The mannose-binding lectin gene CsMBL1 positively regulates resistance to citrus canker

Citrus canker is a serious bacterial disease caused by Xanthomonas citri (Xcc) subsp. citri. It is a significant threat to the citrus industry. Plants have several defense proteins, including mannose-binding lectins (MBL), which modulate innate immune responses by recognizing and binding to carbohydrates on microbial surfaces. However, the role of MBL in citrus canker resistance remains elusive. Previous transcriptomic analysis revealed that in the leaves of the citrus canker-resistant variety Kumquat (Citrus japonica), and the susceptible variety WanJincheng (Citrus sinensis) treated with Xcc, higher levels of CsMBL1 were detected in the former compared to the latter, suggesting that CsMBL1 is associated with resistance to citrus canker. Therefore, this study analyzed and identified a mannose-binding B-lectin gene, CsMBL1, and obtained the following results. Subcellular localization results indicated that CsMBL1 is localized in the nucleus. Phytohormone induction experiments demonstrated that methyl jasmonate (MeJA) downregulated the expression of CsMBL1 in Kumquat, while upregulating it in Wanjincheng. Moreover, its overexpression increased citrus canker resistance, while virus-induced gene silencing (VIGS) increased susceptibility. Biochemical analyses revealed increased reactive oxygen species (ROS) content, reduced peroxidase (POD) activity, and increased respiratory burst oxidase homologs (RBOHs) activity in CsMBL1-overexpressing plants. Additionally, the levels of abscisic acid (ABA) and jasmonic acid (JA) were diminished, while the salicylic acid (SA) level was elevated. These findings suggest that CsMBL1 acts as a positive regulator of citrus canker resistance by modulating JA, ABA, SA, and ROS levels; therefore, CsMBL1 can be targeted for modulation to develop canker-resistant citrus cultivars.