Genome-Wide Screening and Functional Verification of the Tobacco SGR Family Identify Transcription-Factor-Like NtSGRL as a Negative Regulator of Tomato Yellow Leaf Curl Virus Resistance.

Viral infection often induces chlorosis, a symptom closely associated with chlorophyll (Chl) degradation. STAY-GREEN (SGR) proteins are key regulators of Chl breakdown, but their roles in plant virus resistance remain poorly understood. In this study, we identified six NtSGR family genes in Nicotiana tabacum and functionally characterized NtSGRL1 during tomato yellow leaf curl virus (TYLCV) infection. TYLCV drastically upregulates NtSGRL1 (149-fold at 4 days post-inoculation [dpi] vs. control), suggesting its involvement in the tobacco-TYLCV interaction. Subcellular localization revealed that NtSGRL1 is targeted to both the nucleus and chloroplast, and yeast assays confirmed its transcriptional activation activity, indicating potential dual functions. TYLCV accumulation was reduced by 1995-fold in Ntsgrl1 mutants relative to wild-type (WT) plants at 10 dpi. Further assays with tomato spotted wilt virus (TSWV), potato virus Y (PVY) and tobacco mosaic virus (TMV) showed that viral accumulation in Ntsgrl1 mutants was reduced by 62%-99%, suggesting that NtSGRL1 may affect resistance to additional tested viruses. Transcriptomics revealed 1470 differentially expressed genes in TYLCV-infected Ntsgrl1 plants enriched in MAPK signalling and phenylpropanoid biosynthesis pathways. Virus-induced gene silencing (VIGS)-based validation further supported the involvement of these pathways in TYLCV resistance. Overall, this study identifies NtSGRL1 as a critical regulator of antiviral resistance in tobacco and reveals its dual functional roles as both a Chl-related enzyme and a transcriptional regulator. These findings provide new insights into plant antiviral mechanisms and potential genetic resources for breeding virus-resistant crops.