Biology subjects

Sessa, G.

Publications and source records attributed to Sessa, G..

11 recordsLinked to original sources

bioRxiv2026

Auxin and cytokinin antagonistically regulate Arabidopsis root distal stem cell fate through opposing pairs of HD-Zip II transcription factors

The antagonistic action of auxin (IAA) and cytokinin (CK) regulates several aspects of plant development including stem cell specification and maintenance. In the Arabidopsis thaliana root, the fate of distal stem cells (DSCs) is mainly regulated by IAA, while the function of CK is still ill-defined. Here, by investigating the role of the HD-Zip II transcription factor family in distal tissues, we uncover CK as a proliferative input for DSCs, acting in opposition to IAA. We show that opposing pairs of HD-Zip II proteins regulate DSCs fate by integrating IAA and CK signals. The {delta} module, composed of HAT3 and ATHB4, is activated by CK and promotes DSCs division. Conversely, the {gamma} module, constituted by HAT1 and ATHB2, acts downstream of IAA to repress CK biosynthesis and DSCs division. The mutual inhibition between {delta} and {gamma} modules downstream of opposite hormone inputs sustains the precise regulation of DSCs homeostasis.

plant biology↗

bioRxiv2024

Comparative transcriptomic profile reveals candidate genes manipulated by type III effectors of Pantoea agglomerans pv. betae leading to gall formation in beet

Pantoea agglomerans pv. betae (Pab) induces tumor-like galls in beet and gypsophila, a process mediated by the secretion of effector proteins via Pabs type III secretion system (T3SS). The molecular mechanisms underlying Pab-induced gall formation remain largely unexplored. This study delves into the cellular architecture and transcriptional profile of Pab-mediated galls, comparing host responses to wild-type Pab and a T3SS-inactive mutant, hrcC-. Morphological analysis using scanning electron microscopy and cross-sectional visualization of infected beet leaf tissues revealed that Pab-induced gall-like structures are linked to cell hyperplasia and tissue ruptures, contingent on T3SS activity. Comparative transcriptome analysis of wild-type Pab and hrcC- Pab-infected beet leaves at 12 and 48 hours unveiled significant transcriptional reprogramming, with nearly 2,000 differentially expressed genes at 48 hours post inoculation. Enrichment analyses identified the upregulation of pathways related to signal transduction, defense, carbohydrate metabolism, and cell wall modulation in wild-type Pab-infected leaves compared to controls. Particularly notable was the significant upregulation of numerous genes associated with cell wall loosening by wild-type Pab, suggesting an initial rearrangement of cell wall architecture facilitates gall formation. Furthermore, transcriptome analysis demonstrated that wild-type Pab suppresses the expression of the betalain biosynthetic gene DOPA 4,5-DIOXYGENASE, leading to reduced betalain accumulation in infected tissues compared to the mutant strain. These findings offer fresh insights into the transcriptional and physiological manipulation of host tissue during the early stages of Pab-induced gall formation.

plant biology↗

bioRxiv2024

PP2C phosphatase Pic14 negatively regulates tomato Pto/Prf-triggered immunity by inhibiting MAPK activation

