Study on the expression of Nbs-lrr gene analogue (nbs552) in Citrus species infected with ‘candidatus liberibacter asiaticus’ and phytoplasmas

Candidatus Liberibacter asiaticus’ is associated with “huanglongbing” disease in citrus; however the various cultivars/genotypes combinations show a different degree of resistance. Recently, an NBS-LRR resistance transcript (NBS552) has been identified in grapefruit and pomelo trees infected by Ca. L. asiaticus’ under greenhouse conditions, therefore the expression level of this gene in severely HLB-infected orchards was studied in 1,170 symptomatic and asymptomatic trees. The total DNA was extracted from leaf midrib tissues and PCR with the primer set OI1/OI2c was performed to detect Ca. Liberibacter’ species presence. Two phytoplasma universal primer sets were employed to verify the presence of phytoplasmas in the same plants. PCR assays were then carried out to detect the NBS552 gene presence in the collected samples and its expression was analyzed using quantitative RT-PCR and resulted different between Ca. Liberibacter’ species- and phytoplasma-infected citrus plants. These results could confirm that phytoplasma presence is decreasing the NBS552 expression level therefore the expression of this gene may be used for further studies on citrus susceptibility to phloem-restricted prokaryotes.