Differential timing of defense-related responses induced by cerato-platanin and cerato-populin, two non-catalytic fungal elicitors

Expansin-related proteins: biology, microbe-plant interactions and associated plant-defense responses

Expansins, cerato-platanins and swollenins (which we will henceforth refer to as expansin-related proteins) are a group of microbial proteins involved in microbe-plant interactions. Although they share very low sequence similarity, some of their composing domains are near-identical at the structural level. Expansin-related proteins have their target in the plant cell wall, in which they act through a non-enzymatic, but still uncharacterized, mechanism. In most cases, mutagenesis of expansin-related genes affects plant colonization or plant pathogenesis of different bacterial and fungal species, and thus, in many cases they are considered virulence factors. Additionally, plant treatment with expansin-related proteins activate several plant defenses resulting in the priming and protection towards subsequent pathogen encounters. Plant-defence responses induced by these proteins are reminiscent of pattern-triggered immunity or hypersensitive response in some cases. Plant immunity to expansin-related proteins could be caused by the following: (i) protein detection by specific host-cell receptors, (ii) alterations to the cell-wall-barrier properties sensed by the host, (iii) displacement of cell-wall polysaccharides detected by the host. Expansin-related proteins may also target polysaccharides on the wall of the microbes that produced them under certain physiological instances. Here, we review biochemical, evolutionary and biological aspects of these relatively understudied proteins and different immune responses they induce in plant hosts.

The Novel Cerato-Platanin-Like Protein FocCP1 from Fusarium oxysporum Triggers an Immune Response in Plants

Panama disease, or Fusarium wilt, the most serious disease in banana cultivation, is caused by Fusarium oxysporum f. sp. cubense (FOC) and has led to great economic losses worldwide. One effective way to combat this disease is by enhancing host plant resistance. The cerato-platanin protein (CPP) family is a group of small secreted cysteine-rich proteins in filamentous fungi. CPPs as elicitors can trigger the immune system resulting in defense responses in plants. In this study, we characterized a novel cerato-platanin-like protein in the secretome of Fusarium oxysporum f. sp. cubense race 4 (FOC4), named FocCP1. In tobacco, the purified recombinant FocCP1 protein caused accumulation of reactive oxygen species (ROS), formation of necrotic reaction, deposition of callose, expression of defense-related genes, and accumulation of salicylic acid (SA) and jasmonic acid (JA) in tobacco. These results indicated that FocCP1 triggered a hypersensitive response (HR) and systemic acquired resistance (SAR) in tobacco. Furthermore, FocCP1 enhanced resistance tobacco mosaic virus (TMV) disease and Pseudomonas syringae pv. tabaci 6605 (Pst. 6605) infection in tobacco and improved banana seedling resistance to FOC4. All results provide the possibility of further research on immune mechanisms of plant and pathogen interactions, and lay a foundation for a new biological strategy of banana wilt control in the future.

A single amino acid mutation affects elicitor and expansins-like activities of cerato-platanin, a non-catalytic fungal protein

Cerato-platanin (CP) is a non-catalytic, cysteine-rich protein, the first member of the cerato-platanin family. It is a single-domain protein with a double Ψ/β barrel domain resembling the D1 domain of plant and bacterial expansins. Similarly to expansins, CP shows a cell wall-loosening activity on cellulose and can be defined as an expanisin-like protein, in spite of the missing D2 domain, normally present in plant expansins. The weakening activity shown on cellulose may facilitate the CP-host interaction, corroborating the role of CP in eliciting plant defence response. Indeed, CP is an elicitor of primary defences acting as a Pathogen-Associated Molecular Patterns (PAMP). So far, structure-function relationship study has been mainly performed on the bacterial BsEXLX1 expansin, probably due to difficulties in expressing plant expansins in heterologous systems. Here, we report a subcloning and purification method of CP in the engineered E. coli SHuffle cells, which proved to be suitable to obtain the properly folded and biologically active protein. The method also enabled the production of the mutant D77A, rationally designed to be inactive. The wild-type and the mutated CP were characterized for cellulose weakening activity and for PAMP activity (i.e. induction of Reactive Oxygen Species synthesis and phytoalexins production). Our analysis reveals that the carboxyl group of D77 is crucial for expansin-like and PAMP activities, thus permitting to establish a correlation between the ability to weaken cellulose and the capacity to induce defence responses in plants. Our results enable the structural and functional characterization of a mono-domain eukaryotic expansin and identify the essential role of a specific aspartic residue in cellulose weakening.

