Genetic Basis of Resistance to Citrus Canker Disease

Abstract

Citrus canker, caused by the bacterial pathogen Xanthomonas citri subsp. citri (Xcc), is a destructive quarantine disease worldwide. As no commercial cultivars are resistant to the disease and its control is difficult, selection of resistant genotypes becomes the essential solution. A large quantity of citrus genotypes were screened for their behaviors to the pathogen infection during the past decades, unfortunately almost all the tested genotypes are susceptible when they are artificially inoculated with Xcc. The pathogen infects citrus host through attachment on the tissue surface, and then penetration into the tissue for colonization. The successful infection relies on the formation of biofilm, which is affected by different factors including extracellular polymeric substances (EPS) containing mainly extracellular polysaccharides, quorum sensing, etc. The canker disease development depends on the virulent effector PthA4 secreted into citrus cells through the Type III protein secretion system (T3SS). The plant has two layers of defence responses to pathogen attack, i.e., the basal defence realized by pattern recognition receptors (PRRs) to detect microbial- or pathogen-associated molecular patterns (MAMPs or PAMPs) to trigger PAMP-triggered immunity (PTI) and the effector-triggered immunity (ETI) based on the highly specific interaction between products from pathogen avirulence genes (Avr) and products from host resistance genes (R). The XacFhaB, Lipopolysaccharides (LPSs) and flg22 are PAMPs identified in Xcc. The PRR FLS2 was identified in kumquat and mandarin genotypes. The rhizobacteria strains were found to effectively activate plant defence and significantly reduce symptom development in leaves challenged with Xcc. A few resistance genes, like Citrus NPR1 homolog 1 and Avr9/Cf-9 genes, were cloned. Breeding for citrus genotypes resistant to Xcc has continuously carried on for long time. The majority is achieved by genetic transformation, and among the reports, anti-bacterial peptide genes have been widely used, followed by the transferring resistant genes from other plants in citrus. The results, however, indicated only different levels of reduction of susceptibility to Xcc were gained. Further investigation of resistant mechanism and identification of resistant genes are indispensable for breeding of citrus genotypes really resistant to canker disease.