The Arabidopsis E3 ubiquitin ligase PUB4 regulates BIK1 and is targeted by a bacterial type-III effector

Investigating the mechanism of chloroplast singlet oxygen signaling in the Arabidopsis thaliana accelerated cell death 2 mutant

As sessile organisms, plants have evolved complex signaling mechanisms to sense stress and acclimate. This includes the use of reactive oxygen species (ROS) generated during dysfunctional photosynthesis to initiate signaling. One such ROS, singlet oxygen (1O2), can trigger retrograde signaling, chloroplast degradation, and programmed cell death. However, the signaling mechanisms are largely unknown. Several proteins (e.g. PUB4, OXI1, EX1) are proposed to play signaling roles across three Arabidopsis thaliana mutants that conditionally accumulate chloroplast 1O2 (fluorescent in blue light (flu), chlorina 1 (ch1), and plastid ferrochelatase 2 (fc2)). We previously demonstrated that these mutants reveal at least two chloroplast 1O2 signaling pathways (represented by flu and fc2/ch1). Here, we test if the 1O2-accumulating lesion mimic mutant, accelerated cell death 2 (acd2), also utilizes these pathways. The pub4-6 allele delayed lesion formation in acd2 and restored photosynthetic efficiency and biomass. Conversely, an oxi1 mutation had no measurable effect on these phenotypes. acd2 mutants were not sensitive to excess light (EL) stress, yet pub4-6 and oxi1 both conferred EL tolerance within the acd2 background, suggesting that EL-induced 1O2 signaling pathways are independent from spontaneous lesion formation. Thus, 1O2 signaling in acd2 may represent a third (partially overlapping) pathway to control cellular degradation.

A review on ubiquitin ligases: Orchestrators of plant resilience in adversity

Ubiquitin- proteasome system (UPS) is universally present in plants and animals, mediating many cellular processes needed for growth and development. Plants constantly defend themselves against endogenous and exogenous stimuli such as hormonal signaling, biotic stresses such as viruses, fungi, nematodes, and abiotic stresses like drought, heat, and salinity by developing complex regulatory mechanisms. Ubiquitination is a regulatory mechanism involving selective elimination and stabilization of regulatory proteins through the UPS system where E3 ligases play a central role; they can bind to the targets in a substrate-specific manner, followed by poly-ubiquitylation, and subsequent protein degradation by 26 S proteasome. Increasing evidence suggests different types of E3 ligases play important roles in plant development and stress adaptation. Herein, we summarize recent advances in understanding the regulatory roles of different E3 ligases and primarily focus on protein ubiquitination in plant-environment interactions. It also highlights the diversity and complexity of these metabolic pathways that enable plant to survive under challenging conditions. This reader-friendly review provides a comprehensive overview of E3 ligases and their substrates associated with abiotic and biotic stresses that could be utilized for future crop improvement.

Dynamic modulation of nodulation factor receptor levels by phosphorylation-mediated functional switch of a RING-type E3 ligase during legume nodulation

The precise control of receptor levels is crucial for initiating cellular signaling transduction in response to specific ligands; however, such mechanisms regulating nodulation factor (NF) receptor (NFR)-mediated perception of NFs to establish symbiosis remain unclear. In this study, we unveil the pivotal role of the NFR-interacting RING-type E3 ligase 1 (NIRE1) in regulating NFR1/NFR5 homeostasis to optimize rhizobial infection and nodule development in Lotus japonicus. We demonstrated that NIRE1 has a dual function in this regulatory process. It associates with both NFR1 and NFR5, facilitating their degradation through K48-linked polyubiquitination before rhizobial inoculation. However, following rhizobial inoculation, NFR1 phosphorylates NIRE1 at a conserved residue, Tyr-109, inducing a functional switch in NIRE1, which enables NIRE1 to mediate K63-linked polyubiquitination, thereby stabilizing NFR1/NFR5 in infected root cells. The introduction of phospho-dead NIRE1Y109F leads to delayed nodule development, underscoring the significance of phosphorylation at Tyr-109 in orchestrating symbiotic processes. Conversely, expression of the phospho-mimic NIRE1Y109E results in the formation of spontaneous nodules in L. japonicus, further emphasizing the critical role of the phosphorylation-dependent functional switch in NIRE1. In summary, these findings uncover a fine-tuned symbiotic mechanism that a single E3 ligase could undergo a phosphorylation-dependent functional switch to dynamically and precisely regulate NF receptor protein levels.

