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Impacts of Drought on Carrots and Their Symbiotic Fungi

其他書名	From Field Interactions to Gene Expression
出版	University of Wisconsin--Madison, 2019
URL	http://books.google.com.hk/books?id=oGZHzgEACAAJ&hl=&source=gbs_api

註釋In a symbiosis that occurs globally in soils, arbuscular mycorrhizal (AM) fungi engage in mutually beneficial interactions with the roots of plants. Hosts of AM fungi permit and regulate the colonization of their tissues by reorganizing their cellular components to enable the fungus to grow intracellular structures, called arbuscules. These highly branched hyphal projections facilitate exchanges between symbionts, in which the fungus receives lipids and sugars, its only carbon source. Plants benefit from the direct transfer of nutrients as well as from improved tolerance to environmental stress, both of which may be influenced by the genotype of the fungal partner as well as the genotype of the host plant. Water stress poses challenges to plants and agricultural production. AM symbiosis is known to alter plant water management, partially buffering against the negative impacts of drought. Recent work suggests that AM fungi likely transfer water directly to hosts while also modulating hosts' transpiration (by altering carotenoid and hormone processes) and improving hosts' nutritional status (with direct transfers of nitrogen, phosphorus, and potassium). Although much is known of host responses to AM colonization during water stress, relatively little is known of the stress responses in AM fungi. Recent sequencing and annotation of the AM fungal (Rhizophagus irregularis) genome created new opportunities to investigate these organisms. Carrot (Daucus carota) is an excellent plant model for mycorrhizal research. Much of what is known of AM fungal stress biology came out of in vitro experiments in which root organ cultures of carrot supported fungal growth. Carrot invests heavily in the production of its taproot, and it permits a high level of AM colonization. The newly available genome of carrot, its status as a highly nutritious and economically important vegetable, and its high level of carotenoid production make it an intriguing plant model for studies involving AM fungi. The work described below aimed to answer questions about host-fungal interactions during drought with evaluations that range from genotype x genotype experiments to those identifying the changes in gene expression that occur for drought-challenged AM symbionts. Experimental repetition highlighted the folly of extrapolating trends in AM contributions to plant growth within individual experiments. From multiple iterations of field and greenhouse trials, in which we subjected combinations of cultivars of carrot and isolates of AM fungi to late-season water restriction in organic soils, we observed inter- and intraspecific variability among fungal isolates in their contributions to plant growth, but these impacts differed in each trial. Although we observe what could be called 'functional diversity' among AM isolates, it was not possible to elucidate trends in fungal contributions to carrot yield. This may be in part due to the established mycorrhizal networks of native AM populations (in the field) and due to seasonal light availability and differences among soils (in both the greenhouse and field). Breeding histories of plants influenced compatibility with AM fungal inoculants. In the field experiments, although no AM isolate provided a consistent impact, carrot genotypes clearly indicated differential response to inoculation. The open-pollinated, heirloom cultivars exhibited a generally positive mycorrhization response (measured in weights of taproots relative to mock-inoculated controls). On the other hand, hybrid cultivars, bred in and for high-input systems, demonstrated neutral to negative mycorrhization response. This suggests that breeding may be an important consideration if we seek to enhance benefits from AM symbiosis. Gene expression of AM fungi and the carrot hosts contrasted during drought. R. irregularis exhibited a high level of gene upregulation, including for symbiotic genes. Carrot gene expression revealed mostly downregulation in response to drought, with reduced expression of genes involved in the establishment and maintenance of symbiosis. Generally, it seemed that carrot and R. irregularis differed in their carbon management strategies during drought stress.