Type 2C protein phosphatases (PP2Cs) are emerging as important regulators of plant immune responses, although little is known about how they might impact nucleotide-binding, leucine-rich repeat (NLR)-triggered immunity (NTI). We discovered that expression of the PP2C-immunity associated candidate 14 gene (Pic14) is induced upon activation of the Pto/Prf-mediated NTI response in tomato. Pto/Prf recognize the effector AvrPto translocated into plant cells by the pathogen Pseudomonas syringae pv. tomato (Pst) and activate a MAPK cascade and other responses which together confer resistance to bacterial speck disease. Pic14 encodes a PP2C with an N-terminal kinase-interacting motif (KIM) and a C-terminal phosphatase domain. Upon inoculation with Pst-AvrPto, Pto/Prf-expressing tomato plants with loss-of-function mutations in Pic14 developed less speck disease, specifically in older leaves, compared to wild-type plants. Transient expression of Pic14 in leaves of Nicotiana benthamiana and tomato inhibited cell death typically induced by Pto/Prf and the MAPK cascade members M3K and Mkk2. The cell death-suppressing activity of Pic14 was dependent on the KIM and the catalytic phosphatase domain. Pic14 inhibited M3K- and Mkk2-mediated activation of immunity-associated MAPKs and Pic14 was shown to be an active phosphatase that physically interacts with and dephosphorylates Mkk2 in a KIM-dependent manner. Together, our results reveal Pic14 as an important negative regulator of Pto/Prf-triggered immunity by interacting with and dephosphorylating Mkk2. SIGNIFICANCE STATEMENTPlant intracellular immune receptors, typically nucleotide-binding, leucine-rich repeat proteins (NLRs) such as the tomato Prf protein activate NLR-triggered immunity (NTI) in response to specific pathogen virulence proteins. This paper reveals how a protein phosphatase interacts with and dephosphorylates a key signaling component acting downstream of Pto/Prf, likely to moderate negative effects of NTI on growth or other plant processes.

plant biology↗

bioRxiv2024

Related PP2C phosphatases Pic3 and Pic12 negatively regulate immunity in tomato to Pseudomonas syringae

Type 2C protein phosphatases (PP2Cs) constitute a large family in most plant species but relatively few of them have been implicated in immunity. To identify and characterize PP2C phosphatases that affect tomato immunity, we used CRISPR/Cas9 to generate loss-of-function mutations in 11 PP2C-encoding genes whose expression is altered in response to immune elicitors or pathogens. We report that two closely related PP2C phosphatases, Pic3 and Pic12, are involved in regulating resistance to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst). Loss-of-function mutations in Pic3 lead to enhanced resistance to Pst in older but not younger leaves, whereas such mutations in Pic12 resulted in enhanced resistance in both older and younger leaves. Overexpression of Pic3 and Pic12 proteins in leaves of Nicotiana benthamiana inhibited resistance to Pst and this effect was dependent on Pic3/12 phosphatase activity and an N-terminal palmitoylation motif associated with localization to the cell periphery. Pic3 but not Pic12 had a slight negative effect on flagellin-associated reactive oxygen species generation, although their involvement in the response to Pst appeared independent of flagellin. RNA-sequencing analysis of Rio Grande (RG)-PtoR wild-type plants and two independent RG-pic3 mutants revealed that the enhanced disease resistance in RG-pic3 older leaves is associated with increased transcript abundance of multiple defense related genes. RG-pic3/RG-pic12 double mutant plants exhibited stronger disease resistance than RG-pic3 or RG-pic12 single mutants. Together, our results reveal that Pic3 and Pic12 negatively regulate tomato immunity in an additive manner through flagellin-independent pathways.

plant biology↗

bioRxiv2024

A shade-responsive microProtein in the Arabidopsis ATHB2 gene regulates elongation growth and root development

Shade triggers widespread changes in transcript isoform production in Arabidopsis thaliana. Using 5' PEAT sequencing, we identified 377 candidate microprotein-encoding transcripts arising from alternative transcription start sites, including one located within the third exon of ATHB2, a class II HD-ZIP transcription factor central to shade avoidance. The encoded microprotein, ATHB2miP, lacks the DNA-binding homeodomain but retains the leucine zipper domain, through which it forms heterodimers with full-length ATHB2, inhibits its DNA-binding activity, and disrupts its localization to nuclear photobodies. Ectopic expression of ATHB2miP phenocopies loss of ATHB2, deregulating genes involved in auxin signaling, root development, and iron homeostasis. Complementation of the athb2 mutant with a construct carrying a silent mutation at the ATHB2miP start codon results in exaggerated hypocotyl elongation under both white light and shade, demonstrating that ATHB2miP is required to restrain excessive elongation growth. Immunoprecipitation mass spectrometry confirmed that this mutation redirects translation to alternative initiation sites, producing distinct proteoforms absent from wild-type plants. Additionally, ATHB2 suppresses iron uptake genes, and iron availability modulates shade growth responses in an ATHB2-dependent manner. Together, these findings establish ATHB2miP as a regulator of an ATHB2 feedback circuit that integrates light and nutritional signals to coordinate shoot elongation and root development.

