Mechanisms Of Enhanced Plant Drought Tolerance By Arbuscular Mycorrhizal Symbiosis

Drought, a major environmental stress, has adverse effects on plant growth and development. It is predicted that semiarid and arid areas have an inclination to spread due to influence of climatic extremes. Vegetation degrade caused by drought is one of the main reason for the degrade of terrestrial ecosystem in semiarid and arid areas. Arbuscular mycorrhizal fungi (AMF) can establish a symbiotic association with most terrestrial plants and it is widely accepted that they can improve the growth of plants, enhance the absorption of nutrient and water and resist drought stresses by different ways. At present, most studies focused on physiological and molecular mechanisms of drought tolerance and indirect beneficial effects of AM fungi on plant performance under drought stress, while less attention was paid to the other ecological function, such as the functional diversity and effects on soil structure under drought stress. Meanwhile there is no evidence in direct regulation of plant drought tolerance from AM fungal hyphae, because the plant species used in previous studies differing in root hair length did not get rid of influences of root hairs.There are two experimental designs in this study. In the first study a new plant model system-root hairless monogenic mutant(bald root barley, Brb) and its wild type of barley(Hordeum vulgare L.) is used, for the hypothesis that both AM fungi and root hairs are conducive to uptake of water and P under drought condition. In the second study, we conducted an investigation on the influence of different AMF species on the growth of Medicago sativa L. and soil water stable aggregates under simulating drought stress conditions, and searching for AMF with suitable ecological adaptability. The rhizo-box used in this experiment is divided into three compartments:mycorrhizal compartment for both of mycorrhiza and mycelium, plant growth compartment for one Medicago sativa L. planted and hyphal compartment for mycelium only. This study contributes to understanding the potential role of AM fungi in enhancing plant performance in drought tolerance in semiarid and arid environments. The results of the study showed as followed: 1. The enhanced plant drought tolerance by AM and root hairs could be attributed to the improvement of leaf water potential, P uptake and photosynthetic capacity, while the effects of AMF on P uptake and leaf water potential were more pronounced than that of root hairs under drought stress.2. Beneficial effects of Glomus intraradices were greater on the hairless mutant than on the WT genotype under drought stress. Biomass and P concentration in shoot and total plant of mutant inoculated G. intraradices were higher than WT genotype under two soil water conditiongs. The colonization rate of mutant was enhaced by drought.3. Root colonization of Medicago sativa L. by G intraradices were significantly higher than those by the other two fungal species, Acaulospora scrobiculata and Diverspora spurcum irrespective to soil water conditions. Under well watered conditions, inoculation with each of the three AMF species could significantly increase plant biomes and phosphorus concentrations; while drought stresses significantly inhibited plant growth and mycorrhizal colonization, and there were no observable mycorrhizal effects on plant growth under drought stresses, meanwhile, inoculation with D. spurcum even reduced plant biomes, and only with G intraradices there was an increase in plant phosphorus concentrations.4. The concentration of proline in leaves was significant increased by water stress. But plants inoculated by three species AMF respectively showed a lower concentration of proline in leaves.5. The content of T-GRSP inhyphal compartment of plant inoculated G. intraradiceswas significantly enhanced under two soil water content conditions.6. As for mycorrhizal effects on soil structure, AM fungi mainly affected water stable soil aggregates with particle size>2mm, and G intraradices exhibited most prominent effects. Effects of three species of AMF on the number and distribution of soil water stable aggregates in mycorrhizal and hyphal compartment are significantly correlated with the content of T-GRSP in both mycorrhizal and hyphal compartment.