| The amount of sulfur (S) in the environment is radically changing. Anthropogenic pollution has increased the annual worldwide terrestrial deposition of sulfur by more than 300 fold in the last 150 years. However, in recent decades the legislated removal of S at the source has diminished S deposition by more than 6 fold in many areas of the United States, leading to increased reports of S deficiencies in crops. Technology in this area continues to simplify the process of source removal with the goal of achieving pre-industrial levels. Additionally, sulfate, the main form of S assimilated by plants, is highly mobile in soils and can leach into the water system at a rate equal to its addition to the soil surface depending on water addition. In response to these environmental changes, interest in plant S acquisition and assimilation has surged in recent years.;The arbuscular mycorrhizal (AM) fungal symbiosis has co-evolved with plants since their transition to terrestrial environments, and AM fungi are now known to associate with an overwhelming majority of land plants. The major role of the symbiosis is to enhance the acquisition of nutrients in support of plant growth and homeostasis. Although the transfer of P, and to a lesser extent nitrogen, have been thoroughly analyzed, S acquisition by endo-mycorrhizal symbioses is a neglected area of research with often contradictory results. The availability of a monoxenic culture system and development of a model organism for mycorrhizal research, Glomus intraradices, including a worldwide effort to sequence and annotate the genome, has opened the possibility to more definitively analyze S in relation to an AM fungal symbiosis. The goal of this thesis was to quantify and analyze the transfer of S in relation to plant demand, and to pioneer research in the study of the regulation of S assimilation in AMF.;The model AMF Glomus intraradices transferred physiologically significant amounts of oxidized and reduced S forms to host roots, suggesting that S acquisition is a likely role of this fungus in natural ecosystems. The assimilation of sulfate by the symbiotic fungus was less regulated by reduced S sources than was the transfer to the host roots. Additionally, the simultaneous assimilation of sulfate and met or cys by pre-symbiotic mycelium is novel among studied fungi, whose sulfate assimilatory pathways are often completely diminished in response to reduced S sources. The regulation of sulfate acquisition, reduction, and assimilation was found to be greatly affected by ammonia assimilation and intracellular GSH concentration, and, unlike other fungi, less affected by reduced S forms cys and met. During germination, the addition of met led to very similar gene expression patterns compared to the addition of GSH, which did not correlate with uptake data. In general, changes in gene expression were not reflective of changes in S uptake with the exception of the down-regulation of a putative high affinity sulfate permease by cys addition in both the symbiotic and pre-symbiotic tissue. Unlike other fungi studied, transcriptional regulation does not appear to be the primary point of control for S acquisition by germinating spores of G. intraradices. |