plant biology↗

bioRxiv2023

HD-Zip II transcription factors control distal stem cell fate in Arabidopsis roots by linking auxin signaling to the FEZ/SOMBRERO pathway

In multicellular organisms, specialized tissues are generated by specific populations of stem cells through cycles of asymmetric cell divisions, where one daughter undergoes differentiation and the other maintains proliferative properties. In Arabidopsis thaliana roots, the columella - a gravity-sensing tissue which protects and defines the position of the stem cell niche - represents a typical example of a tissue whose organization is exclusively determined by the balance between proliferation and differentiation. The columella derives from a single layer of stem cells through a binary cell fate switch that is precisely controlled by multiple, independent regulatory inputs. Here, we show that the HD-Zip II transcription factors HAT3, ATHB4, and AHTB2, redundantly regulate columella stem cell fate and patterning in the Arabidopsis root. The HD-Zip II transcription factors promote columella stem cell proliferation by acting as effectors of the FEZ/SMB circuit and, at the same time, by interfering with auxin signaling to counteract hormone-induced differentiation. Overall, our work shows that HD-Zip II connect two opposing parallel inputs to fine tune the balance between proliferation and differentiation in columella stem cells. Summary statementHD-Zip II transcription factors redundantly regulate columella stem cells activity by counteracting auxin-mediated differentiation downstream of the FEZ/SMB proliferative input

plant biology↗

bioRxiv2023

An N-terminal delivery domain defines a new class of polymorphic T6SS effectors in Enterobacterales

The type VI secretion system (T6SS), a widespread protein delivery apparatus, plays a role in bacterial competition by delivering toxic effectors into neighboring cells. Identifying new T6SS effectors and deciphering the mechanism that governs their secretion remain major challenges. Here, we report two orphan, antibacterial T6SS effectors in the pathogen Pantoea agglomerans (Pa). These effectors share an N-terminal domain, PIX, that defines a widespread class of polymorphic T6SS effectors in Enterobacterales. We show that the PIX domain is necessary and sufficient for T6SS-mediated effector secretion and that PIX binds to a specialized Pa VgrG protein, outside of its C-terminal toxic domain. Our findings underline the importance of identifying and characterizing new delivery domains in polymorphic toxin classes as a tool to reveal novel effectors and shed light on effector delivery mechanisms.

microbiology↗

bioRxiv2022

Tomato brassinosteroid-signaling kinase Bsk830 is a component of flagellin signaling that regulates pre-invasion immunity

Detection of bacterial flagellin by the tomato receptors Flagellin sensing 2 (Fls2) and Fls3 triggers activation of pattern-triggered immunity (PTI). Tomato signaling components associated or downstream of flagellin receptors are largely unknown. We investigated the involvement of tomato brassinosteroid-signaling kinase 830 (Bsk830) in PTI triggered by flagellin perception. Bsk830 localized to the plasma membrane and interacted with Fls2 and Fls3. Consistent with a role in flagellin- induced signaling, CRISPR/Cas9-generated tomato bsk830 mutants were impaired in ROS accumulation induced by the flagellin-derived flg22 and flgII-28 peptides. In addition, bsk830 mutants developed larger populations of Pseudomonas syringae pv. tomato (Pst) strain DC3000 than wild-type plants, whereas no differences were observed in plants infected with the flagellin deficient Pst DC3000{Delta}fliC. bsk830 mutants failed to close stomata when infected with Pst DC3000 and Pseudomonas fluorescens, and were more susceptible to Pst DC3000 than wild-type plants when inoculated by dipping, but not by vacuum-infiltration, indicating involvement of Bsk830 in pre-invasion immunity. Analysis of gene expression profiles in bsk830 mutants detected a reduced number of differentially expressed genes and altered expression of jasmonic acid (JA)-related genes. In support of deregulation of JA response in bsk830 mutants, these plants were similarly susceptible to Pst DC3000 and to the Pst DC3118 strain, which is deficient in coronatine production, and more resistant to the necrotrophic fungus Botrytis cinerea following PTI activation. These results indicate that tomato Bsk830 is required for a subset of flagellin-triggered PTI responses and support a model in which Bsk830 negatively regulates JA signaling during PTI activation.

