The fungal microbiome as a driver of plant adaptation and ecology
All plants in natural ecosystems are thought to be symbiotic with fungal endophytes. There are 4 classes of fungal endophytes that differ ecological roles with regard to plant adaptation and fitness. Class 2 endophytes confer abiotic stress tolerance to plants in a habitat-specific manner. We endeavored to genetically characterize class 2 endophytes that adapt plants to varying degrees of stress across environmental gradients. Endophytes were isolated from native and invasive plant species (Leymus mollis, Spartina anglica, Halogeten glomerotus, Ceratoides lanata) growing across salt gradients in coastal and desert ecosystems. Plants were chosen based on soil conductivity levels (low medium and high), surface sterilized, sectioned and plated on an artificial medium to allow fungal emergence and growth. Fungi emerging from plants were taxonomically identified using classical microscopic techniques and DNA sequence of the ribosomal ITS regions. The ecological functionality of the fungi was assessed in plants exposed to salt stress in laboratory, greenhouse and field studies.
All plants growing across environmental gradients were colonized with a limited number (1-5) of dominant class 2 endophytes. Unexpectedly, we found that individual plant species growing across salt gradients modulated there symbiotic associations so that different fungal species were isolated at locations imposing different levels of stress. A detailed analysis of coastal Leymus mollis plants revealed that the endophytes conferred salt tolerance in a micro-habitat specific manner and were required for optimal plant fitness. Fungal isolates from plants growing on the beaches conferred high levels of salt tolerance and that ability decreased in species from plants growing in lower salinity soils. We conclude that plants modulate symbiotic associations with different fungal species to optimize fitness at micro-habitat scales across environmental gradients. The potential significance of these findings in regard to climate change and global vegetation patterns will be discussed.