Interconnection between actin cytoskeleton and plant defense signaling

Cross-regulation of cytoskeleton and calcium signaling at plant-pathogen interface

During plant-pathogen interactions, cytoskeleton and calcium signaling work independently as well as in coordination with each other for developing preformed and induced defense responses. A cell wall (CW) - plasma membrane (PM) - cytoskeleton (CS) continuum is maintained by coordination of cytoskeleton and calcium signaling. The current review is focused on the current knowledge of cytoskeleton‑calcium cross-regulation during plant-pathogen interactions. Implications of recent technological developments in the existing toolkit that can address the outstanding questions of cytoskeleton‑calcium coordination plant immunity are also discussed.

Research on the Molecular Interaction Mechanism between Plants and Pathogenic Fungi

Plant diseases caused by fungi are one of the major threats to global food security and understanding the interactions between fungi and plants is of great significance for plant disease control. The interaction between pathogenic fungi and plants is a complex process. From the perspective of pathogenic fungi, pathogenic fungi are involved in the regulation of pathogenicity by surface signal recognition proteins, MAPK signaling pathways, transcription factors, and pathogenic factors in the process of infecting plants. From the perspective of plant immunity, the signal pathway of immune response, the signal transduction pathway that induces plant immunity, and the function of plant cytoskeleton are the keys to studying plant resistance. In this review, we summarize the current research progress of fungi–plant interactions from multiple aspects and discuss the prospects and challenges of phytopathogenic fungi and their host interactions.

An Integrated Proteomics and Metabolomics Strategy for the Mechanism of Calcium Oxalate Crystal-Induced Kidney Injury

Renal fibrosis is the pathological repair reaction of the kidney to chronic injury, which is an important process of chronic kidney disease (CKD) progressing to end-stage renal failure. Nephrolithiasis is one of the most common renal diseases, with waist and abdomen pain, hematuria, urinary tract infection, and other clinical symptoms, which can increase the risk of renal fibrosis. Oxalate crystal-induced kidney injury is an early stage of nephrolithiasis; it is of great significance to explore the mechanism for the prevention and treatment of nephrolithiasis. A rodent model of calcium oxalate (CaOx) crystal-induced kidney injury was used in the present study, and a network analysis method combining proteomics and metabolomics was conducted to reveal the mechanism of crystal kidney injury and to provide potential targets for the intervention of nephrolithiasis. Using the metabolomics method based on the UHPLC-Q/TOF-MS platform and the iTRAQ quantitative proteomics method, we screened a total of 244 metabolites and 886 proteins from the kidney tissues that had significant changes in the Crystal group compared with that in the Control group. Then, the ingenuity pathway analysis (IPA) was applied to construct a protein-to-metabolic regulatory network by correlating and integrating differential metabolites and proteins. The results showed that CaOx crystals could induce inflammatory reactions and oxidative stress through Akt, ERK1/2, and P38 MAPK pathways and affect amino acid metabolism and fatty acid β-oxidation to result in kidney injury, thus providing an important direction for the early prevention and treatment of nephrolithiasis.

Tomato Prosystemin Is Much More than a Simple Systemin Precursor

Simple Summary

Prosystemin is a 200 amino acid precursor that releases, upon wounding and biotic attacks, an 18 amino acid peptide called Systemin. This peptide was traditionally considered as the principal actor of the resistance of tomato plants induced by triggering multiple defense pathways in response to a wide range of biotic/abiotic stress agents. Recent findings from our group discovered the disordered structure of Prosystemin that promotes the binding of different molecular partners and the possible activation of multiple stress-related pathways. All of our recent findings suggest that Prosystemin could be more than a simple precursor of Systemin peptide. Indeed, we hypothesized that it contains other sequences able to activate multiple stress-related responses. To verify this hypothesis, we produced a truncated Prosystemin protein deprived of the Systemin peptide and the relative deleted gene. Experiments with transgenic tomato plants overexpressing the truncated Prosystemin and with plants exogenously treated with the recombinant truncated protein demonstrated that both transgenic and treated plants modulated the expression of defense-related genes and were protected against a noctuid moth and a fungal pathogen. Taken together, our results demonstrated that Prosystemin is not a mere scaffold of Systemin, but itself contains other biologically active regions.