Deciphering the mechanism of E3 ubiquitin ligases in plant responses to abiotic and biotic stresses and perspectives on PROTACs for crop resistance

With global climate change, it is essential to find strategies to make crops more resistant to different stresses and guarantee food security worldwide. E3 ubiquitin ligases are critical regulatory elements that are gaining importance due to their role in selecting proteins for degradation in the ubiquitin-proteasome proteolysis pathway. The role of E3 Ub ligases has been demonstrated in numerous cellular processes in plants responding to biotic and abiotic stresses. E3 Ub ligases are considered a class of proteins that are difficult to control by conventional inhibitors, as they lack a standard active site with pocket, and their biological activity is mainly due to protein-protein interactions with transient conformational changes. Proteolysis-targeted chimeras (PROTACs) are a new class of heterobifunctional molecules that have emerged in recent years as relevant alternatives for incurable human diseases like cancer because they can target recalcitrant proteins for destruction. PROTACs interact with the ubiquitin-proteasome system, principally the E3 Ub ligase in the cell, and facilitate proteasome turnover of the proteins of interest. PROTAC strategies harness the essential functions of E3 Ub ligases for proteasomal degradation of proteins involved in dysfunction. This review examines critical advances in E3 Ub ligase research in plant responses to biotic and abiotic stresses. It highlights how PROTACs can be applied to target proteins involved in plant stress response to mitigate pathogenic agents and environmental adversities.

Receptor-like cytoplasmic kinases: orchestrating plant cellular communication

The receptor-like kinase (RLK) family of receptors and the associated receptor-like cytoplasmic kinases (RLCKs) have expanded in plants because of selective pressure from environmental stress and evolving pathogens. RLCKs link pathogen perception to activation of coping mechanisms. RLK-RLCK modules regulate hormone synthesis and responses, reactive oxygen species (ROS) production, Ca2+ signaling, activation of mitogen-activated protein kinase (MAPK), and immune gene expression, all of which contribute to immunity. Some RLCKs integrate responses from multiple receptors recognizing distinct ligands. RLKs/RLCKs and nucleotide-binding domain, leucine-rich repeats (NLRs) were found to synergize, demonstrating the intertwined genetic network in plant immunity. Studies in arabidopsis (Arabidopsis thaliana) have provided paradigms about RLCK functions, but a lack of understanding of crop RLCKs undermines their application. In this review, we summarize current understanding of the diverse functions of RLCKs, based on model systems and observations in crop species, and the emerging role of RLCKs in pathogen and abiotic stress response signaling.

Cell wall-mediated root development is targeted by a soil-borne bacterial pathogen to promote infection

Plant pathogens manipulate host development, facilitating colonization and proliferation. Ralstonia solanacearum is a soil-borne bacterial pathogen that penetrates roots and colonizes plants through the vascular system, causing wilting and death. Here, we find that RipAC, an effector protein from R. solanacearum, alters root development in Arabidopsis, promoting the formation of lateral roots and root hairs. RipAC interacts with CELLULOSE SYNTHASE (CESA)-INTERACTIVE PROTEIN 1 (CSI1), which regulates the activity of CESA complexes at the plasma membrane. RipAC disrupts CESA-CSI1 interaction, leading to a reduction in cellulose content, root developmental alterations, and a promotion of bacterial pathogenicity. We find that CSI1 also associates with the receptor kinase FERONIA, forming a complex that negatively regulates immunity in roots; this interaction, however, is not affected by RipAC. Our work reveals a bacterial virulence strategy that selectively affects the activities of a host target, promoting anatomical alterations that facilitate infection without causing activation of immunity.

Plant U-Box 4 regulates chloroplast stress signaling and programmed cell death via Salicylic acid modulation

