Huanglongbing (HLB) is a destructive disease of citrus. The disease is caused by the phloemlimited fastidious proteobacterium Candidatus Liberibacter asiaticus, which is transmitted by the Asian citrus psyllid (Diaphorina citri). The symptoms of HLB have been related by callose accumulation in the phloem sieve plates. The key class of enzymes for callose synthesis is the Callose Synthases. The callose synthase genes (calS) expression is modulated for biotic and abiotic stresses. In this study, nine C. sinensis calS genes (CscalS) were identified and the expression patterns were analyzed in CaLas inoculated and healthy plants. At 120 days after inoculation (dpi), CscalS2 and CscalS7 were significantly up-regulated in the HLB positive plants. At 360 dpi CscalS7 and CscalS12 were significantly up-regulated in the HLB positive plants. Our results confirmed that CaLas infection is accompanied by the increased deposition of callose in the phloem sieve tubes and accumulation of starch in the leaves. It is suggested that the increased deposition of callose in the phloem sieve tubes is a hypersensitivity reaction, inhibiting phloem transport to consequently reducing the bacterial colonization via phloem, what possibly contribute to the starch accumulation in the leaves and the development of HLB symptoms. It is also proposed that CscalS2, CscalS7 and CscalS12 are involved in citrus defense against CaLas, forming a complex in the phloem.

BackgroundRNA helicases are enzymes that catalyze the separation of double-stranded RNA (dsRNA) using the free energy of ATP binding and hydrolysis. DEAD/DEAH families participate in many different aspects of RNA metabolism, including RNA synthesis, RNA folding, RNA-RNA interactions, RNA localization and RNA degradation. Several important bacterial DEAD/DEAH-box RNA helicases have been extensively studied. In this study, we characterize the ATP-dependent RNA helicase encoded by the hrpB (XAC0293) gene using deletion and genetic complementation assays. We provide insights into the function of the hrpB gene in Xanthomonas citri subsp. citri by investigating the roles of hrpB in biofilm formation on abiotic surfaces and host leaves, cell motility, host virulence of the citrus canker bacterium and growth in planta.ResultsThe hrpB gene is highly conserved in the sequenced strains of Xanthomonas. Mutation of the hrpB gene (∆hrpB) resulted in a significant reduction in biofilms on abiotic surfaces and host leaves. ∆hrpB also exhibited increased cell dispersion on solid medium plates. ∆hrpB showed reduced adhesion on biotic and abiotic surfaces and delayed development in disease symptoms when sprayed on susceptible citrus leaves. Quantitative reverse transcription-PCR assays indicated that deletion of hrpB reduced the expression of four type IV pili genes. The transcriptional start site of fimA (XAC3241) was determined using rapid amplification of 5′-cDNA Ends (5′RACE). Based on the results of fimA mRNA structure predictions, the fimA 5′ UTR may contain three different loops. HrpB may be involved in alterations to the structure of fimA mRNA that promote the stability of fimA RNA.ConclusionsOur data show that hrpB is involved in adherence of Xanthomonas citri subsp. citri to different surfaces. In addition, to the best of our knowledge, this is the first time that a DEAH RNA helicase has been implicated in the regulation of type IV pili in Xanthomonas.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-016-0655-1) contains supplementary material, which is available to authorized users.

Huanglongbing (HLB), caused mainly by ‘Candidatus Liberibacter asiaticus’ (CLas), is the most devastating citrus disease because all commercial species are susceptible. HLB tolerance has been observed in Poncirus trifoliata and their hybrids. A wide-ranging transcriptomic analysis using contrasting genotypes regarding HLB severity was performed to identify the genetic mechanism associated with tolerance to HLB. The genotypes included Citrus sinensis, Citrus sunki, Poncirus trifoliata and three distinct groups of hybrids obtained from crosses between C. sunki and P. trifoliata. According to bacterial titer and symptomatology studies, the hybrids were clustered as susceptible, tolerant and resistant to HLB. In P. trifoliata and resistant hybrids, genes related to specific pathways were differentially expressed, in contrast to C. sinensis, C. sunki and susceptible hybrids, where several pathways were reprogrammed in response to CLas. Notably, a genetic tolerance mechanism was associated with the downregulation of gibberellin (GA) synthesis and the induction of cell wall strengthening. These defense mechanisms were triggered by a class of receptor-related genes and the induction of WRKY transcription factors. These results led us to build a hypothetical model to understand the genetic mechanisms involved in HLB tolerance that can be used as target guidance to develop citrus varieties or rootstocks with potential resistance to HLB.

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