The LIN and LINL E3 ligases function redundantly in arbuscular mycorrhizal symbiosis and nodulation of Medicago truncatula

LUMPY INFECTION (LIN) is known to direct the polar growth of infection threads during nodulation in Medicago. However, the role of LIN in the arbuscular mycorrhizal (AM) symbiosis has yet to be characterized. Here, we identified a novel lin allele mutant lin-6 (FN9104) in Medicago that exhibited impaired nodulation and reduced efficiency of AM symbiosis. LIN and its four LIN-like homologs (LINL1–4) are involved in both nodulation and AM symbiosis in Medicago. RNAi knockdown assays in both lin-6/LINL1-3-RNAi hairy roots and lin-4 linl1-1 double-mutant roots demonstrated that LIN and LINL1–3 exhibit functional redundancy in the AM symbiosis. Furthermore, the U-box domain, Armadillo-like domain, and WD40 repeat domain of LIN are essential for its functions in nodulation and mycorrhizal symbiosis, and the U-box domains of LIN and LINL1 exhibit E3 ubiquitin ligase activity in vitro. Interestingly, the interactions of LIN and LINL1 with DELLAs, scaffold proteins in the common symbiosis signaling pathway (CSSP), rely on their U-box domain. Our findings revalidate that LIN is a key component of the CSSP, redundant with LINLs in AM symbiosis. The U-box-mediated DELLA interaction suggests LIN's E3 ligase activity may regulate this central signaling hub to enable intracellular accommodation in root endosymbiosis.