plant biology↗

bioRxiv2021

A dynamic antibacterial T6SS in Pantoea agglomerans pv. betae delivers a lysozyme-like effector to antagonize competitors

The type VI secretion system (T6SS) is deployed by numerous Gram-negative bacteria to deliver toxic effectors into neighboring cells. The genome of Pantoea agglomerans pv. betae (Pab) phytopathogenic bacteria contains a gene cluster (T6SS1) predicted to encode a complete T6SS. Using secretion and competition assays, we found that T6SS1 in Pab is a functional antibacterial system that allows this pathogen to outcompete rival plant-associated bacteria found in its natural environment. Computational analysis of the T6SS1 gene cluster revealed that antibacterial effector and immunity proteins are encoded within three dynamic genomic islands that harbor arrays of orphan immunity genes or toxin and immunity cassettes. Functional analysis demonstrated that the specialized antibacterial effector VgrG contains a C-terminal catalytically active glucosaminidase domain that is used to degrade prey peptidoglycan. Moreover, we confirmed that a bicistronic unit at the end of the T6SS1 cluster encodes a novel antibacterial T6SS effector and immunity pair. Together, these results demonstrate that Pab T6SS1 is an antibacterial system delivering a lysozyme-like effector to eliminate competitors, and indicate that this bacterium contains novel T6SS effectors. Significance StatementIn this work, we describe the identification of a Pantoea agglomerans T6SS as an antibacterial determinant used by this phytopathogen to outcompete bacterial rivals. Furthermore, we provide an in-depth analysis of the T6SS gene cluster and the putative effector and immunity genes that comprise it, and we propose explanations for its dynamic evolution and effector diversification in Pantoea strains. Lastly, we experimentally validate two predicted effector and immunity pairs, and we demonstrate that one is a potent lysozyme-like toxin.

microbiology↗

bioRxiv2019

Show me your secret(ed) weapons: a multifaceted approach reveals novel type III-secreted effectors of a plant pathogenic bacterium