Overexpression of MoSM1, encoding for an immunity-inducing protein from Magnaporthe oryzae, in rice confers broad-spectrum resistance against fungal and bacterial diseases

Potential of MoSM1, encoding for a cerato-platanin protein from Magnaporthe oryzae, in improvement of rice disease resistance was examined. Transient expression of MoSM1 in rice leaves initiated hypersensitive response and upregulated expression of defense genes. When transiently expressed in tobacco leaves, MoSM1 targeted to plasma membrane. The MoSM1-overexpressing (MoSM1-OE) transgenic rice lines showed an improved resistance, as revealed by the reduced disease severity and decreased in planta pathogen growth, against 2 strains belonging to two different races of M. oryzae, causing blast disease, and against 2 strains of Xanthomonas oryzae pv. oryzae, causing bacterial leaf blight disease. However, no alteration in resistance to sheath blight disease was observed in MoSM1-OE lines. The MoSM1-OE plants contained elevated levels of salicylic acid (SA) and jasmonic acid (JA) and constitutively activated the expression of SA and JA signaling-related regulatory and defense genes. Furthermore, the MoSM1-OE plants had no effect on drought and salt stress tolerance and on grain yield. We conclude that MoSM1 confers a broad-spectrum resistance against different pathogens through modulating SA- and JA-mediated signaling pathways without any penalty on abiotic stress tolerance and grain yield, providing a promising potential for application of MoSM1 in improvement of disease resistance in crops.

IMA Genome-F 7: Draft genome sequences for Ceratocystis fagacearum, C. harringtonii, Grosmannia penicillata, and Huntiella bhutanensis

Draft genomes for the fungi Ceratocystis fagacearum, C. harringtonii, Grosmannia penicillata, and Huntiella bhutanensis are presented. Ceratocystis fagacearum is a major causal agent of vascular wilt of oaks and other trees in the family Fagaceae. Ceratocystis harringtonii, previously known as C. populicola, causes disease in Populus species in the USA and Canada. Grosmannia penicillata is the causal agent of bluestain of sapwood on various conifers, including Picea spp. and Pinus spp. in Europe. Huntiella bhutanensis is a fungus in Ceratocystidaceae and known only in association with the bark beetle Ips schmutzenhorferi that infests Picea spinulosa in Bhutan. The availability of these genomes will facilitate further studies on these fungi.

PAMP Activity of Cerato-Platanin during Plant Interaction: An -Omic Approach

Cerato-platanin (CP) is the founder of a fungal protein family consisting in non-catalytic secreted proteins, which work as virulence factors and/or as elicitors of defense responses and systemic resistance, thus acting as PAMPs (pathogen-associated molecular patterns). Moreover, CP has been defined an expansin-like protein showing the ability to weaken cellulose aggregates, like the canonical plant expansins do. Here, we deepen the knowledge on CP PAMP activity by the use of a multi-disciplinary approach: proteomic analysis, VOC (volatile organic compound) measurements, and gas exchange determination. The treatment of Arabidopsis with CP induces a differential profile either in protein expression or in VOC emission, as well changes in photosynthetic activity. In agreement with its role of defense activator, CP treatment induces down-expression of enzymes related to primary metabolism, such as RuBisCO, triosephosphate isomerase, and ATP-synthase, and reduces the photosynthesis rate. Conversely, CP increases expression of defense-related proteins and emission of some VOCs. Interestingly, CP exposure triggered the increase in enzymes involved in GSH metabolism and redox homeostasis (glutathione S-transferase, thioredoxin, Cys-peroxiredoxin, catalase) and in enzymes related to the “glucosinolate-myrosinase” system, which are the premise for synthesis of defence compounds, such as camalexin and some VOCs, respectively. The presented results are in agreement with the accepted role of CP as a PAMP and greatly increase the knowledge of plant primary defences induced by a purified fungal elicitor.

