de Miranda, Marcelo Pedreira

Phloem feeders, such as the psyllid Diaphorina citri ( D. citri ), feed on plants by inserting their stylet bundle followed by probing the apoplast before reaching the phloem. The psyllids secrete watery saliva containing various proteins into the phloem, which can act as effectors to facilitate their feeding or modulate host defense responses. Concomitantly, feeding is the main mode of transmitting the Candidatus Liberibacter asiaticus ( C Las) bacteria to the phloem. C Las produces several effectors that have been hypothesized to contribute to Huanglongbing (HLB) virulence. Here, we aimed to identify putative effector proteins from both D. citri and C Las. To achieve this, we used different omics techniques on different tissues and organs from both plants and insects. More specifically, we performed transcriptomics on the heads of healthy and C Las-infected D. citri and proteomics of artificial diet and of phloem of four different plant species fed on by healthy and C Las-infected D. citri. Subsequently, we used various criteria and bioinformatics tools to predict putative effectors. This resulted in the identification of four proteins from D. citri [ferritin, prisilkin, CG31997-PA, and pterin-4-alpha-carbinolamine dehydratase-like protein (PCBD)] and two Sec-dependent effectors from C Las, CLIBASIA_04560 and CLIBASIA_05320, that were used for further functional studies. The expression of these six proteins in Nicotiana benthamiana modified the ROS burst triggered by flagellin, indicating that they can indeed function as effector proteins in planta . In addition, expression of the psyllid effectors in planta significantly reduced the growth of Pseudomonas syringae pv. tabaci ( Pta ).

AbstractProduction of citrus, the main fruit tree crop worldwide, is severely threatened by Huanglongbing (HLB), for which as yet a cure is not available. Spread of this bacterial disease in America and Asia is intimately connected with dispersal and feeding of the insect vector Diaphorina citri, oligophagous on rutaceous host plants. Effective control of this psyllid is an important component in successful HLB management programs. Volatiles released from the non-host guava have been shown to be repellent to the psyllid and to inhibit its response to citrus odour. By analysing VOC emission from guava we identified one volatile compound, (E)-β-caryophyllene, which at certain doses exerts a repellent effect on D. citri. Non-host plant rejection mediated by (E)-β-caryophyllene is demonstrated here by using Arabidopsis over-expression and knock-out lines. For the first time, results indicate that genetically engineered Arabidopsis plants with modified emission of VOCs can alter the behaviour of D. citri. This study shows that transgenic plants with an inherent ability to release (E)-β-caryophyllene can potentially be used in new protection strategies of citrus trees against HLB.