Coordinated ubiquitination of OsHIPP56 and blast fungus MAX effectors by OsFBX388 regulates rice immunity

Protein ubiquitination by E3 ligases is crucial for plant growth and stress resistance. However, the mechanisms by which E3 ligases regulate rice immunity remain largely unclear. A previous study identified OsFBX388 as an E3 ligase that negatively regulates rice blast resistance. Here, we demonstrate that OsFBX388 interacts with OSK25 via its N-terminal F-box domain, thereby is a component of the SCF complex. Using its C-terminal domain, OsFBX388 targets OsHIPP56, a heavy metal-associated isoprenylated plant protein, for ubiquitination and degradation by the 26S proteasome. Meanwhile, knockout of OsFBX388 or overexpression of OsHIPP56 leads to substantial accumulation of OsHIPP56. Moreover, OsHIPP56 overexpression enhances resistance to multiple blast fungus strains and to bacterial blight without compromising yield. The HMA domain of OsHIPP56 is structurally similar to the integrated domains (IDs) of the sensor NLRs RGA5 and Pikp-1, which recognize the MAX effectors AvrPia and AvrPik-D from the blast fungus, respectively. We found that OsHIPP56 interacts with both AvrPia and AvrPik-D. Notably, AvrPia and AvrPik-D are also targeted by OsFBX388 for ubiquitination and degradation by the 26S proteasome. The interactions between OsHIPP56 and these MAX effectors accelerate the OsFBX388-mediated degradation of both OsHIPP56 and the MAX effectors. Furthermore, OsHIPP56 interacts with the HMA domains of the NLR receptors RGA5 and Pikp-1. Co-expression of OsHIPP56 with the NLR pairs RGA4/RGA5 or Pikp-1/Pikp-2 triggers cell death in rice protoplasts. Collectively, these findings reveal a coordinated regulatory mechanism in rice immunity, where an E3 ligase mediates the ubiquitination of both a host target and pathogen effectors. • The E3 ligase OsFBX388 mediates ubiquitination and degradation of rice target protein OsHIPP56 and MAX effectors derived from Magnaporthe oryzae. • OsHIPP56 overexpressing enhances resistance to rice blast and bacterial blight without compromising yield. • OsHIPP56 interacts with the MAX effectors AvrPia and AvrPik-D from Magnaporthe oryzae. • OsHIPP56 is recognized by sensor NLRs to trigger cell death induced by helper NLRs.