Under fungal attack on a metalliferous soil: ROS or not ROS? Insights from Silene paradoxa L. growing under copper stress

We investigated how the adaptation to metalliferous environments can influence the plant response to biotic stress. In a metallicolous and a non-metallicolous population of Silene paradoxa the induction of oxidative stress and the production of callose and volatiles were evaluated in the presence of copper and of the PAMP fungal protein cerato-platanin, separately and in combination. Our results showed incompatibility between the ordinary ROS-mediated response to fungal attack and the acquired mechanisms of preventing oxidative stress in the tolerant population. A similar situation was also demonstrated by the sensitive population growing in the presence of copper but, in this case, with a lack of certain responses, such as callose production. In addition, in terms of the joint behaviour of emitted volatiles, multivariate statistics showed that not only did the populations respond differently to the presence of copper or biotic stress, but also that the biotic and abiotic stresses interacted in different ways in the two populations. Our results demonstrated that the same incompatibility of hyperaccumulators in ROS-mediated biotic stress signals also seemed to be exhibited by the excluder metallophyte, but without the advantage of being able to rely on the elemental defence for plant protection from natural enemies.

Cerato-platanin induces resistance in Arabidopsis leaves through stomatal perception, overexpression of salicylic acid- and ethylene-signalling genes and camalexin biosynthesis

Microbe-associated molecular patterns (MAMPs) lead to the activation of the first line of plant defence. Few fungal molecules are universally qualified as MAMPs, and proteins belonging to the cerato-platanin protein (CPP) family seem to possess these features. Cerato-platanin (CP) is the name-giving protein of the CPP family and is produced by Ceratocystis platani, the causal agent of the canker stain disease of plane trees (Platanus spp.). On plane tree leaves, the biological activity of CP has been widely studied. Once applied on the leaf surface, CP acts as an elicitor of defence responses. The molecular mechanism by which CP elicits leaves is still unknown, and the protective effect of CP against virulent pathogens has not been clearly demonstrated. In the present study, we tried to address these questions in the model plant Arabidopsis thaliana. Our results suggest that stomata rapidly sense CP since they responded to the treatment with ROS signalling and stomatal closure, and that CP triggers salicylic acid (SA)- and ethylene (ET)-signalling pathways, but not the jasmonic acid (JA)-signalling pathway, as revealed by the expression pattern of 20 marker genes. Among these, EDS1, PAD4, NPR1, GRX480, WRKY70, ACS6, ERF1a/b, COI1, MYC2, PDF1.2a and the pathogenesis-related (PR) genes 1–5. CP rapidly induced MAPK phosphorylation and induced the biosynthesis of camalexin within 12 hours following treatment. The induction of localised resistance was shown by a reduced susceptibility of the leaves to the infection with Botrytis cinerea and Pseudomonas syringae pv. tomato. These results contribute to elucidate the key steps of the signalling process underlying the resistance induction in plants by CP and point out the central role played by the stomata in this process.

Cerato-platanins: a fungal protein family with intriguing properties and application potential

Cerato-platanin proteins are small, secreted proteins with four conserved cysteines that are abundantly produced by filamentous fungi with all types of lifestyles. These proteins appear to be readily recognized by other organisms and are therefore important factors in interactions of fungi with other organisms, e.g. by stimulating the induction of defence responses in plants. However, it is not known yet whether the main function of cerato-platanin proteins is associated with these fungal interactions or rather a role in fungal growth and development. Cerato-platanin proteins seem to unify several biochemical properties that are not found in this combination in other proteins. On one hand, cerato-platanins are carbohydrate-binding proteins and are able to bind to chitin and N-acetylglucosamine oligosaccharides; on the other hand, they are able to self-assemble at hydrophobic/hydrophilic interfaces and form protein layers, e.g. on the surface of aqueous solutions, thereby altering the polarity of solutions and surfaces. The latter property is reminiscent of hydrophobins, which are also small, secreted fungal proteins, but interestingly, the surface-activity-altering properties of cerato-platanins are the opposite of what can be observed for hydrophobins. The so far known biochemical properties of cerato-platanin proteins are summarized in this review, and potential biotechnological applications as well as implications of these properties for the biological functions of cerato-platanin proteins are discussed.

