Plant Growth Promotion Driven by a Novel Caulobacter Strain

Soil microbial communities hold great potential for sustainable and ecologically compatible agriculture. Although numerous plant-beneficial bacterial strains from a wide range of taxonomic groups have been reported, very little evidence is available on the plant-beneficial role of bacteria from the genus Caulobacter. Here, the mode of action of a Caulobacter strain, designated RHG1, which had originally been identified through a microbial screen for plant growth-promoting (PGP) bacteria in maize (Zea mays), is investigated in Arabidopsis thaliana. RHG1 colonized both roots and shoots of Arabidopsis, promoted lateral root formation in the root, and increased leaf number and leaf size in the shoot. The genome of RHG1 was sequenced and was utilized to look for PGP factors. Our data revealed that the bacterial production of nitric oxide, auxins, cytokinins, or 1-aminocyclopropane-1-carboxylate deaminase as PGP factors could be excluded. However, the analysis of brassinosteroid mutants suggests that an unknown PGP mechanism is involved that impinges directly or indirectly on the pathway of this growth hormone.

From prediction to function: Current practices and challenges towards the functional characterization of type III effectors

The type III secretion system (T3SS) is a well-studied pathogenicity determinant of many bacteria through which effectors (T3Es) are translocated into the host cell, where they exercise a wide range of functions to deceive the host cell's immunity and to establish a niche. Here we look at the different approaches that are used to functionally characterize a T3E. Such approaches include host localization studies, virulence screenings, biochemical activity assays, and large-scale omics, such as transcriptomics, interactomics, and metabolomics, among others. By means of the phytopathogenic Ralstonia solanacearum species complex (RSSC) as a case study, the current advances of these methods will be explored, alongside the progress made in understanding effector biology. Data obtained by such complementary methods provide crucial information to comprehend the entire function of the effectome and will eventually lead to a better understanding of the phytopathogen, opening opportunities to tackle it.

Keeping in Touch with Type-III Secretion System Effectors: Mass Spectrometry-Based Proteomics to Study Effector-Host Protein-Protein Interactions

Manipulation of host cellular processes by translocated bacterial effectors is key to the success of bacterial pathogens and some symbionts. Therefore, a comprehensive understanding of effectors is of critical importance to understand infection biology. It has become increasingly clear that the identification of host protein targets contributes invaluable knowledge to the characterization of effector function during pathogenesis. Recent advances in mapping protein-protein interaction networks by means of mass spectrometry-based interactomics have enabled the identification of host targets at large-scale. In this review, we highlight mass spectrometry-driven proteomics strategies and recent advances to elucidate type-III secretion system effector-host protein-protein interactions. Furthermore, we highlight approaches for defining spatial and temporal effector-host interactions, and discuss possible avenues for studying natively delivered effectors in the context of infection. Overall, the knowledge gained when unravelling effector complexation with host factors will provide novel opportunities to control infectious disease outcomes.

Exploring the Tomato Root Protein Network Exploited by Core Type 3 Effectors from the Ralstonia solanacearum Species Complex

Proteomics has become a powerful approach for the identification and characterization of type III effectors (T3Es). Members of the Ralstonia solanacearum species complex (RSSC) deploy T3Es to manipulate host cells and to promote root infection of, among others, a wide range of solanaceous plants such as tomato, potato, and tobacco. Here, we used TurboID-mediated proximity labeling (PL) in tomato hairy root cultures to explore the proxeomes of the core RSSC T3Es RipU, RipD, and RipB. The RipU proxeome was enriched for multiple protein kinases, suggesting a potential impact on the two branches of the plant immune surveillance system, being the membrane-localized PAMP-triggered immunity (PTI) and the RIN4-dependent effector-triggered immunity (ETI) complexes. In agreement, a transcriptomics analysis in tomato revealed the potential involvement of RipU in modulating reactive oxygen species (ROS) signaling. The proxeome of RipB was putatively enriched for mitochondrial and chloroplast proteins and that of RipD for proteins potentially involved in the endomembrane system. Together, our results demonstrate that TurboID-PL in tomato hairy roots represents a promising tool to study Ralstonia T3E targets and functioning and that it can unravel potential host processes that can be hijacked by the bacterial pathogen.

Cataloguing Protein Complexes In Planta Using TurboID-Catalyzed Proximity Labeling

Mapping protein-protein interactions is crucial to understand protein function. Recent advances in proximity-dependent biotinylation (BioID) coupled to mass spectrometry (MS) allow the characterization of protein complexes in diverse plant models. Here, we describe the use of BioID in hairy root cultures of tomato and provide detailed information on how to analyze the data obtained by MS.