An endolysin gene from Candidatus Liberibacter asiaticus confers dual resistance to huanglongbing and citrus canker

The most damaging citrus diseases are Huanglongbing(HLB)and citrus canker,which are caused by Candidatus Liberibacter asiaticus(CaLas)and Xanthomonas citri pv.citri(Xcc),respectively.Endolysins from bacteriophages are a possible option for disease resistance in plant breeding.Here,we report improvement of citrus resistance to HLB and citrus canker using the LasLYS1 and LasLYS2 endolysins from CaLas.LasLYS2 demonstrated bactericidal efficacy against several Rhizobiaceae bacteria and Xcc,according to inhibition zone analyses.The two genes,driven by a strong promoter from Cauliflower mosaic virus,35S,were integrated into Carrizo citrange via Agrobacterium-mediated transformation.More than 2 years of greenhouse testing indicated that LasLYS2 provided substantial and long-lasting resistance to HLB,allowing transgenic plants to retain low CaLas titers and no obvious symptoms while also clearing CaLas from infected plants in the long term.LasLYS2 transgenic plants with improved HLB resistance also showed resistance to Xcc,indicating that LasLYS2 had dual resistance to HLB and citrus canker.A microbiome study of transgenic plants revealed that the endolysins repressed Xanthomonadaceae and Rhizobiaceae populations in roots while increasing Burkholderiaceae and Rhodanobacteraceae populations,which might boost the citrus defense response,according to transcriptome analysis.We also found that Lyz domain 2 is the key bactericidal motif of LasLYS1 and LasLYS2.Four endolysins with potential resistance to HLB and citrus canker were found based on the structures of LasLYS1 and LasLYS2.Overall,the work shed light on the mechanisms of resistance of CaLas-derived endolysins,providing insights for designing endolysins to develop broad-spectrum disease resistance in citrus.

Production of marker-free transgenic plants from mature tissues of navel orange using a Cre/loxP site-recombination system

Genetic transformation with mature material as the explants could shorten the transgenic period and avoid seed dependence compared with genetic transformation using the epicotyl seedling stem segments as the receptor. Here, we constructed an Agrobacterium tumefaciensmediated transformation for generation of marker-free transgenic plants from navel orange(Citrus sinensis Osbeck) mature stems using a CreloxP recombination system. To efficiently recover the regenerated buds from mature tissues, five recovery methods were compared: in vitro micrografting of 0.1-0.5(1-2 weeks), > 0.5 cm(3-4 weeks) and > 1 cm long lignified bud and in vitro micrografting of explants with a bud and rooting regenerated bud. The data showed that in vitro micrografting of > 1 cm long regenerated bud with expanded leaves after one month of continuous culture for lignification was the optimal solution for plant recovery from mature tissues. Transgenic plants without selectable marker genes were created from navel orange(Citrus sinensis Osbeck) tissue using a transformation vector PLI-35SPR1aCB containing a Cre/loxP system recombination together with genes encoding the selectable marker isopentenyl transferase(IPT) and an anti-bacterial peptide(PR1aCB).Using IPT positive selection, the transformation efficiency determined by PCR was 0.9%, and in total, 20 transgenic plants were obtained.Southern blotting confirmed further their transgenicity. PCR and sequencing analysis demonstrated that both the Cre and IPT genes had been successfully removed from the transgenic plants(deletion efficiency 100%). Over all, using Cre/loxP system recombination together with the IPT positive selection, marker-free transgenic plants can be recovered efficiently from mature tissues of navel orange(Citrus sinensis Osbeck), which provides a potential method for production of transgenic plants from citrus mature tissue.

CitGVD:a comprehensive database of citrus genomic variations

Citrus is one of the most important commercial fruit crops worldwide.With the vast genomic data currently available for citrus fruit,genetic relationships,and molecular markers can be assessed for the development of molecular breeding and genomic selection strategies.In this study,to permit the ease of access to these data,a web-based database,the citrus genomic variation database(CitGVD,http://citgvd.cric.cn/home)was developed as the first citrusspecific comprehensive database dedicated to genome-wide variations including single nucleotide polymorphisms(SNPs)and insertions/deletions(INDELs).The current version(V1.0.0)of CitGVD is an open-access resource centered on 1,493,258,964 high-quality genomic variations and 84 phenotypes of 346 organisms curated from in-house projects and public resources.CitGVD integrates closely related information on genomic variation annotations,related gene annotations,and details regarding the organisms,incorporating a variety of built-in tools for data accession and analysis.As an example,CitGWAS can be used for genome-wide association studies(GWASs)with SNPs and phenotypic data,while CitEVOL can be used for genetic structure analysis.These features make CitGVD a comprehensive web portal and bioinformatics platform for citrus-related studies.It also provides a model for analyzing genome-wide variations for a wide range of crop varieties.