Genetic analysis of salicylic acid-mediated defenses responses and histopathology in the huanglongbing pathosystem

The emergence of citrus huanglongbing (HLB) has been a constraint for worldwide citrus growers. HLB disease is associated to species of the biothrophic bacteria Candidatus Liberibacter spp. (CaL). In this study, we assessed the transcriptional status of salicylic acid (SA) genes and associated defenses between two contrasting citrus genotypes during challenge with Ca. Liberibacter asiaticus or Ca. Liberibacter americanus. Citrus sinensis exhibited the most evident alterations in gene expression of evaluated genes, when compared with Poncirus trifoliata. Upstream pathway SA genes showed a slight upward regulation in C. sinensis. Salicylic acid biosynthesis and accumulation might be impaired as we observed a low expression level of SA biosynthesis related genes. Moreover, genes associated to SA metabolism showed a slight induction. These results may account for the absence of significant downstream defense response related to salicylic acid. Leaf anatomical analysis revealed callose accumulation in both HLB infected, C. sinensis and P. trifoliata sieve tube elements (STE), although only C. sinensis exhibited collapsed STE. Our data corroborate other studies and suggest that the SA biosynthesis and metabolism related genes might be involved in the contrasting response to CaL in different citrus genotypes. Additionally, we suggest that collapsed STE might have a prominent implication in symptomatology of highly susceptible plants.