| Arbuscular mycorrhizal (AM) fungi that could form mutualistic symbioses with host plants are ubiquitous components in most agroecosystems and inhabit both plant roots and surrounding soils. The symbioses play an important role in plant growth, development and productivity due to their benefits including improving the host plants uptake of water and mineral elements and increasing the resistance to stresses and the adaptability of new surroundings. Most varieties of citrus are cultivated in hilly areas in southern China, and fairly dependent on AM fungi to achieve a good growth and development. In this research, the development characterics of different structures of AM associated with citrus were investigated in orchards under various ecological conditions and the species of AM fungi in citrus rhizosphere and colonizing the citrus roots were identified by using the spore morphologic or molecular biology methods. The main results in this study are as follows:(1) The microenvironment of citrus mycorrhizosphere in orchards.The citrus AM development in different soil depths were investigated in orchards. The results showed that the citrus hyphal and arbuscular colonization rates which were largest (69.72% nd 43.83%, respectively) in (20-40) cm soil layers were signicantly affected by the soil depth; while the vesicular colonization rate did not chang notably with the increase of soil depth. The arbuscular colonization rate was significant (P< 0.05) positively correlated with the numbers of bacteria (r= 0.483) and fungi (r= 0.529) and activites of soil invertase (r= 0.448), urease (r= 0.529) and phosphatase (r= 0.518).(2) Effects of altitude and seasons on the citrus AM development.The AM status associated with citrus trees in orchards along an altitudinal gradient was investigated in each season. The results exhibited that citrus AM development that was better in lower altitude (< 400 m) orchards than in higher altitude (> 600 m) ones fluctuated remarkably with the season alternation. The PCA indicated that the microorganism factors including AM activity and microbe numbers were first impacted with the altitude and season changes, then, were the edaphic facors. The seasonality of mycorrhizosphere ecology was more obvious in higher altitude orchards compared to lower ones according to the principle component plots. The AM activities were generally higher in summer and autumn seasons in all investegaed orchards. The soil organic matter was significant positively correlated with AM colonization (r= 0.762, P< 0.01), spore density (r= 0.619, P< 0.01), hyphal length density (r= 0.513, P< 0.05) and the numbers of bacteria (r= 0.454, P< 0.05), actinomycetes (r= 0.644, P< 0.01) and fungi (r= 0.451, P< 0.05).(3) The species diversity of AM fungi in citrus rhizosphere.The species diversity and community distribution pattern of AM fungi were studied in Three Gorges Region orchards along an altitudinal gradient. A total of eighteen species of AM fungi belonging to five genuses were identified in all soil samples. Of them, Glomus had the most relative abundance and isolation frequence, and G.aggregatum and G. mosseae were the dominant species in citrus rhizosphere. Compared with other nature systems, the species richness ranging from 8～19 was lower in citrus orchard system. The CCA indicated that G. aggregatum, G. caledonium, G. etunicatum, G. mosseae and G. intraradices were similar in distribution pattern and frequently observed in lower altitude, moisture, pH and nutrition orchards, while Gigaspora albida and Scutellospora sp.l observed in higher altitude, moisture and lower pH and nutrition orchards.(4) The molecular diversity of AM fungi colonizing citrus roots.The molecular diversity of AM fungi colonizing citrus rootstocks of trifoliate orange(Poncirus trifoliata L. Raf.) and red tangerine(Citrus reticulata Blanco) was analyzed by the small subunit ribosomal DNA (SSU rDNA) genes in this experiment. Phylogenetic analysis revealed that the sequences clustered in at least 11 discrete sequence groups all belonging to the genus Glomus. The clades of GLO1 (31.65%) and GLO2 (22.78%) classed wih Uncultured Glomus were the most common in roots of trifoliate orange, while the clade of GLO6 (28.75%) classed with G.intraradices、G. faciculatum and G. irregulare was the most common in roots of red tangerine. The diversiy indices of AM fungi colonizing the roots of trifoliate orange and red tangerine were 1.69 and 1.65, respectively, which showed no significant difference.(5) Effects of native AM fungi on citrus growth and gene expression.Experiment was carried out in pot culture and effects of native and exotic G. mosseae on growth and gene expression of trifoliate orange(Poncirus trifoliata L. Raf.) under low P condition were studied. The results showed that either native or exotic AM fungi could improve the citrus growth, but the growth enhancing effect of native AM fungi was better than the exotic one. The microarray was employed to analyze the differential gene expression pattern of trifoliate orange after being inoculated AM fungi. The results exhibited that differentially expressed genes were mainly involved in metabolic process (33.3%), response to stimulus (13.4%), transferase activity (14.8%), regulation of biological progress (10.0%), transporter activity (9.1%) and so on. Compared with non-AM fungi treatment, two phosphate transporter genes expression changed significantly in citrus roots colonized by AM fungi. One low affinity inorganic phosphate transporter gene expression was down-regulated, while another high affinity inorganic phosphate transporter up-regulated. |
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