HPE1, an Effector from Zebra Chip Pathogen Interacts with Tomato Proteins and Perturbs Ubiquitinated Protein Accumulation

The gram-negative bacterial genus Liberibacter includes economically important pathogens, such as ‘Candidatus Liberibacter asiaticus’ that cause citrus greening disease (or Huanglongbing, HLB) and ‘Ca. Liberibacter solanacearum’ (Lso) that cause zebra chip disease in potato. Liberibacter pathogens are fastidious bacteria transmitted by psyllids. Pathogen manipulation of the host’ and vector’s immune system for successful colonization is hypothesized to be achieved by Sec translocon-dependent effectors (SDE). In previous work, we identified hypothetical protein effector 1 (HPE1), an SDE from Lso, that acts as a suppressor of the plant’s effector-triggered immunity (ETI)-like response. In this study, using a yeast two-hybrid system, we identify binding interactions between tomato RAD23 proteins and HPE1. We further show that HPE1 interacts with RAD23 in both nuclear and cytoplasmic compartments in planta. Immunoblot assays show that HPE1 is not ubiquitinated in the plant cell, but rather the expression of HPE1 induced the accumulation of other ubiquitinated proteins. A similar accumulation of ubiquitinated proteins is also observed in Lso infected tomato plants. Finally, earlier colonization and symptom development following Lso haplotype B infection are observed in HPE1 overexpressing plants compared to wild-type plants. Overall, our results suggest that HPE1 plays a role in virulence in Lso pathogenesis, possibly by perturbing the ubiquitin-proteasome system via direct interaction with the ubiquitin-like domain of RAD23 proteins.

Potato Zebra Chip: An Overview of the Disease, Control Strategies, and Prospects

Potato (Solanum tuberosum L.) is an important food crop worldwide. As the demand for fresh and processed potato products is increasing globally, there is a need to manage and control devastating diseases such as zebra chip (ZC). ZC disease causes major yield losses in many potato-growing regions and is associated with the fastidious, phloem-limited bacterium Candidatus Liberibacter solanacearum (CLso) that is vectored by the potato-tomato psyllid (Bactericera cockerelli Šulc). Current management measures for ZC disease mainly focus on chemical control and integrated pest management strategies of the psyllid vector to limit the spread of CLso, however, they add to the costs of potato production. Identification and deployment of CLso and/or the psyllid resistant cultivars, in combination with integrated pest management, may provide a sustainable long-term strategy to control ZC. In this review, we provide a brief overview of the ZC disease, epidemiology, current management strategies, and potential new approaches to manage ZC disease in the future.

Candidatus Liberibacter solanacearum (zebra chip).

Candidatus Liberibacter solanacearum (Lso) is a phloem-limited, Gram-negative, unculturable bacterium that is primarily spread by psyllid insect vectors. It is considered very invasive due to its ability to be transported primarily in infective psyllids (Munyaneza et al., 2007a; 2010a,b; 2012a,b; Munyaneza, 2012; Alfaro-Fernandez et al., 2012a,b). It has been shown that Lso distribution in the Americas, New Zealand and Europe follows the distribution of its known psyllid vectors (Munyaneza, 2010; 2012).In New Zealand, where Lso was introduced along with Bactericera cockerelli, supposedly from Western USA in early 2000s, the bacterium had already spread to both North and South Island by the time it was first documented in 2006 (Gill, 2006). It is clear that introduction of the psyllid vectors of Lso into new regions is likely to result in the rapid spread of this bacterium. Lso and several of its vectors are already on several alert lists, including the EPPO A1 Regulated Quarantine Plant Pests.

Parámetros agronómicos del tomate (Solanum lycopersicum L.) inoculado con “Candidatus Liberibacter solanacearum” y tratados con fosfitos

La enfermedad del permanente del tomate o zebra chip cau- sada por “Candidatus Liberibacter solanacearum” (Ca. L. solanacearum), es de gran importancia en las regiones productoras de tomate en México, la cual causa pérdidas de entre el 60 y 100% de la producción. El objetivo del trabajo fue evaluar el efecto de seis fosfitos (FOS), ácido salicílico y acibenzolar s-metil sobre el crecimiento del cultivo de tomate y la severidad de Ca. L. solanacearum. Las plantas de tomate se infectaron con Ca. L. solanacearum mediante el confinamiento con adultos de B. cockerelli portadores de esta bacteria. Como respuesta se evaluaron la disminución de la severidad de la enfermedad y nueve parámetros agronómicos. La aspersión de fosfitos redujo la severidad de los síntomas de la enfermedad ocasionada por Ca. L. solanacearum, mejorando las variables altura de planta, peso del fruto, peso de planta y peso de raíz. En cambio, la aspersión de ácido salicílico y acibenzolar s-metil no redujeron los efectos de la enfermedad en el cultivo de tomate.

Liberibacter, A Preemptive Bacterium: Apoptotic Response Repression in the Host Gut at the Early Infection to Facilitate Its Acquisition and Transmission

“Candidatus Liberibacter solanacearum” (Lso) is a phloem-limited Gram-negative bacterium that infects crops worldwide. In North America, two haplotypes of Lso (LsoA and LsoB) are transmitted by the potato psyllid, Bactericera cockerelli (Šulc), in a circulative and persistent manner. Both haplotypes cause damaging plant diseases (e.g., zebra chip in potatoes). The psyllid gut is the first organ Lso encounters and could be a barrier for its transmission. However, little is known about the psyllid gut immune responses triggered upon Lso infection. In this study, we focused on the apoptotic response in the gut of adult potato psyllids at the early stage of Lso infection. We found that there was no evidence of apoptosis induced in the gut of the adult potato psyllids upon infection with either Lso haplotype based on microscopic observations. However, the expression of the inhibitor of apoptosis IAPP5.2 gene (survivin-like) was significantly upregulated during the period that Lso translocated into the gut cells. Interestingly, silencing of IAPP5.2 gene significantly upregulated the expression of two effector caspases and induced apoptosis in the psyllid gut cells. Moreover, RNA interference (RNAi) of IAPP5.2 significantly decreased the Lso titer in the gut of adult psyllids and reduced their transmission efficiency. Taken together, these observations suggest that Lso might repress the apoptotic response in the psyllid guts by inducing the anti-apoptotic gene IAPP5.2 at an early stage of the infection, which may favor Lso acquisition in the gut cells and facilitate its transmission by potato psyllid.

Plant hairy roots enable high throughput identification of antimicrobials against Candidatus Liberibacter spp.

A major bottleneck in identifying therapies to control citrus greening and other devastating plant diseases caused by fastidious pathogens is our inability to culture the pathogens in defined media or axenic cultures. As such, conventional approaches for antimicrobial evaluation (genetic or chemical) rely on time-consuming, low-throughput and inherently variable whole-plant assays. Here, we report that plant hairy roots support the growth of fastidious pathogens like Candidatus Liberibacter spp., the presumptive causal agents of citrus greening, potato zebra chip and tomato vein greening diseases. Importantly, we leverage the microbial hairy roots for rapid, reproducible efficacy screening of multiple therapies. We identify six antimicrobial peptides, two plant immune regulators and eight chemicals which inhibit Candidatus Liberibacter spp. in plant tissues. The antimicrobials, either singly or in combination, can be used as near- and long-term therapies to control citrus greening, potato zebra chip and tomato vein greening diseases.