Genome and secretome analysis of the hemibiotrophic fungal pathogen, Moniliophthora roreri, which causes frosty pod rot disease of cacao: mechanisms of the biotrophic and necrotrophic phases

BACKGROUND

The basidiomycete Moniliophthora roreri is the causal agent of Frosty pod rot (FPR) disease of cacao (Theobroma cacao), the source of chocolate, and FPR is one of the most destructive diseases of this important perennial crop in the Americas. This hemibiotroph infects only cacao pods and has an extended biotrophic phase lasting up to sixty days, culminating in plant necrosis and sporulation of the fungus without the formation of a basidiocarp.

RESULTS

We sequenced and assembled 52.3 Mb into 3,298 contigs that represent the M. roreri genome. Of the 17,920 predicted open reading frames (OFRs), 13,760 were validated by RNA-Seq. Using read count data from RNA sequencing of cacao pods at 30 and 60 days post infection, differential gene expression was estimated for the biotrophic and necrotrophic phases of this plant-pathogen interaction. The sequencing data were used to develop a genome based secretome for the infected pods. Of the 1,535 genes encoding putative secreted proteins, 1,355 were expressed in the biotrophic and necrotrophic phases. Analysis of the data revealed secretome gene expression that correlated with infection and intercellular growth in the biotrophic phase and invasive growth and plant cellular death in the necrotrophic phase.

CONCLUSIONS

Genome sequencing and RNA-Seq was used to determine and validate the Moniliophthora roreri genome and secretome. High sequence identity between Moniliophthora roreri genes and Moniliophthora perniciosa genes supports the taxonomic relationship with Moniliophthora perniciosa and the relatedness of this fungus to other basidiomycetes. Analysis of RNA-Seq data from infected plant tissues revealed differentially expressed genes in the biotrophic and necrotrophic phases. The secreted protein genes that were upregulated in the biotrophic phase are primarily associated with breakdown of the intercellular matrix and modification of the fungal mycelia, possibly to mask the fungus from plant defenses. Based on the transcriptome data, the upregulated secreted proteins in the necrotrophic phase are hypothesized to be actively attacking the plant cell walls and plant cellular components resulting in necrosis. These genes are being used to develop a new understanding of how this disease interaction progresses and to identify potential targets to reduce the impact of this devastating disease.

Functional diversification of cerato-platanins in Moniliophthora perniciosa as seen by differential expression and protein function specialization

Cerato-platanins (CP) are small, cysteine-rich fungal-secreted proteins involved in the various stages of the host-fungus interaction process, acting as phytotoxins, elicitors, and allergens. We identified 12 CP genes (MpCP1 to MpCP12) in the genome of Moniliophthora perniciosa, the causal agent of witches' broom disease in cacao, and showed that they present distinct expression profiles throughout fungal development and infection. We determined the X-ray crystal structures of MpCP1, MpCP2, MpCP3, and MpCP5, representative of different branches of a phylogenetic tree and expressed at different stages of the disease. Structure-based biochemistry, in combination with nuclear magnetic resonance and mass spectrometry, allowed us to define specialized capabilities regarding self-assembling and the direct binding to chitin and N-acetylglucosamine (NAG) tetramers, a fungal cell wall building block, and to map a previously unknown binding region in MpCP5. Moreover, fibers of MpCP2 were shown to act as expansin and facilitate basidiospore germination whereas soluble MpCP5 blocked NAG6-induced defense response. The correlation between these roles, the fungus life cycle, and its tug-of-war interaction with cacao plants is discussed.