| In the present study, we investigated the arbuscular mycorrhizal fungi（AMF） resources in rhizosphere soils of five major tree species, including Robinia pseudoacacia, Juniperus communis, Populus cathayana, Hippophae rhamnoides, and Salix matsudana, and evaluated the relationships between AMF colonization, soil and climatic factors in the semi-arid Loess Plateau, northwest China. We also determined the effects of AMF on growth, photosynthesis, antioxidant enzyme activities and other physiological and biochemical indices, and gene expression of antioxidant enzymes and aquaporins of Robinia pseudoacacia subjected to artificial drought stress, in attempt to reflect the functions of AMF comprehensively. The main results are as follows:1. AMF resources in the semi-arid Loess Plateau and its relationships with soil and climatic factorsAll the 21 AMF species in eight genera were isolated from rhizosphere soils of five major tree species（R. pseudoacacia, J. communis, P. cathayana, H. rhamnoides, and S. matsudana）, of which seven belonged to Funneliformis, and Funneliformis was the dominant AMF genus. Claroideoglomus etunicatum and Glomus reticulatum were the dominant AMF species. Soil urease, catalase and total phosphorus were the strongest direct factors that affected the total colonization and spore density, while invertase was the strongest direct factor that affected AMF species richness, the Shannon-Wiener diversity index and the Shannon evenness index. Among six different regions in the semi-arid Loess Plateau, a total of 23 AMF species in nine genera were isolated from rhizosphere soils of R. pseudoacacia, of which six belonged to Funneliformis, and Funneliformis was the dominant AMF genus. Rhizophagus intraradices, Fun. dimorphicum, Rhizophagus aggregatum, Fun. monosporum, Fun. multiforum, Fun. geosporum and C. etunicatum were the dominant AMF species. Path analyses revealed that precipitation was one of the strongest direct factors that affected AMF status and AMF diversity in the semi-arid ecosystem.2. Effects of AMF on photosynthesis of R. pseudoacacia under drought stressInfluences of R. irregularis colonization on growth, chlorophyll, gas exchange parameters, chlorophyll fluorescence parameters were investigated in R. pseudoacacia, which subjected to artificial drought stress. The results showed that mycorrhizal R. pseudoacacia had higher chlorophyll a（120.8%）, carotenoids（44.7%）, Fm（28.6%）, q P（3.4%）, NPQ（4.2%）, and ΦPSII（7.0%） under well watered（WW） condition. Under drought stress（DS） condition, R. pseudoacacia had higher chlorophyll a（69.5%）, carotenoids（265.0%）, Fm（12.8%）, q P（4.3%）, NPQ（3.8%）, and ΦPSII（7.5%）. There were significant differences between mycorrhizal and non-mycorrhizal R. pseudoacacia. Mycorrhizal R. pseudoacacia also had higher net photosynthetic rate and stomatal conductance, with increasement of 16.4% and 12.8% under WW, and 14.3% and 8.1% under DS. This demonstrated that AMF symbiosis enhanced the PSII photochemical activity, photosynthetic electron transport capacity in light-adapted leaves, and maintained a high potential of light energy use efficiency and photosynthesis. Mycorrhizal R. pseudoacacia seedlings grow and do photosynthesis better.3. Effects of AMF on anoxidation of R. pseudoacacia under drought stressInfluences of R. irregularis colonization on anoxidation were investigated in R. pseudoacacia, which subjected to artificial drought stress. Results showed that the O2, H2O2 and MDA contents of R. pseudoacacia in roots decreased by 20.5%, 33.1% and 26.8% when they formed symbiosis with R. irregularis. This indicated that AMF detoxified the oxidative stress by drought. R. irregularis colonization can improve the activity of superoxide dismutase（SOD）, peroxidase（POD）, catalase（CAT）, ascorbate peroxidase（APX） and glutathione reductase（GR） in leaves and roots of R. pseudoacacia under WW and DS conditions. Under WW, SOD, POD, CAT, APX, GR activities in leaves and roots of R. pseudoacacia increased by 53.6% and 33.3%, 20.4% and 16.3%, 20.9% and 10.3%, 67.4% and 36.4%, 2.7% and 11.0%, respectively. Under DS, SOD, POD, CAT, APX, GR activities in leaves and roots of R. pseudoacacia increased by 1.1% and 27.2%, 25.7% and 23.5%, 13.5% and 9.8%, 78.3% and 58.2%, 22.7% and 14.4%, respectively. Among which, there were significant differences in SOD, POD, CAT, APX, GR activities between mycorrhizal and non-mycorrhizal R. pseudoacacia seedlings. This indicated that mycorrhizal R. pseudoacacia seedlings enhanced their antioxidant enzyme activities to scavenge the reactive oxygen species induced by drought.4. Effects of AMF on gene expression of antioxidant enzymes in R. pseudoacacia under drought stressWe investigated the effects of R. irregularis on gene expression of Cu/Zn-SOD, APX and GR in R. pseudoacacia by real-time fluorescent quantitative PCR（q RT-PCR） technique. R. irregularis colonization up-regulated the expression of Cu/Zn-SOD, APX and GR genes in roots, stems and leaves of R. pseudoacacia. We found that R. irregularis colonization upregulated the expression of APX（3.31-fold in roots）, APX（3.17-fold in stems）, APX（3.06-fold in leaves）, and GR（3.92-fold in leaves） under WW. We observed that R. irregularis colonization upregulated the expression of Cu/Zn-SOD（1.47-fold in roots）, Cu/Zn-SOD（1.62-fold in stems）, Cu/Zn-SOD（1.49-fold in leaves）, and GR（1.96-fold in roots） under DS. This indicated that mycorrhizal R. pseudoacacia seedlings regulated the active oxygen metabolism by raising the gene expression of Cu/Zn-SOD, APX and GR in roots, stems and leaves of R. pseudoacacia, and enhanced drought resistance.5. Effects of AMF on gene expression of R. pseudoacacia aquaporins under drought stressInfluences of R. irregularis colonization on gene expression of aquaporins were tested in R. pseudoacacia roots, stems and leaves by using q RT-PCR when seedlings subjected to artificial drought stress. Six full-length AQP c DNAs were isolated from R. pseudoacacia, named Rp TIP1;1, Rp TIP1;3, Rp TIP2;1, Rp PIP1;1, Rp PIP1;3, and Rp PIP2;1. The fulllength sequences of the Rp AQPs genes are 825–1201 bp, with 750–870 bp open reading frame encoding 251–289 amino acids. The predicted protein molecular weights are between 25.37 and 31.06 k Da and the isoelectric points are in the range of 5.09–8.96. Rp AQPs share the highest sequence identity（89%–97%） with Medicago truncatula. A phylogenetic analysis of deduced amino acid sequences demonstrated that putative proteins coded by these Rp AQP genes belong to the water channel protein family. Expression analyses revealed higher Rp PIP expression in roots whilst Rp TIP expression was higher in leaves, except for Rp TIP1;3. Under DS, R. irregularis colonization upregulated the gene expression of Rp TIP2;1（54.7%） and Rp PIP2;1（79.4%） in roots; Rp TIP1;1（1.17-fold）, Rp TIP2;1（1.83-fold）, and Rp PIP2;1（1.15-fold） in stems; Rp TIP2;1（1.44-fold）, and Rp PIP2;1（1.61-fold） in mature leaves. Different aquaporin genes play distinct roles in the control of water transportation for a plant. Mycorrhizal symbioses enhance the adaptability of R. pseudoacacia to drought stress by promoting the expression of aquaporin genes. |
PDF Full Text Request