In response to environmental stress, chloroplasts generate reactive oxygen species, including singlet oxygen (1O2), which regulates nuclear gene expression (retrograde signaling), chloroplast turnover, and programmed cell death (PCD). Yet, the central signaling mechanisms and downstream responses remain poorly understood. The Arabidopsis thaliana plastid ferrochelatase two (fc2) mutant conditionally accumulates 1O2 and involves Plant U-Box 4 (PUB4), a cytoplasmic E3 ubiquitin ligase, in propagating these signals. To gain insights into 1O2 signaling pathways, we compared transcriptomes of fc2 and fc2 pub4 mutants. The accumulation of 1O2 in fc2 plants broadly repressed genes involved in chloroplast function and photosynthesis, while 1O2 induced genes and transcription factors involved in abiotic and biotic stress, the biosynthesis of jasmonic acid (JA), and Salicylic acid (SA). Elevated JA and SA levels were observed in stressed fc2 plants, but were not responsible for PCD. pub4 reversed the majority of 1O2-induced gene expression in fc2 and reduced the JA content, but maintained elevated levels of SA even in the absence of 1O2 stress. Reducing SA levels in fc2 pub4 restored 1O2 signaling and light sensitivity. Together, this work demonstrates that SA plays a protective role during photo-oxidative stress and that PUB4 mediates 1O2 signaling by modulating its levels.

The OsCPK17-OsPUB12-OsRLCK176 module regulates immune homeostasis in rice

Plant immunity is fine-tuned to balance growth and defense. However, little is yet known about molecular mechanisms underlying immune homeostasis in rice (Oryza sativa). In this study, we reveal that a rice calcium-dependent protein kinase (CDPK), OsCPK17, interacts with and stabilizes the receptor-like cytoplasmic kinase (RLCK) OsRLCK176, a close homolog of Arabidopsis thaliana BOTRYTIS-INDUCED KINASE 1 (AtBIK1). Oxidative burst and pathogenesis-related gene expression triggered by pathogen-associated molecular patterns are significantly attenuated in the oscpk17 mutant. The oscpk17 mutant and OsCPK17-silenced lines are more susceptible to bacterial diseases than the wild-type plants, indicating that OsCPK17 positively regulates rice immunity. Furthermore, the plant U-box (PUB) protein OsPUB12 ubiquitinates and degrades OsRLCK176. OsCPK17 phosphorylates OsRLCK176 at Ser83, which prevents the ubiquitination of OsRLCK176 by OsPUB12 and thereby enhances the stability and immune function of OsRLCK176. The phenotypes of the ospub12 mutant in defense responses and disease resistance show that OsPUB12 negatively regulates rice immunity. Therefore, OsCPK17 and OsPUB12 reciprocally maintain OsRLCK176 homeostasis and function as positive and negative immune regulators, respectively. This study uncovers positive cross talk between CDPK- and RLCK-mediated immune signaling in plants and reveals that OsCPK17, OsPUB12, and OsRLCK176 maintain rice immune homeostasis.

Evolutionary trajectory of pattern recognition receptors in plants

Cell-surface receptors play pivotal roles in many biological processes, including immunity, development, and reproduction, across diverse organisms. How cell-surface receptors evolve to become specialised in different biological processes remains elusive. To shed light on the immune-specificity of cell-surface receptors, we analyzed more than 200,000 genes encoding cell-surface receptors from 350 genomes and traced the evolutionary origin of immune-specific leucine-rich repeat receptor-like proteins (LRR-RLPs) in plants. Surprisingly, we discovered that the motifs crucial for co-receptor interaction in LRR-RLPs are closely related to those of the LRR-receptor-like kinase (RLK) subgroup Xb, which perceives phytohormones and primarily governs growth and development. Functional characterisation further reveals that LRR-RLPs initiate immune responses through their juxtamembrane and transmembrane regions, while LRR-RLK-Xb members regulate development through their cytosolic kinase domains. Our data suggest that the cell-surface receptors involved in immunity and development share a common origin. After diversification, their ectodomains, juxtamembrane, transmembrane, and cytosolic regions have either diversified or stabilised to recognise diverse ligands and activate differential downstream responses. Our work reveals a mechanism by which plants evolve to perceive diverse signals to activate the appropriate responses in a rapidly changing environment.

Plant U-box E3 ligases PUB20 and PUB21 negatively regulate pattern-triggered immunity in Arabidopsis

