| The bacterial agents of citrus canker (Xanthomonas citri ssp. citri, Xcc) and Huanglongbing (HLB) (Candidatus Liberibacter asiaticus, CLas) have caused tremendous economic losses to the citrus industry. Pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) is important to plant immunity. In this study, comparison of the defense response of citrus canker resistant and susceptible genotypes to the Xcc-derived PAMP, flg22 (Xflg22), and of HLB tolerant and sensitive genotypes to the CLas-derived flg22 (Lflg22), was performed by analyzing the expression of several citrus defense-associated genes. The Xflg22 treatment significantly induced the expression of EDS1, NDR1, PBS1, RAR1, SGT1, PAL1, NPR2 and NPR3 in the resistant 'Nagami' kumquat but not in the highly susceptible 'Duncan' grapefruit. Similarly, Lflg22 triggered gene transcriptional reprogramming in the HLB tolerant 'Sun Chu Sha' mandarin but not in the HLB sensitive 'Duncan' grapefruit. Moreover, Xflg22 induced a stronger oxidative burst and inhibited Xcc growth only in the canker resistant genotypes, and a flagellin-deficient Xcc strain (XccDeltafliC) showed greater growth increase in the apoplast relative to wild-type Xcc in 'Nagami' kumquat than in 'Duncan' grapefruit. These results indicate that flg22 initiates PTI in canker resistant and HLB tolerant genotypes, but not in susceptible/sensitive ones, and that a robust induction of PTI is an important component of citrus resistance to bacterial diseases. In citrus, two flg22 receptor candidate genes (CiFLS2-1 and CiFLS2-2) were identified using the citrus genomic databases. The two FLS2s were sequenced from 7 citrus species and interestingly, only one FLS2 ( kqFLS2-1) was identified from 'Nagami' kumquat. For the protein function analysis of citrus FLS2s, we selected kqFLS2-1 (the only FLS2 in canker highly resistant 'Nagami' kumquat), mdFLS2-2 (highly Xflg22-inducible FLS2 in resistant 'Sun Chu Sha' mandarin) and gfFLS2-2 (highly Xflg22-inducible FLS2 in susceptible 'Duncan' grapefruit), and expressed them using an Agrobacterium-mediated transient expression system in 'Duncan' grapefruit. It was found that leaves expressing kqFLS2-1 and mdFLS2-2 conferred higher responsiveness to Xflg22 treatment than the ones expressing gfFLS2-2. This result suggests that kqFLS2-1 and mdFLS2-2 have potential for engineering enhanced PTI in susceptible citrus genotypes for the control of citrus canker and other bacterial diseases. |
