Arbuscular mycorrhizal fungi (AMF, subphylum Glomeromycotina) are ubiquitous soil organisms and plant root symbionts that are well-recognized for increasing plant access to nutrients in soil. Yet, AMF also perform other essential functions for plants and ecosystems. The first two chapters of this dissertation focus on two important additional roles of AMF: improving plant-water relations and soil properties. Chapter 1 investigates the ability of AMF to directly transport water to plants and examines the potential transport pathways involved. Chapter 2 assesses the capacity of AMF to transport plant-fixed carbon (C) to soil and affect soil microbial communities. In turn, AMF are affected by their environment. Chapter 3 evaluates molecular methods for studying the ecology of AMF, then explores the potential drivers that shape their diversity and distribution.Plants partnered with AMF commonly have an improved ability to cope with drought stress. The role of AMF in plant-water relations is often attributed to indirect mechanisms, such as enhancing plant nutrition and osmoregulation, but AMF may also directly transport water to plants. AMF consume up to 20% of plant photosynthetic C and grow extensive hyphal networks into the soil, where they interact with minerals and other microbial taxa. AMF thus have the potential to influence plant water dynamics, soil C dynamics, and soil microbial communities. In turn, environmental conditions, such as precipitation, soil characteristics, and microorganisms, have the potential to shape AMF communities.
Chapters 1 and 2 report the results of a greenhouse experiment that was conceived to explore the role of AMF in water transport to plants and C transport to soil. For these purposes, I designed microcosms with two compartments separated by a 3.2 mm air gap. One compartment had plants inoculated with AMF while the other had soil with bonemeal to encourage AMF to cross the air gap. Each side of the air gap was covered by mesh that allowed AMF, but not roots, to pass through. The purpose of the air gap was to prevent water (and the nutrients or dissolved organic C it may contain) from travelling between compartments unless it was transported by hyphae. This design allowed us to add isotopically-labeled compounds to one compartment and measure how much label AMF transported to the other compartment.
In chapter 1, we explored the role of AMF in water transport to plants. We tracked H218O containing a fluorescent dye from soil inaccessible to roots to the host plant Avena barbata, an annual grass, via the AMF Rhizophagus intraradices. We found that AMF accessed H218O and transported it across the air gap to host plants, leading to an increase in plant transpiration. In addition, we determined water was transported via AMF on the outside of hyphae cell membrane, either outside the cell wall or within the cell wall matrix.
In chapter 2, we investigated the role of AMF in C transport to soil. We tracked 13CO2 from the air fixed by A. barbata to the soil via R. intraradices and examined the effects of AMF on soil microbial communities beyond the rhizosphere. We found that AMF moved a substantial amount of C from plants to the soil inaccessible to root. About a third of that C occurred as aggregate- occluded and mineral-associated forms. Furthermore, AMF C inputs modified the bacterial community, potentially enhancing AMF access to nitrogen and phosphorus.
In turn, biotic and abiotic environmental conditions have the potential to shape the diversity and distribution of AMF. Increasingly diverse AMF communities often enhance ecosystem-level processes, yet, understanding the relative roles of different drivers in shaping AMF biodiversity patterns is an ongoing challenge. The development of molecular methods has made it possible to identify AMF taxa directly from soil and plant roots, but primers routinely used for molecular investigation of AMF may have different specificity and potential to describe the diversity of AMF phylogenetic lineages.
In Chapter 3, we compared SSU/18S primers WANDA/AML2 and ITS2 primers 5.8S-Fun/ITS- Fun in terms of their abilities to discover and identify AMF taxa as well as their efficacy to assess the ecology of AMF in field soil communities. First, we used biological mock communities as a base to compare primers performance using Illumina based techniques. Then, we used natural communities from soil samples collected across three California grasslands to further investigate differences in primer performance on field samples. Finally, we investigated AMF species richness and community composition at the three sites and identified drivers that shape AMF biodiversity in grasslands.
Our greenhouse experiment contributed to a better understanding of the processes involved in water transport to plants and C transport to soil by AMF. We provided direct evidence that AMF can act as extensions of the root evapotranspiration pathway, with plant transpiration driving water flow along hyphae outside of the hyphal cell membrane. Our findings indicate that AMF are important for the management of plant drought tolerance and have implications and potential applications in the context of climate change, as the frequency, duration, and intensity of droughts are predicted to increase in many regions worldwide.
AMF also moved a substantial amount of C from plants beyond the rhizosphere, and a third of that C occurred as aggregate-occluded and mineral-associated forms, which may have longer residence times in soil. Soil C storage is a vital ecosystem service resulting from interactions of many processes. Soil quality is paramount to the health of natural and agroecosystems, and will be improved if more C is accumulated as soil organic matter due to an increased flow and persistence of plant-fixed C to soil via AMF.
