丛枝菌根真菌和溶磷细菌互作促进枳磷吸收的作用机制

Citrus is an important fruit crop in southern China. Most citrus trees are cultivated in hilly and mountainous areas. The soil available phosphorus is easily fixed, resulting in its lower content in citrus orchards, which heavily restricts the high yield and quality of citrus fruit. Consequently, an excessive amount of phosphorus fertilizer is often used. However, this practice is usually facing to the lower phosphorus utilizing efficiency and deterioration of the soil ecosystem due to phosphorus accumulation in orchards. Recently, increasing attention has been paid how to use the soil microorganisms to improve citrus phosphorus uptake. So, experiment was carried out in trifoliate orange (Poncirus trifoliata L. Raf.) pot culture system with simultaneous inoculation of native arbuscular mycorrhizal fungi (AMF, Glomus sp1. and Glomus sp.2) screened in citrus rhizosphere and phosphate-solubilizing bacteria (PSB1 and PSB2). The effect of AMF and PSB interaction on trifoliate orange phosphorus uptake under phosphorus stress was researched to select the best combination of AMF and PSB in promoting the citrus growth. In this study, the mechanism of AMF and PSB interaction on enhancing trifoliate orange phosphorus uptake will be explored by analyzing the differences of trifoliate orange root morphology and mycorrhizal symbiosis status, physiological and ecological effects in mycorrhizosphere and phosphorus stress response gene expressions in roots among various treatments. The present study has important theoretical values for increasing citrus phosphorus uptake efficiency and protecting citrus orchards ecology and environment.

柑橘是我国南方重要的果树，多在丘陵山地栽培，橘园土壤有效磷易固定，造成其含量较低，限制了柑橘的优质高产。通过施入磷肥解决通常面临肥料利用效率低，磷素积累造成橘园土壤生态恶化等问题。因此，通过应用土壤微生物肥料来帮助柑橘吸收利用土壤难溶性磷成为了研究热点。本项目应用从柑橘根际筛选得到的两株优势丛枝菌根真菌（Glomus sp.1和Glomus sp.2）、两株溶磷细菌（PSB1和PSB2）及柑橘常用砧木枳（Poncirus trifoliata L. Raf.）的栽培体系为研究对象，在磷胁迫下研究AMF与PSB互作对枳吸收利用土壤难溶性磷的影响，筛选出AMF与PSB的最佳促生效应组合；并通过分析不同处理间菌根共生状况、菌根际生理生态效应和枳根系磷胁迫响应基因表达的差异，阐明AMF与PSB互作促进枳磷吸收的作用机制。本研究对于提高柑橘对磷的利用效率，保护橘园生态环境具有重要的理论价值。

柑橘在我国南部山区广泛种植，为了提高柑橘的品质和产量，磷肥被大量施入橘园土壤中，但磷肥利用效率低，易被土壤固定。因此，研究应用土壤有益微生物帮助柑橘活化吸收土壤难溶性磷就显得尤为重要，而单一微生物的接种效应通常较低。基于此，本项目筛选了不同磷含量基质中促进枳生长的最优丛枝菌根真菌与促生细菌的接种组合，发现在低磷基质中联合接种丛枝菌根真菌（Rhizophagus intraradices，Ri）与促生细菌（Paenibacillus mucilaginosus，Pm）比单接种Ri及Pm能更高效的促进枳的生长；同时分析了不同基质中接种Ri和Pm对枳菌根共生状况、根际微域环境等生理生态的影响，发现在低磷基质中混合接种相较于单接种能进一步提高枳根系的菌根侵染率、根际菌丝长度、溶磷菌数量及土壤酶活性，这说明Ri与Pm能互相促进对方的繁殖，提高根际生物活性，协同发挥促生效果。然而，在高磷基质中混合接种的促生效应与单接种Ri无显著差异，这说明菌根促生效应受到Pm接种与基质含磷量的双重影响。低磷基质中混合接种与单接种Ri能降低根系主根长而增加总根长，而单接种Pm对根系的主根长没有显著影响；另外，对比单接种Pm，混合接种与单接种Ri能更加显著的调控枳叶片抗氧化酶、光合相关酶及蔗糖代谢酶活性，这说明混合接种中菌根缓解磷胁迫的促生效应更大。在菌根真菌Ri响应Pm接种的基因中，鉴定到3232个表达水平显著差异的基因。大量菌根响应基因属于跨膜转运蛋白、细胞壁几丁质代谢和调控水解酶活性等生物学过程，其中多数磷酸盐、镁离子及锌离子转运子基因受接种Pm诱导出现上调表达，利于Ri菌丝对基质矿质营养的吸收，而细胞壁几丁质合成过程受到正调控，能够促进Ri分生无性孢子和菌丝辐射生长。除此之外，Pm接种还正调控Ri碳氮化合物跨膜转运，以促使Ri将从宿主中得到的碳氮营养通过菌丝广泛得分泌到土壤环境中作为Pm营养物质的主要来源，最终扩大Pm的侵染范围。本研究为柑橘生产中利用微生物提高橘树对磷的利用效率奠定了坚实的理论基础，同时为认识根际微生物之间的相互作用关系提供了重要的启示。