| Maize(Zea mays L.) is not only an important food crop, but also an important source of forage, industrial raw materials. It plays an essential role in food supplies and economic development. Arbuscular mycorrhizal fungi (AMF) is the most common Endomycorrhiza fungi, and is able to form a mutually beneficial symbiosis-arbuscular mycorrhiza (AM) with more than 80% of terrestrial plants. AM promotes host plants to absorb water and mineral nutrients, especially for phosphorus. As a consequence, it improves plants nutrition. AM also enhances plants resistance to biotic and abiotic stress to improve the yield and quality of plants. At the present time, researches between AMF and maize are concentrated attention on resistance to salt, drought, heavy metal pollution, and so on, however study of symbiosis efficiency between maize and AMF has not been reported yet. So far, genes controlling the symbiosis efficiency between maize and AMF have not been identified from maize germplasm. With the development of the single nucleotide polymorphisms (SNPs), based on linkage disequilibrium (LD), genome-wide association studies (GWAS) offers a new method to analyze the complex quantitative traits. In this study, GWAS was used to study the symbiosis efficiency between the 380 maize inbred lines and AMF, as a consequence, we located a major gene and did functional verification. This work provided a research target for molecular marker breeding basing on genetic resources.In this study, maize B73 could establish a good symbiotic relationship with R. irregularis as same as the tomato, which root infection rate was about 50%. Similarly to promote the plant growth for tomato, R. irregularis was able to increase significantly the stem length and shoot fresh weight of B73. Meanwhile, R. irregularis could also significantly increase resistance of B73 to Rhizoctonia solani. The roots infection rate of both commercial maize Zhendan 958 and Xianyu 335 was evaluated to less than 10%. Pot experiments and field experiments had been confirmed that R. irregularis had no significant effect on stem length, stem diameter and yields of both Zhengdan958 and Xianyu335. These results indicated that we first needed to genetically modify the symbiosis efficiency to make full use of mycorrhizal fungi on maize. In order to identify the key maize genes influencing the symbiotic relationship between arbuscular mycorrhizal fungi and maize, we selected the 380 maize inbred lines and inoculated with R. irregularis, then investigated the symbiosis efficiency. We found that the AMF infection rate was dramatically ranged from 0% to 100% with large genetic variations.Root infection rate was used as the genetic phenotype. GWAS was used to study the root infection rate of 303 maize inbred lines with the genome-wide 550,000 SNPs. Q model was the optimum the best model among the model GLM, Q, K, Q+K for GWAS analysis. As a consequence, in Q model,27 significant SNPs had been identified to associate with root infection rate which distributed on five genes(MSERG 1～5,p<1×10-4) located on the chromosomes 2,4,5,7. Those five genes were regarded as the candidate genes controlling the symbiosis efficiency between maize and R. irregularis. MSERG 4 with the most significance level, was selected as the investigation subject for complementary text. Three allelic mutants of MSERG 4 were selected out from maize mutants library. Homozygote mutants of MSERG 4 were charactered and then inoculated with R. irregularis. Compared with the wild type, the AMF infection rate was increased more significantly for the maize mutants MSERG 4-1, MSERG 4-2 and MSERG 4-3. This suggested that MSERG 4 was very important to control the symbiosis between maize and R. irregularis. Also in maize mutants, the contents of inorganic phosphorus were decreased in leaves and roots, while were significantly increased in stems. These results demonstrated that MSERG 4 was negatively regulate the symbiotic relationship between maize and R. irregularis, meanwhile it might affect the distribution of inorganic phosphorus in mycorrhizal maize. This implied MSERG 4 was likely to influence maize to absorb inorganic phosphorus to control symbiosis efficiency.As AMF symbiosis with plants had a broad spectrum, we evaluated the homologous genes of MSERG 4 in rice, tomato and tobacco by homologous alignment. In order to investigate whether the function regulating symbiosis efficiency of MSERG 4 was conserved in different species. We used mutants or RNAi genetic transformation to identify the function of MSERG 4 homologues. Right now, the mutants of rice, and transgenic plant of tomato and tobacco has constructed which will accumulate material for functional analysis in the future. |
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