Transcriptome-wide Identification of CDR Family in Citrus Latifolia and its Expression During HLB Disease

Huanglongbing (HLB) is the most devastating disease in citrus production around the world. Currently, there are no HLB-resistant citrus species or genotypes, however, Persian lime (Citrus latifolia Tan.) has been reported as the most tolerant citrus species to this disease, but their molecular mechanisms of tolerance have not been characterized yet. It has been suggested that the high expression of Constitutive Disease Resistance (CDR) genes is associated with HLB tolerance in Poncirus trifoliata, a close relative of citrus species, therefore, the aim of this work was to identify CDR gene family members in the Persian lime transcriptome and to determine if HLB induces their expression. Transcripts with one or two catalytic domains were searched, then, phylogeny for CDR genes was carried out with the selected transcripts from C. latifolia, P. trifoliata, and C. sinensis. Similarity among proteins was estimated using alignments, identity matrix, UPGMA, and conserved motifs analyses. Once C. latifolia CDR genes were identified, their expression was determined in the transcriptome data and validated by RT-PCR in asymptomatic and HBL-symptomatic leaves. A total of 146 sequences with one or two catalytic domains were found, and seven CDR clades were recovered. However, the transcriptome and RT-PCR data did not show significant differences in the expression of these ClCDR genes between HLB-symptomatic and asymptomatic plants. Therefore, although our results demonstrated the presence of CDR genes in the Persian lime genome, they also suggest that HLB tolerance is not correlated with the increased expression of this gene family in this species.