Abstract

Systemin (Sys) is an octadecapeptide, which upon wounding, is released from the carboxy terminus of its precursor, Prosystemin (ProSys), to promote plant defenses. Recent findings on the disordered structure of ProSys prompted us to investigate a putative biological role of the whole precursor deprived of the Sys peptide. We produced transgenic tomato plants expressing a truncated ProSys gene in which the exon coding for Sys was removed and compared their defense response with that induced by the exogenous application of the recombinant truncated ProSys (ProSys_(1-178), the Prosystemin sequence devoid of Sys region). By combining protein structure analyses, transcriptomic analysis, gene expression profiling and bioassays with different pests, we demonstrate that truncated ProSys promotes defense barriers in tomato plants through a hormone-independent defense pathway, likely associated with the production of oligogalacturonides (OGs). Both transgenic and plants treated with the recombinant protein showed the modulation of the expression of genes linked with defense responses and resulted in protection against the lepidopteran pest Spodoptera littoralis and the fungus Botrytis cinerea. Our results suggest that the overall function of the wild-type ProSys is more complex than previously shown, as it might activate at least two tomato defense pathways: the well-known Sys-dependent pathway connected with the induction of jasmonic acid biosynthesis and the successive activation of a set of defense-related genes, and the ProSys_(1-178)-dependent pathway associated with OGs production leading to the OGs mediate plant immunity.

A Modified Actin (Gly65Val Substitution) Expressed in Cotton Disrupts Polymerization of Actin Filaments Leading to the Phenotype of Ligon Lintless-1 (Li1) Mutant

Dynamic remodeling of the actin cytoskeleton plays a central role in the elongation of cotton fibers, which are the most important natural fibers in the global textile industry. Here, a high-resolution mapping approach combined with comparative sequencing and a transgenic method revealed that a G65V substitution in the cotton actin Gh_D04G0865 (GhACT17D in the wild-type) is responsible for the Gossypium hirsutum Ligon lintless-1 (Li₁) mutant (GhACT17DM). In the mutant GhACT17DM from Li₁ plant, Gly65 is substituted with valine on the lip of the nucleotide-binding domain of GhACT17D, which probably affects the polymerization of F-actin. Over-expression of GhACT17DM, but not GhACT17D, driven by either a CaMV35 promoter or a fiber-specific promoter in cotton produced a Li₁-like phenotype. Compared with the wild-type control, actin filaments in Li₁ fibers showed higher growth and shrinkage rates, decreased filament skewness and parallelness, and increased filament density. Taken together, our results indicate that the incorporation of GhACT17DM during actin polymerization disrupts the establishment and dynamics of the actin cytoskeleton, resulting in defective fiber elongation and the overall dwarf and twisted phenotype of the Li₁ mutant.

Actin depolymerization is able to increase plant resistance against pathogens via activation of salicylic acid signalling pathway

The integrity of the actin cytoskeleton is essential for plant immune signalling. Consequently, it is generally assumed that actin disruption reduces plant resistance to pathogen attack. Here, we demonstrate that actin depolymerization induced a dramatic increase in salicylic acid (SA) levels in Arabidopsis thaliana. Transcriptomic analysis showed that the SA pathway was activated due to the action of isochorismate synthase (ICS). The effect was also confirmed in Brassica napus. This raises the question of whether actin depolymerization could, under particular conditions, lead to increased resistance to pathogens. Thus, we explored the effect of pretreatment with actin-depolymerizing drugs on the resistance of Arabidopsis thaliana to the bacterial pathogen Pseudomonas syringae, and on the resistance of an important crop Brassica napus to its natural fungal pathogen Leptosphaeria maculans. In both pathosystems, actin depolymerization activated the SA pathway, leading to increased plant resistance. To our best knowledge, we herein provide the first direct evidence that disruption of the actin cytoskeleton can actually lead to increased plant resistance to pathogens, and that SA is crucial to this process.

Molecular mechanism of Arabidopsis thaliana profilins as antifungal proteins

BACKGROUND

It remains an open question whether plant phloem sap proteins are functionally involved in plant defense mechanisms.

METHODS

The antifungal effects of two profilin proteins from Arabidopsis thaliana, AtPFN1 and AtPFN2, were tested against 11 molds and 4 yeast fungal strains. Fluorescence profiling, biophysical, and biochemical analyses were employed to investigate their antifungal mechanism.

RESULTS

Recombinant AtPFN1 and AtPFN2 proteins, expressed in Escherichia coli, inhibited the cell growth of various pathogenic fungal strains at concentrations ranging from 10 to 160 μg/mL. The proteins showed significant intracellular accumulation and cell-binding affinity for fungal cells. Interestingly, the AtPFN proteins could penetrate the fungal cell wall and membrane and act as inhibitors of fungal growth via generation of cellular reactive oxygen species and mitochondrial superoxide. This triggered the AtPFN variant-induced cell apoptosis, resulting in morphological changes in the cells.

