Arbuscular mycorrhizal (AM) fungal diversity of arid lands : from AM fungal species to AM fungal communities

One of the widespread constituents of soil communities are the symbiotic arbuscular mycorrhizal (AM) fungi which associate in a mutualistic symbiosis with 80% of all land plants. They were shown to positively influence plant nutrition, plant productivity and improve their host plants’ tolerance to biotic and abiotic stresses. It was shown that AM fungal communities vary among broadly defined habitat types and further, that differences in AM fungal communities occur between different continents and climatic zones. This thesis highlights, for the first time, the particularity of the AM fungal communities in one location including four different habitat types of Southern Arabia. Using morphological and molecular methods, three AM fungal species new-to-science were described. These are Diversispora omaniana, Septoglomus nakheelum and Rhizophagus arabicus. In addition, four previously described AM fungal species were detected and isolated from the Arabian Peninsula for the first time. These are Claroideoglomus drummondii, Diversispora aurantia, Diversispora spurca and Funneliformis africanum.
The isolated and characterized AM fungal species were then used in two different research approaches. The first approach was to gain basic knowledge on physiological aspects of the isolated species through two experiments.
The first experiment was conducted in experimental microcosms with sorghum as host plant to evaluate the interacting effects of water regime and the presence of a potentially invasive AM fungal species, Rhizophagus irrigularis, in an assemblage of our AM fungal species (called native AM fungal assemblage). The community structure of the AM fungi colonizing the roots was assessed by determining the transcript abundance of the large ribosomal subunit (rLSU) of each individual AM fungus, using real-time PCR and species-specific primers. Our results showed that both water regime and the introduction of an invasive AM fungal species strongly altered the structure of the native AM fungal assemblage accompanied by a reduction of the plants’ drought tolerance as reflected by a lower accumulation of plant biomass and reduced amount of extraradical mycelium. Especially in arid environments, where the occurring AM fungal species are adapted to the environmental conditions, such changes can have a great impact on ecosystem functioning.
In a second experiment, we wanted to investigate functional characteristics of R. arabicus, one of our newly discovered AM fungal species endemic to the desert ecosystem, in comparison to R. irregularis, a well-studied and established lab AM fungus isolated from an agricultural field in Switzerland. As the plants’ drought tolerance is highly dependent on the associated AM fungal species we tested the two closely related AM fungi of contrasting climates for their ability to promote drought tolerance of sorghum by comparing transpiration rates, growth and nutrition of sorghum plants. Additionally, we estimated their ability to produce extraradical hyphae and their efficiency to extract nutrients from moist and dry soil to reveal their performance under contrasting conditions. Functional traits as extraradical hyphal formation and nutrient uptake efficiency, which were affected by the applied stress conditions, we observed huge differences between the two AM fungi where Rhizophagus arabicus seems to improve the plants’ drought tolerance more than R. irregularis. These differences might have resulted from specific adaptations to environmental conditions prevalent at the place where the AM fungal species originated from. R. arabicus was isolated from a hyper-arid environment and therefore might be better adapted to dry conditions. In a second part of this experiment, the aquaporin (AQPs) genes of Sorghum bicolor, our model plant were investigated. AQPs are pore-forming integral membrane proteins, located in various membranes of plant cells, and were recognized to play an important role in root water transport. First, we identified and characterized all AQP encoding genes in the fully-sequenced genome of sorghum. Additionally, the regulation of AQP transcript expression of mycorrhizal and non-mycorrhizal sorghum under well-watered and drought condition was assessed. Bioinformatic analyses of the S. bicolor genome revealed 35 genes coding for AQPs. Gene expression studies showed that the selected AQP genes were differentially regulated, either by mycorrhization, by water regime or by their interaction.
The last part of this thesis describes experiments that have been conducted to evaluate the abilities of some of the propagated AM fungal strains in promoting the seedling growth of two important plant species: Phoenix dactylifera (Nakhal) and Prosopis cineraria (Ghaf). The application of AM fungal inocula in horticulture, agriculture and revegetation programs became more prominent within the last decades as the number of studies demonstrating improved plant growth after inoculation has steadily increased. Especially in arid and semiarid ecosystems, the use of AM fungal inocula is of particular interest due to additional challenges the plants have to face in these climates such as drought, soil salinity and low nutrient availability. Furthermore, it was shown that the mycorrhizal potential in those soils is comparatively low and the additional application of AM fungi may lead to an improved establishment and functionality of the symbiosis. In our experiments some of the isolated AM fungal species or combinations of species were able to enhance the plants’ growth under nursery conditions. The plants have been transplanted to a desert field site in Oman for continuous long term observations.
This PhD thesis displays a full circle of isolation of AM fungi from nature, over their identification and propagation, with their subsequent application in basic and applied experiments. Further experiments assessing functional traits and characteristics of those particular AM fungi would be of high interest. Furthermore, the isolated AM fungi can open new doors in the field of conservation and revegetation in that unique and fascinating part of the world.