The CsNPR1 gene expression modulation in citrus and understanding the defense mechanism against Huanglongbing by screening CsNPR1-interacting proteins

Abstract Huanglongbing (HLB) disease, also known as citrus greening, is a severe threat for citrus fruit cultivation industry. Till date, neither any effective radical method to control HLB nor HLB resistant cultivar is developed. The aim of this study was to provide clues for unraveling the molecular regulatory mechanism of citrus in response to HLB, and to provide reference gene for breeding of HLB resistant cultivar. One-year-old plants of Shatian pomelo (Citrus grandis (L.) Osbeck) were treated with different hormones and adverse stresses. We found the disease resistance gene, CsNPR1, expression was modulated by gibberellin, indole-3-acetic acid, and salicylic acid, whereas significant expression changes also occurred under adverse stresses including drought, salt, and mechanical injury. Additionally, we enhanced the expression of CsNPR1 in citrus and evaluated plant resistance against HLB. Finally, to investigate CsNPR1 interacting proteins, a high-quality citrus cDNA library was constructed. The interacting proteins were screened and verified by yeast two-hybrid (Y2H) and Bimolecular Fluorescence Complementation (BiFC) assay. Two CsNPR1-interacting proteins (CsHSPRO2 and CsHIR2) were screened and verified. The results showed that the transgenic plants overexpressing CsNPR1 exhibited enhance tolerance against HLB. Thus, CsNPR1 plays a role in stress and disease defense in citrus through interaction with CsHSPRO2 or CsHIR2 proteins.