Data_Sheet_1_Role of Cell Wall Polyphosphates in Phosphorus Transfer at the Arbuscular Interface in Mycorrhizas.PDF

Arbuscular mycorrhizal fungi provide plants with soil mineral nutrients, particularly phosphorus. In this symbiotic association, the arbuscular interface is the main site for nutrient exchange. To understand phosphorus transfer at the interface, we analyzed the subcellular localization of polyphosphate (polyP) in mature arbuscules of Rhizophagus irregularis colonizing roots of Lotus japonicus wild-type (WT) and H+-ATPase ha1-1 mutant, which is defective in phosphorus acquisition through the mycorrhizal pathway. In both, the WT and the ha1-1 mutant, polyP accumulated in the cell walls of trunk hyphae and inside fine branch modules close to the trunk hyphae. However, many fine branches lacked polyP. In the mutant, most fine branch modules showed polyP signals compared to the WT. Notably, polyP was also observed in the cell walls of some fine branches formed in the ha1-1 mutant, indicating phosphorus release from fungal cells to the apoplastic regions. Intense acid phosphatase (ACP) activity was detected in the periarbuscular spaces around the fine branches. Furthermore, double staining of ACP activity and polyP revealed that these had contrasting distribution patterns in arbuscules. These observations suggest that polyP in fungal cell walls and apoplastic phosphatases may play an important role in phosphorus transfer at the symbiotic interface in arbuscules.

丛枝菌根真菌（Arbuscular mycorrhizal fungi）可为植物提供土壤矿质养分，尤以磷元素为重。在该共生体系中，丛枝界面（arbuscular interface）是养分交换的核心位点。为解析该界面处的磷转运机制，我们分析了多聚磷酸盐（polyphosphate, polyP）在定殖百脉根（Lotus japonicus）野生型（wild-type, WT）与H+-ATPase ha1-1突变体根系的根内根孢囊霉（Rhizophagus irregularis）成熟丛枝中的亚细胞定位——该突变体无法通过菌根途径完成磷素获取。在野生型与ha1-1突变体样本中，多聚磷酸盐均在主干菌丝（trunk hyphae）的细胞壁以及紧邻主干菌丝的细枝模块内部积累，但多数细枝并未检测到多聚磷酸盐信号。相较于野生型，突变体中多数细枝模块呈现出多聚磷酸盐信号。值得注意的是，在ha1-1突变体形成的部分细枝的细胞壁中也观测到了多聚磷酸盐，这提示磷素可从真菌细胞释放至质外体区域（apoplastic regions）。在细枝周围的丛枝周间隙（periarbuscular spaces）中，检测到了强烈的酸性磷酸酶（acid phosphatase, ACP）活性。进一步的酸性磷酸酶活性与多聚磷酸盐双染色实验显示，二者在丛枝中呈现截然相反的分布模式。上述观测结果表明，真菌细胞壁中的多聚磷酸盐与质外体酸性磷酸酶可能在丛枝共生界面的磷转运过程中发挥关键作用。