In Brazil, Huanglongbing (HLB) is caused by Candidatus Liberibacter americanus (CaLam) and Ca. L. asiaticus (CaLas). Both species are vectored by the Asian citrus psyllid and are restricted to the phloem of infected citrus, where they promote a severe imbalance in the translocation of nutrients and other important substances along the plant. Several studies of the transcriptional response of citrus to HLB have been reported, but only for infection caused by CaLas. This study evaluated the transcriptional reprogramming of a susceptible genotype (Pera sweet orange) challenged with CaLam, using a customized 385K microarray chip. The analyses showed that a large number of genes and biological processes were significantly altered upon CaLam infection. Among the changes we highlight induction of zinc transporters, modulation of enzymes related to sugar metabolism, decreased photosynthesis, induction of several defense-related genes and modulation of enzymes regulating ROS production. Several biological processes reported as differentially modulated upon infection with CaLas responded similarly to CaLam. The large number of receptor-like proteins, PR genes, NBS-LRR and transcription factors (such as WRKY and MYB) found showed that even a susceptible citrus genotype is able to actively respond to infection by CaLam, as reported for CaLas. Twenty candidate genes were selected for validation in symptomatic and asymptomatic PCR-positive leaves of Hamlin sweet orange infected with CaLas or CaLam. Finally, using in silico approaches, we compared our results with all published studies using CaLas to hypothesize a global feature of the defense/susceptibility mechanisms of citrus in response to the bacteria. These results have been explored in selection of target genes for genetic engineering to control HLB. Also, further transcriptome (RNAseq) experiments using tolerant and susceptible citrus genotypes infected with CaLam or CaLas using different time points are in progress to investigate the dynamics of expression of these genes during early stages of infection.

In Brazil, Huanglongbing (HLB) is caused by Candidatus Liberibacter americanus (CaLam) and Ca. L. asiaticus (CaLas). Both species are vectored by the Asian citrus psyllid and are restricted to the phloem of infected citrus, where they promote a severe imbalance in the translocation of nutrients and other important substances along the plant. Several studies of the transcriptional response of citrus to HLB have been reported, but only for infection caused by CaLas. This study evaluated the transcriptional reprogramming of a susceptible genotype (Pera sweet orange) challenged with CaLam, using a customized 385K microarray chip. The analyses showed that a large number of genes and biological processes were significantly altered upon CaLam infection. Among the changes we highlight induction of zinc transporters, modulation of enzymes related to sugar metabolism, decreased photosynthesis, induction of several defense-related genes and modulation of enzymes regulating ROS production. Several biological processes reported as differentially modulated upon infection with CaLas responded similarly to CaLam. The large number of receptor-like proteins, PR genes, NBS-LRR and transcription factors (such as WRKY and MYB) found showed that even a susceptible citrus genotype is able to actively respond to infection by CaLam, as reported for CaLas. Twenty candidate genes were selected for validation in symptomatic and asymptomatic PCR-positive leaves of Hamlin sweet orange infected with CaLas or CaLam. Finally, using in silico approaches, we compared our results with all published studies using CaLas to hypothesize a global feature of the defense/susceptibility mechanisms of citrus in response to the bacteria. These results have been explored in selection of target genes for genetic engineering to control HLB. Also, further transcriptome (RNAseq) experiments using tolerant and susceptible citrus genotypes infected with CaLam or CaLas using different time points are in progress to investigate the dynamics of expression of these genes during early stages of infection.

Several studies have addressed transcriptional changes in Citrus sinensis samples in response to Candidatus Liberibacter asiaticus (CaLas) with the objective to reveal the mechanisms underlying the development of Huanglongbing (HLB) and identify possible strategies to manage the disease. The aim of this work was to provide data using NGS technology (RNAseq) for a comprehensive analysis of differential expression changes in C. sinensis leaves and branches induced by HLB, caused either by CaLas or CaLam. Four treatments were evaluated; each of them consisted of RNA bulks extracted from five C. sinensis HLB symptomatic leaves or branches inoculated with CaLam or CaLas. The samples were subjected to RNAseq sequencing and the differential expression analyses were performed with Cuffdiff. In parallel, we performed a simple parametric test based on the mean and standard deviation to select statistically significant differentially expressed genes (DEG), named RSDA (Relative standard deviation analysis). For this approach, we considered standard deviation values of <0.7, and p-value = 0.01. Several genes associated with disease response, transcription factors involved in the activation of pathways such as the jasmonic acid, salicylic acid and ethylene, as well as genes involved in oxidative stress proved to be differentially expressed in our analyses. In leaves, we identified genes belonging to the WRKY transcription factors, ankyrin repeat family, NB-ARC domain-containing disease resistance, ethylene-forming enzymes and chaperones. In branches, we found many cytochromes, as well as genes involved in callose deposition, AP2/B3 transcriptional factor family and LEA proteins as being differentially expressed. Validation by RT-qPCR was performed for ten DEG.