KEY MESSAGE

Plant U-box E3 ligases PUB20 and PUB21 are flg22-triggered signaling components and negatively regulate immune responses. Plant U-box proteins (PUBs) constitute a class of E3 ligases that are associated with various stress responses. Among the class IV PUBs featuring C-terminal Armadillo (ARM) repeats, PUB20 and PUB21 are closely related homologs. Here, we show that both PUB20 and PUB21 negatively regulate innate immunity in plants. Loss of PUB20 and PUB21 function leads to enhanced resistance to surface inoculation with the virulent bacterium Pseudomonas syringae pv. tomato DC3000 (Pst DC3000). However, the resistance levels remain unaffected after infiltration inoculation, suggesting that PUB20 and PUB21 primarily function during the early defense stages. The enhanced resistance to Pst DC3000 in PUB mutant plants (pub20-1, pub21-1, and pub20-1/pub21-1) correlates with extensive flg22-triggered reactive oxygen production, strong MPK3 activation, and enhanced transcriptional activation of early immune response genes. Additionally, PUB mutant plants (except pub21-1) exhibit constitutive stomatal closure after Pst DC3000 inoculation, implying the significant role of PUB20 in stomatal immunity. Comparative analyses of flg22 responses between PUB mutants and wild-type plants reveals that the robust activation of the pattern-induced immune responses may enhance resistance against Pst DC3000. Notably, the hypersensitivity responses triggered by RPM1/avrRpm1 and RPS2/avrRpt2 are independent of PUB20 and PUB21. These results suggest that PUB20 and PUB21 knockout mutations affect bacterial invasion, likely during the early stages, acting as negative regulators of plant immunity.

Root colonization by beneficial rhizobacteria

Rhizosphere microbes play critical roles for plant's growth and health. Among them, the beneficial rhizobacteria have the potential to be developed as the biofertilizer or bioinoculants for sustaining the agricultural development. The efficient rhizosphere colonization of these rhizobacteria is a prerequisite for exerting their plant beneficial functions, but the colonizing process and underlying mechanisms have not been thoroughly reviewed, especially for the nonsymbiotic beneficial rhizobacteria. This review systematically analyzed the root colonizing process of the nonsymbiotic rhizobacteria and compared it with that of the symbiotic and pathogenic bacteria. This review also highlighted the approaches to improve the root colonization efficiency and proposed to study the rhizobacterial colonization from a holistic perspective of the rhizosphere microbiome under more natural conditions.

High air humidity dampens salicylic acid pathway and NPR1 function to promote plant disease

The occurrence of plant disease is determined by interactions among host, pathogen, and environment. Air humidity shapes various aspects of plant physiology and high humidity has long been known to promote numerous phyllosphere diseases. However, the molecular basis of how high humidity interferes with plant immunity to favor disease has remained elusive. Here we show that high humidity is associated with an "immuno-compromised" status in Arabidopsis plants. Furthermore, accumulation and signaling of salicylic acid (SA), an important defense hormone, are significantly inhibited under high humidity. NPR1, an SA receptor and central transcriptional co-activator of SA-responsive genes, is less ubiquitinated and displays a lower promoter binding affinity under high humidity. The cellular ubiquitination machinery, particularly the Cullin 3-based E3 ubiquitin ligase mediating NPR1 protein ubiquitination, is downregulated under high humidity. Importantly, under low humidity the Cullin 3a/b mutant plants phenocopy the low SA gene expression and disease susceptibility that is normally observed under high humidity. Our study uncovers a mechanism by which high humidity dampens a major plant defense pathway and provides new insights into the long-observed air humidity influence on diseases.

Suppression of NLR-mediated plant immune detection by bacterial pathogens

The plant immune system is constituted of two functionally interdependent branches that provide the plant with an effective defense against microbial pathogens. They can be considered separate since one detects extracellular pathogen-associated molecular patterns by means of receptors on the plant surface, while the other detects pathogen-secreted virulence effectors via intracellular receptors. Plant defense depending on both branches can be effectively suppressed by host-adapted microbial pathogens. In this review we focus on bacterially driven suppression of the latter, known as effector-triggered immunity (ETI) and dependent on diverse NOD-like receptors (NLRs). We examine how some effectors secreted by pathogenic bacteria carrying type III secretion systems can be subject to specific NLR-mediated detection, which can be evaded by the action of additional co-secreted effectors (suppressors), implying that virulence depends on the coordinated action of the whole repertoire of effectors of any given bacterium and their complex epistatic interactions within the plant. We consider how ETI activation can be avoided by using suppressors to directly alter compromised co-secreted effectors, modify plant defense-associated proteins, or occasionally both. We also comment on the potential assembly within the plant cell of multi-protein complexes comprising both bacterial effectors and defense protein targets.

