Response of plants and fungal root endophytes to experimentally increased resource availability and physical stress in an intertidal wetland
Plant-fungal symbioses can have strong consequences for community and ecosystem processes, and they are sensitive to variation in abiotic factors. While the functions of mycorrhizal fungi and their interactions with plants are well established, the role of other root colonizing fungi such as dark septate endophytes (DSE), which lack specialized structures for nutritional transfer, is less clear. Despite the ubiquity of DSE in many extreme and stressful ecosystems, and their potential role in plant nutrition, the responses of DSE to nutrient availability and physical stress have rarely been tested in the field. We conducted a one-year field experiment to investigate how a symbiosis between the salt marsh plant Spartina alterniflora and DSE fungi responded to changes in resource availability (nutrient levels) and physical stress (salinity).
Plant shoot density and height increased in response to nutrient enrichment, consistent with past experiments in nutrient-limited marsh systems. However, the response of some plant traits depended on physical stress. There was a positive relationship between plant shoot growth and root colonization by DSE. Increased resource availability decreased the prevalence of DSE hyphae, but it did not influence fungal reproductive structures, which were marginally increased by salinity. Our results are consistent with the view that plant-DSE interactions are based in part on enhanced nutritional condition of plants by fungi. The hyphal response of DSE to increased nutrient levels is consistent with similar observations of mycorrhizal fungi. The ultimate outcome of this interaction for plants however, may also depend on ambient stress. That DSE were not negatively influenced by salinity stress is consistent with prior research demonstrating the ability of these fungi to tolerate stressful conditions. Our results highlight the importance of DSE for plants in stressful environments, where they may play roles similar to other root colonizing fungi. Further, these findings are suggestive of the importance of this poorly understood symbiosis.