the foundational plant in northern Gulf coast salt marshes affected by the spill. We compared bacterial and fungal endophytes isolated from plants in reference areas to isolates from plants collected in areas with residual oil that has persisted for more than three years after the DWH spill. DNA sequence-based estimates showed that oil exposure shifted endophyte diversity and community structure. Plants from oiled areas exhibited near total loss of leaf fungal endophytes. Root fungal endophytes exhibited a more modest decline and little change was observed in endophytic bacterial diversity or abundance, though a shift towards hydrocarbon metabolizers was found in plants from oiled sites. These results show that plant-endophyte symbioses can be disrupted by stressor exposure, and indicate that symbiont community disassembly in marsh plants is an enduring outcome of the DWH spill.</p></div

The results of <i>in vitro</i> growth assays showed that oil reduced the growth of some endophyte strains, but increased growth for others.

<p>(A) Mean (±1 standard error) percent change in colony growth shows that bacteria tended to be more oil-philic while fungi were relatively oil-phobic. The dotted zero line represents no difference in growth on oiled versus unoiled media. (B) The top panel shows growth differences for an oil-phobic fungus isolated from a leaf, while the bottom panel shows an oil-phobic fungus isolated from a root.</p

Endophyte operational taxonomic units (OTUs) that predominated in isolates from <i>S</i>. <i>alterniflora</i> roots and leaves, with putative taxonomic assignments (NCBI Genbank accession numbers, % identity and % sequence cover given in Table B of S1 File).

<p>*Average abundance is presented for each OTU for oiled and reference areas. Average abundance was calculated using SIMPER in PRIMER v.7. All abundance values for each OTU within a group, (<i>e</i>.<i>g</i>., all abundance values for <i>Bacillus pumilus</i> within Bay Jimmy Roots from oiled areas) were summed and this total divided by the number of samples within the group.</p><p>^§ Percent contribution is a measure of the amount of variation explained by each OTU separately within each group (Bay Jimmy Roots).</p><p>^¶ Cumulative percent contribution is the running total variation accounted for by a given set of OTUs within a group. For example, the first four OTUs listed within the Bay Jimmy roots group, together account for 28.44% of the total variation within this group, and the sixteen OTUs listed for this group account for 72.04% of the total variation within Bay Jimmy roots.</p><p>Endophyte operational taxonomic units (OTUs) that predominated in isolates from <i>S</i>. <i>alterniflora</i> roots and leaves, with putative taxonomic assignments (NCBI Genbank accession numbers, % identity and % sequence cover given in Table B of <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0122378#pone.0122378.s001" target="_blank">S1 File</a>).</p

Map of study area.

<p>(A) The location of <i>S</i>. <i>alterniflora</i> collections for oiled (yellow circles) and unoiled reference (red circles) areas at Bay Jimmy; and (B) the location of <i>S</i>. <i>alterniflora</i> collections for oiled and unoiled reference areas at Fourchon. Specific GPS coordinates for each area at both sites are provided in Table A in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0122378#pone.0122378.s001" target="_blank">S1 File</a>. Images were obtained from the Louisiana Oil Spill Coordinator's Office (LOSCO), 20000120, Louisiana Land and Water Interface, Geographic NAD83, LOSCO (2000) (landwater_interface_LOSCO_1992). Metadata for these maps are available at <a href="http://lagic.lsu.edu/data/losco/landwater_interface_losco_1992.html" target="_blank">http://lagic.lsu.edu/data/losco/landwater_interface_losco_1992.html</a>.</p

Ordinations of endophyte community data.

<p>(A) Nonmetric multidimensional scaling using centroids for foliar and root communities from oiled and unoiled reference areas in Bay Jimmy and Fourchon. Arrows indicate the direction of community change between oiled and unoiled reference conditions. (B) Canonical Analysis of Principal Coordinates on unconstrained data illustrating community differences according to study location, oiling and tissue type with superimposed vectors of OTUs driving community differences.</p

Linear contrasts for foliar and root endophyte diversity and abundance according to treatment.

<p>(A) Fungal endophyte diversity in leaves and roots; (B) bacterial endophyte diversity in leaves and roots; (C) fungal endophyte abundance in leaves and roots; and (D) bacterial endophyte abundance in leaves and roots. Black bars correspond to oiled areas and grey bars correspond to unoiled reference areas. * = significant differences. Error bars are standard errors.</p

Rarefaction curves for the number of operational taxonomic units (OTUs) by group.

<p>The dotted line corresponds to samples from oiled areas, the solid line represents samples from unoiled reference areas, and the dashed line corresponds to all samples combined.</p