CONCLUSION

PFNs may play a critical role as antifungal proteins in the Arabidopsis defense system against fungal pathogen attacks.

GENERAL SIGNIFICANCE

The present study indicates that two profilin proteins, AtPFN1 and AtPFN2, can act as natural antimicrobial agents in the plant defense system.

Transcriptome profiling of Cucumis metuliferus infected by Meloidogyne incognita provides new insights into putative defense regulatory network in Cucurbitaceae

Root-knot nematodes (RKN) represent extensive challenges to Cucurbitaceae crops. However, Cucumis metuliferus (Cm) is known to be resistant to Meloidogyne incognita (Mi) infections. Thus, analysis of differentially expressed genes may lead to a comprehensive gene expression profiling of the incompatible Cm-Mi interaction. In this study, the time-course transcriptome of Cm against Mi infection was monitored using RNA-Seq. More than 170000 transcripts were examined in Cm roots, and 2430 genes were subsequently identified as differentially expressed in response to Mi infection. Based on function annotation and orthologs finding, the potential mechanism of transcriptional factor, cytoskeleton, pathogen-related genes and plant hormone were assessed at the transcription level. A comparison of gene expression levels between Mi-infected Cm and cucumber plants revealed that cytoskeleton-related genes are key regulators of Cm resistance to Mi. We herein discuss the dual nature of cytoskeleton-related genes in the susceptibility and resistance of plant hosts to Mi. Our observations provide novel insights into the responses of Cm to Mi at the transcriptome level. The data generated in this study may be useful for elucidating the mechanism underlying resistance to RKNs in cucurbitaceous crops.

Gene coding for an elongation factor is involved in resistance against powdery mildew in common bean

Genetic control of the resistance response against powdery mildew in common bean was studied combining genetic, genomic and transcriptomic analyses. A candidate resistance gene in cultivar Porrillo Sintetico was proposed. The species causing the fungal disease powdery mildew (PM) in the local common bean crop was identified as Erysiphe polygoni through the molecular analysis of the internal transcribed spacer region. A genetic analysis of the resistance in cultivar Porrillo Sintetico was conducted using different F_2:3 populations, and a dominant gene conferring total resistance against a local PM isolate was physically located between 84,188 and 218,664 bp of chromosome Pv04. An in silico analysis of this region, based on the common bean reference sequence, revealed four genes candidate to be involved in the resistance reaction. Relative expression levels of these genes after PM infection showed a significant over-expression of the candidate gene Phvul.004G001500 in the resistant genotype Porrillo Sintetico. This gene was re-sequenced in the parental genotypes X2776 and Porrillo Sintetico to explain their different phenotypic responses against PM. Several substitutions where identified in exon regions, all of them synonymous, so differences in the produced amino acid sequence were not expected. However, a total of 37 mutations were identified in non-coding regions of the gene sequence, suggesting that intron variation could be responsible for the different gene expression levels after PM infection. No evidence of other regulatory mechanisms, such as alternative splicing or methylation, was identified. Candidate resistance gene Phvul.004G001500 codes for an elongation factor that is not a typical gene related to recognition of specific pathogens in plants, suggesting its involvement in the resistance through plant immune system.

Genetic fine mapping and candidate gene analysis of the Gossypium hirsutum Ligon lintless-1 (Li1) mutant on chromosome 22(D)

Ligon lintless-1 (Li1) is a Gossypium hirsutum mutant that is controlled by a dominant gene that arrests the development of cotton fiber after anthesis. Two F2 mapping populations were developed from mutant (Li1 × H7124) F1 plants in 2012 and 2013; each was composed of 142 and 1024 plants, respectively. Using these populations, Li1 was mapped to a 0.3-cM region in which nine single-strand conformation polymorphism markers co-segregated with the Li1 locus. In the published G. raimondii genome, these markers were mapped to a region of about 1.2 Mb (the Li1 region) and were separated by markers that flanked the Li1 locus in the genetic map, dividing the Li1 region into three segments. Thirty-six genes were annotated by the gene prediction software FGENESH (Softberry) in the Li1 region. Twelve genes were candidates of Li1, while the remaining 24 genes were identified as transposable elements, DNA/RNA polymerase superfamily or unknown function genes. Among the 12 candidate genes, those encoding ribosomal protein s10, actin protein, ATP synthase, and beta-tubulin 5 were the most-promising candidates of the Li1 mutant because the function of these genes is closely related to fiber development. High-throughput RNA sequencing and quantitative PCR revealed that these candidate genes had obvious differential gene expression between mutant and wild-type plants at the fiber elongation stage, strengthening the inference that they could be the most likely candidate gene of the Li1 mutant phenotype.