Differential expression of Isochorismate synthase in Catharanthus roseus during ‘Candidatus Liberibacter asiaticus’ infection

Abstract

Periwinkle facilitates study of the non-culturable pathogen ‘Candidatus Liberibacter asiaticus’ (CaLas), the causal agent of Huanglongbing (HLB) in citrus. Isochorismate synthase (ICS) is involved in the defense responses to pathogens through the salicylic acid (SA) signaling pathway in some plants. In this study, crICS from Catharanthus roseus was identified. The full-length cDNA of crICS is 2206 bp long, with an open reading frame (ORF) encoding a putative protein of 322 amino acids, including a conserved chorismate binding enzyme motif. Phylogenetic analysis revealed the crICS protein to cluster together with those of Rubia cordifolia and Coffea canephora to form a separate clade. Results of a qRT-PCR analysis indicated that crICS was constitutively expressed in leaf, stem, and root tissue of healthy periwinkle plants. During CaLas infection, crICS expression levels increased gradually and reached a peak at 22 days after inoculation (DAI), and then decreased gradually but were still higher than the expression level for the control group (CK) until the end of the experiment. To investigate whether crICS may act to mediate resistance through the SA signaling pathway in periwinkle, we sprayed SA on leaves, and found that SA increased crICS expression levels, but did not decrease the cumulative CaLas-positive rate of infected plants. The results suggest that crICS most likely mediates resistance to CaLas through the SA signaling pathway.