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‘Duncan’ Grapefruit (Citrus paradisi Macf.) and ‘Sun Chu Sha’ mandarin (C. reticulalta Blanco) represent two citrus genotypes that have different levels of tolerance to citrus greening or huanglongbing (HLB), a bacterial disease caused by Candidatus Liberibacter sp. In this study, the response of the two genotypes to the conserved 22 amino acid domain of the Liberibacter flagellin (Lflg22), a Pathogen-Associated Molecular Pattern (PAMP), were compared. The expression levels of citrus defense-associated genes including AZI1, EDS1, NDR1, SGT1, RAR1, PAL1, ICS1, PAL1, NPR1, NPR2, NPR3, PR1 and RdRp in response to Lflg22 were analyzed. The HLB moderately tolerant Sun Chu Sha showed a stronger response to Lflg22 than the HLB-sensitive grapefruit. These results suggest that differences in the levels of PAMP-triggered Immunity (PTI) between the two genotypes are associated with the observed levels of HLB tolerance. Interestingly, although the Ca. L. asiaticus flagellin gene has been shown to be functional, no flagellum has been observed in this bacterium.

The bacterial agent of citrus canker disease ( Xanthomonas citri ssp. citri, Xcc) has caused tremendous economic losses to the citrus industry around the world. Pathogen-associated molecular pattern ( PAMP)-triggered immunity ( PTI) is important to plant immunity. In this study, we compared the defence responses of citrus canker-resistant and citrus canker-susceptible genotypes to the Xcc-derived PAMP flg22 ( Xflg22) by analysing the expression of 20 citrus defence-associated genes. We showed that, in the most resistant genotype, 'Nagami' kumquat, there was significant induction of several defence genes ( EDS1, NDR1, PBS1, RAR1, SGT1, PAL1, NPR2 and NPR3) as early as 6 h and up to 72 h after Xflg22 treatment. At the other end of the spectrum, highly susceptible 'Duncan' grapefruit showed no induction of the same defence genes, even 120 h after treatment. Citrus genotypes with partial levels of resistance showed intermediate levels of transcriptional reprogramming that correlated with their resistance level. Xflg22 also triggered a rapid oxidative burst in all genotypes which was higher and accompanied by the induction of PTI marker genes ( WRKY22 and GST1) only in the more resistant genotypes. Pretreatment with Xflg22 prior to Xcc inoculation inhibited bacterial growth in kumquat, but not in grapefruit. A flagellin-deficient Xcc strain ( XccΔfliC) showed greater growth increase relative to wild-type Xcc in kumquat than in grapefruit. Taken together, our results indicate that Xflg22 initiates strong PTI in canker-resistant genotypes, but not in susceptible ones, and that a robust induction of PTI is an important component of citrus resistance to canker. [ABSTRACT FROM AUTHOR]