Functional characterization of SIZ1 homologs reveals reduced TSWV susceptibility of mutant Nicotiana benthamiana lines

The SIZ1 gene, encoding a SUMO E3 ligase, is known to regulate various developmental and stress response pathways in plants. Despite considerable research efforts, the specific function of SIZ1 in plants remains largely unknown. In this study, we investigated the role of the SIZ1 gene in plant resistance by utilizing gene-edited Nicotiana benthamiana lines with modified SIZ1 gene to assess resistance to Tomato spotted wilt virus (TSWV), which is a significant agricultural pathogen causing substantial crop losses across various host species. Our findings showed that N. benthamiana SIZ1 (NbSIZ1) mutants exhibited delayed symptom development when challenged with TSWV, indicating reduced susceptibility to TSWV infection. The mutations in the SIZ1 gene appeared to activate salicylic acid (SA) signaling pathways, leading to increased SA accumulation and the establishment of constitutive defense responses. Specifically, we observed enhanced expression of N. benthamiana pathogenesis-related (PR1), isochorismate synthase 1 (ICS1) and nonexpresser of pathogenesis-related gene 1 (NPR1) genes in mutant lines compared to wild-type and overexpression lines and thus contributed to improved resistance against TSWV in SIZ1 mutant plants. In conclusion, these observations indicate that SIZ1 negatively modulates the SA signaling pathway, and its loss of function can lead to increased immune responses, contributing to reduced disease susceptibility in plants. Our study sheds light on the role of NbSIZ1 in combating TSWV infection and highlights the potential of SIZ1 as an important genetic resource for future molecular breeding efforts to develop TSWV-resistant crops.