<scp>MdPOB1L</scp> Positively Regulates Disease Resistance Against <i>Botryosphaeria dothidea</i> by Manipulating <scp>MdNPR1</scp> Protein Stability in <i>Malus domestica</i>

E3 ubiquitin ligase is a key component of the ubiquitin-proteasome system, which is deeply involved in multiple aspects of plant growth and development, including in plant defence responses. POZ/BTB containing-protein1 (POB1) is a type of BTB-BACK domain-containing E3 ligase, which was previously reported to be a negative regulator of defence responses in multiple plant species. In this report, we identified MdPOB1-like (MdPOB1L) as a positive regulator in defence responses against Botryosphaeria dothidea by manipulating protein stability of MdNPR1, a master regulator in salicylic acid (SA) signalling pathway, in apple (Malus domestica). We first found that MdPOB1L is evolutionarily close to PbPOB1 from Pyrus bretschneideri, and it was localised in the nucleus in the epidermal cells of Nicotiana benthamiana leaves. Reverse transcription-quantitative PCR analysis revealed that MdPOB1L was inducible upon B. dothidea infection or SA treatment. Further investigation demonstrated that overexpressing MdPOB1L enhanced disease resistance to B. dothidea in both apple calli and fruits, while repressing its transcription displayed the opposite phenotype. Moreover, overexpressing MdPOB1L promoted the transcription of SA-responsive pathogenesis-related (PR) genes and MdNPR1. We further demonstrated that MdPOB1L interacted with MdNPR1 in the nucleus and promoted its ubiquitination and degradation through the proteasome pathway, which might contribute to the turnover of MdNPR1 in the nucleus, leading to enhanced downstream responses of the SA signalling pathway. Therefore, we demonstrate here that MdPOB1L positively regulates defence responses against B. dothidea by manipulating MdNPR1 protein stability in apple.