Many Gram-negative plant and animal pathogenic bacteria employ a type III secretion system (T3SS) to secrete protein effectors into the cells of their hosts and promote disease. The plant pathogen Acidovorax citrulli requires a functional T3SS for pathogenicity. As with Xanthomonas and Ralstonia spp., an AraC-type transcriptional regulator, HrpX, regulates expression of genes encoding T3SS components and type III-secreted effectors (T3Es) in A. citrulli. A previous study reported eleven T3E genes in this pathogen, based on the annotation of a sequenced strain. We hypothesized that this was an underestimation. Guided by this hypothesis, we aimed at uncovering the T3E arsenal of the A. citrulli model strain, M6. We carried out a thorough sequence analysis searching for similarity to known T3Es from other bacteria. This analysis revealed 51 A. citrulli genes whose products are similar to known T3Es. Further, we combined machine learning and transcriptomics to identify novel T3Es. The machine learning approach ranked all A. citrulli M6 genes according to their propensity to encode T3Es. RNA-Seq revealed differential gene expression between wild-type M6 and a mutant defective in HrpX. Data combined from these approaches led to the identification of seven novel T3E candidates, that were further validated using a T3SS-dependent translocation assay. These T3E genes encode hypothetical proteins, do not show any similarity to known effectors from other bacteria, and seem to be restricted to plant pathogenic Acidovorax species. Transient expression in Nicotiana benthamiana revealed that two of these T3Es localize to the cell nucleus and one interacts with the endoplasmic reticulum. This study not only uncovered the arsenal of T3Es of an important pathogen, but it also places A. citrulli among the \"richest\" bacterial pathogens in terms of T3E cargo. It also revealed novel T3Es that appear to be involved in the pathoadaptive evolution of plant pathogenic Acidovorax species.\n\nAuthor summaryAcidovorax citrulli is a Gram-negative bacterium that causes bacterial fruit blotch (BFB) disease of cucurbits. This disease represents a serious threat to cucurbit crop production worldwide. Despite the agricultural importance of BFB, the knowledge about basic aspects of A. citrulli-plant interactions is rather limited. As many Gram-negative plant and animal pathogenic bacteria, A. citrulli employs a complex secretion system, named type III secretion system, to deliver protein virulence effectors into the host cells. In this work we aimed at uncovering the arsenal of type III-secreted effectors (T3Es) of this pathogen by combination of bioinformatics and experimental approaches. We found that this bacterium possesses at least 51 genes that are similar to T3E genes from other pathogenic bacteria. In addition, our study revealed seven novel T3Es that seem to occur only in A. citrulli strains and in other plant pathogenic Acidovorax species. We found that two of these T3Es localize to the plant cell nucleus while one partially interacts with the endoplasmic reticulum. Further characterization of the novel T3Es identified in this study may uncover new host targets of pathogen effectors and new mechanisms by which pathogenic bacteria manipulate their hosts.

microbiology↗

bioRxiv2019

Mai1 protein acts between host recognition of pathogen effectors and MAPK signaling

The molecular mechanisms acting between host recognition of pathogen effectors by NOD-like receptor (NLR) proteins and mitogen-activated protein kinase (MAPK) signaling cascades are unknown. MAPKKK (M3K) activates MAPK signaling leading to programmed cell death (PCD) associated with NLR-triggered immunity. We identified a tomato M3K-interacting protein, SlMai1, that has 80% amino acid identity with Arabidopsis brassinosteroid kinase 1 (AtBsk1). SlMai1 has a protein kinase domain and a C-terminal tetratricopeptide repeat domain which interacts with the kinase domain of M3K. Virus-induced gene silencing of Mai1 homologs in Nicotiana benthamiana increased susceptibility to Pseudomonas syringae and compromised PCD induced by four NLR proteins. PCD was restored by expression of a synthetic SlMai1 gene that resists silencing. Expression of AtBsk1 did not restore PCD in Mai1-silenced plants, suggesting SlMai1 is functionally divergent from AtBsk1. PCD caused by overexpression of M3K or MKK2 was unaffected by Mai1 silencing indicating Mai1 acts upstream of these proteins. Co-expression of Mai1 with M3K in leaves enhanced MAPK phosphorylation and accelerated PCD. These findings reveal Mai1 as a molecular link acting between host recognition of pathogens and MAPK signaling. Author SummaryPlants use intracellular immune receptors to detect and respond to specific effector proteins which pathogens translocate into the host cell as part of their infection process. Localized programmed cell death (PCD) involving a mitogen-activated protein kinase (MAPK) cascade is an important host response associated with effector-triggered immunity, although the molecular connections between immune receptors and MAPK signaling is unknown. The Mai1 protein was found to act downstream of multiple immune receptors in Nicotiana benthamiana and to physically interact with MAPKKK. The Mai1-MAPKKK interaction enhances MAPK phosphorylation, triggers PCD and promotes disease resistance.

plant biology↗