The Plant Ubiquitin-Proteasome System as a Target for Microbial Manipulation

The plant immune system perceives pathogens to trigger defense responses. In turn, pathogens secrete effector molecules to subvert these defense responses. The initiation and maintenance of defense responses involve not only de novo synthesis of regulatory proteins and enzymes but also their regulated degradation. The latter is achieved through protein degradation pathways such as the ubiquitin-proteasome system (UPS). The UPS regulates all stages of immunity, from the perception of the pathogen to the execution of the response, and, therefore, constitutes an ideal candidate for microbial manipulation of the host. Pathogen effector molecules interfere with the plant UPS through several mechanisms. This includes hijacking general UPS functions or perturbing its ability to degrade specific targets. In this review, we describe how the UPS regulates different immunity-related processes and how pathogens subvert this to promote disease.

Ubiquitination from the perspective of plant pathogens

The constant battle of survival between pathogens and host plants has played a crucial role in shaping the course of their co-evolution. However, the major determinants of the outcome of this ongoing arms race are the effectors secreted by pathogens into host cells. These effectors perturb the defense responses of plants to promote successful infection. In recent years, extensive research in the area of effector biology has reported an increase in the repertoire of pathogenic effectors that mimic or target the conserved ubiquitin-proteasome pathway. The role of the ubiquitin-mediated degradation pathway is well known to be indispensable for various aspects of a plant's life, and thus targeting or mimicking it seems to be a smart strategy adopted by pathogens. Therefore, this review summarizes recent findings on how some pathogenic effectors mimic or act as one of the components of the ubiquitin-proteasome machinery while others directly target the plant's ubiquitin-proteasome system.

BIK1 protein homeostasis is maintained by the interplay of different ubiquitin ligases in immune signaling

Pathogen-associated molecular patterns (PAMPs) trigger plant innate immunity that acts as the first line of inducible defense against pathogen infection. A receptor-like cytoplasmic kinase BOTRYTIS-INDUCED KINASE 1 (BIK1) functions as a signaling hub immediately downstream of multiple pattern recognition receptors (PRRs). It is known that PLANT U-BOX PROTEIN 25 (PUB25) and PUB26 ubiquitinate BIK1 and mediate BIK1 degradation. However, how BIK1 homeostasis is maintained is not fully understood. Here, we show that two closely related ubiquitin ligases, RING DOMAIN LIGASE 1 (RGLG1) and RGLG2, preferentially associate with the hypo-phosphorylated BIK1 and promote the association of BIK1 with the co-receptor for several PRRs, BRI1-ASSOCIATED RECEPTOR KINASE1 (BAK1). PUB25 interacts with RGLG2 and mediates its degradation. In turn, RGLG2 represses the ubiquitin ligase activity of PUB25. RGLG1/2 suppress PUB25-mediated BIK1 degradation, promote BIK1 protein accumulation, and positively regulate immune signaling in a ubiquitin ligase activity-dependent manner. Our work reveals how BIK1 homeostasis is maintained by the interplay of different ubiquitin ligases.

Symbiont-host interactome mapping reveals effector-targeted modulation of hormone networks and activation of growth promotion

Plants have benefited from interactions with symbionts for coping with challenging environments since the colonisation of land. The mechanisms of symbiont-mediated beneficial effects and similarities and differences to pathogen strategies are mostly unknown. Here, we use 106 (effector-) proteins, secreted by the symbiont Serendipita indica (Si) to modulate host physiology, to map interactions with Arabidopsis thaliana host proteins. Using integrative network analysis, we show significant convergence on target-proteins shared with pathogens and exclusive targeting of Arabidopsis proteins in the phytohormone signalling network. Functional in planta screening and phenotyping of Si effectors and interacting proteins reveals previously unknown hormone functions of Arabidopsis proteins and direct beneficial activities mediated by effectors in Arabidopsis. Thus, symbionts and pathogens target a shared molecular microbe-host interface. At the same time Si effectors specifically target the plant hormone network and constitute a powerful resource for elucidating the signalling network function and boosting plant productivity.

