侵染线：Endosymbiotic rhizobia enter nodule primordia via a specialized transcellular compartment known as the infection thread (IT). The IT elongates by polar tip growth, following the path of the migrating nucleus along and within the root hair cell.

提出问题：Rho-family ROP GTPases are known to regulate the polarized growth of cells, but their role in regulating polarized IT growth is poorly understood.

结果1-LjSPK1：Here we show that LjSPK1, a DOCK family guanine nucleotide exchange factor (GEF), interacts with three type I ROP GTPases.

结果2-LjROP6：Genetic analyses showed that these three ROP GTPases are involved in root hair development, but only LjROP6 is required for IT formation after rhizobia inoculation.

结果3-突变体：Misdirected ITs formed in the root hairs of Ljspk1 and Ljrop6 mutants.

结果4-LjSPK1-LjROP6：We show that LjSPK1 functions as a GEF that activates LjROP6. LjROP6 enhanced the plasma membrane localization LjSPK1 in Nicotiana benthamiana leaf cells and Lotus japonicusroot hairs, and LjSPK1 and LjROP6 interact at the plasma membrane.

结论：Taken together, these results shed light on how the LjROP6-LjSPK1 module mediates the polarized growth of ITs in L. japonicus.