CsLOB1 impairs kumquat immunity to citrus canker by reprogramming cell development and defense responses

Abstract Citrus canker, caused by Xanthomonas citri subsp. citri ( Xcc ), represents a significant threat to the global citrus industry. LATERAL ORGAN BOUNDARIES 1 ( LOB1 ) has been identified as a key gene involved in the development of citrus canker in susceptible varieties. However, the role and molecular mechanisms of LOB1 in disease-resistant varieties, such as Kumquat ( Fortunella spp.), remain poorly understood. In this study, transgenic kumquat plants with either Citrus sinensis LOB1 ( CsLOB1 ) overexpression or RNAi-mediated silencing were obtained to investigate the function and molecular basis of LOB1 in kumquat resistance to Xcc . Overexpression of CsLOB1 in kumquats resulted in dwarf phenotype with reduced leaf size, increased branching, and numerous pustule-like bulges resembling citrus canker. Microscopic analysis revealed that these bulges were due to the excessive proliferation of mesophyll cells, along with spongy cell enlargement and palisade cell shortening. Resistance assays showed that CsLOB1 overexpression promoted kumquat susceptibility to Xcc , while the resistance remained stable in the RNAi lines. RNA-seq analysis revealed that CsLOB1 significantly upregulated immune response-related genes in kumquat. Furthermore, CsLOB1 was shown to regulate kumquat immunity through modulation of indole- 3-acetic acid-amido synthetase 3.17 ( GH3.17 ), elongation factor tu receptor ( EFR ), mitogen-activated protein kinase kinase 5 ( MKK5 ) and ENHANCED DISEASE SUSCEPTIBILITY 1 ( EDS1 ). Taken together, these findings demonstrate that CsLOB1 not only promotes citrus canker development by altering cellular states and hormone signaling but also activates immune responses in kumquat. Our work offers insights into harnessing LOB1 in the breeding of canker-resistant varieties in citrus.