The RING-Finger Protein NbRFP1 Contributes to Regulating the Host Hypersensitive Response Induced by Oat Dwarf Virus RepA

Our previous study identified that the RepA protein encoded by the oat dwarf virus (ODV) was responsible for inducing a strong hypersensitive response (HR) during the virus infection in non-host tobacco plants. However, little was known about the molecular mechanism of the RepA-elicited HR. Here, a RING-finger protein, which is described as NbRFP1 and is mainly located in the cytoplasm and nucleus in Nicotiana benthamiana cells, was confirmed to interact with RepA. In addition, the accumulation level of NbRFP1 in N. benthamiana leaves was enhanced by either ODV infection or by only RepA expression. The knockdown of NbRFP1 by a TRV-mediated virus-induced gene silencing markedly delayed the ODV or RepA-elicited HR. By contrast, the overexpression of NbRFP1 in N. benthamiana conferred enhanced resistance to ODV infection and promoted RepA-induced HR. Further mutation analysis showed that a RING-finger domain located in NbRFP1 plays important roles in modulating RepA-induced HR, as well as in mediating the interaction between NbRFP1 and RepA.

K63-linked ubiquitin chains are a global signal for endocytosis and contribute to selective autophagy in plants

In contrast to other eukaryotic model organisms, the closely related ubiquitin (Ub)-conjugating enzymes UBC35 and UBC36 are the main sources of K63-linked Ub chains in Arabidopsis.¹ Although K63-linked chains have been associated with the regulation of vesicle trafficking, definitive proof for their role in endocytosis was missing. We show that the ubc35 ubc36 mutant has pleiotropic phenotypes related to hormone and immune signaling. Specifically, we reveal that ubc35-1 ubc36-1 plants have altered turnover of integral membrane proteins including FLS2, BRI1, and PIN1 at the plasma membrane. Our data indicates that K63-Ub chains are generally required for endocytic trafficking in plants. In addition, we show that in plants K63-Ub chains are involved in selective autophagy through NBR1, the second major pathway delivering cargoes to the vacuole for degradation. Similar to autophagy-defective mutants, ubc35-1 ubc36-1 plants display an accumulation of autophagy markers. Moreover, autophagy receptor NBR1 interacts with K63-Ub chains, which are required for its delivery to the lytic vacuole.² Together, we show that K63-Ub chains act as a general signal required for the two main pathways delivering cargo to the vacuole and thus, to maintain proteostasis.

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.

Proposal to classify Ralstonia solanacearum phylotype I strains as Ralstonia nicotianae sp. nov., and a genomic comparison between members of the genus Ralstonia

A Gram-negative, aerobic, rod-shaped, motile bacterium with multi-flagella, strain RS^T, was isolated from bacterial wilt of tobacco in Yuxi city of Yunnan province, China. The strain contains the major fatty acids of C_16:0, summed feature 3 (C_16:1 ω7c and/or C_16:1 ω6c), and summed feature 8 (C_18:1 ω7c and/or C_18:1 ω6c). The polar lipid profile of strain RS^T consists of diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine, and unidentified aminophospholipid. Strain RS^T contains ubiquinones Q-7 and Q-8. 16S rRNA gene sequence (1,407 bp) analysis showed that strain RS^T is closely related to members of the genus Ralstonia and shares the highest sequence identities with R. pseudosolanacearum LMG 9673^T (99.50%), R. syzygii subsp. indonesiensis LMG 27703^T (99.50%), R. solanacearum LMG 2299^T (99.28%), and R. syzygii subsp. celebesensis LMG 27706^T (99.21%). The 16S rRNA gene sequence identities between strain RS^T and other members of the genus Ralstonia were below 98.00%. Genome sequencing yielded a genome size of 5.61 Mbp and a G + C content of 67.1 mol%. The genomic comparison showed average nucleotide identity (ANIb) values between strain RS^T and R. pseudosolanacearum LMG 9673^T, R. solanacearum LMG 2299^T, and R. syzygii subsp. indonesiensis UQRS 627^T of 95.23, 89.43, and 91.41%, respectively, and the corresponding digital DNA-DNA hybridization (dDDH) values (yielded by formula 2) were 66.20, 44.80, and 47.50%, respectively. In addition, strains belonging to R. solanacearum phylotype I shared both ANIb and dDDH with strain RS^T above the species cut-off values of 96 and 70%, respectively. The ANIb and dDDH values between the genome sequences from 12 strains of R. solanacearum phylotype III (Current R. pseudosolanacearum) and those of strain RS^T were below the species cut-off values. Based on these data, we concluded that strains of phylotype I, including RS^T, represent a novel species of the genus Ralstonia, for which the name Ralstonia nicotianae sp. nov. is proposed. The type strain of Ralstonia nicotianae sp. nov. is RS^T (=GDMCC 1.3533^T = JCM 35814^T).