Unveiling the dynamic expression of PR-1 during Musa spp. infection by Fusarium oxysporum fsp. Cubense: a cloning and characterization study

BACKGROUND

Pathogen-related proteins (PR) are pivotal in plant defense, combating diverse biotic and abiotic stresses. While multiple gene families contribute to banana resistance against Fusarium oxysporum f sp. cubense (Foc), Pseudocercospora eumusae, and Pratylenchus coffeae, the significance of PR-1 genes in defense is paramount.

METHODS

Three PR-1 genes, up-regulated under diverse biotic stresses, were cloned from both resistant and susceptible cultivars of Foc, P. eumusae, and P. coffeae. Molecular characterization, phylogenetic analysis, and docking studies with the Foc TR4 CP gene were conducted.

RESULTS

Through transcriptomic and real-time studies, three PR-1 genes (Ma02_g15050, Ma02_g15060, and Ma04_g34800) from Musa spp. were identified. These genes exhibited significant up-regulation in resistant cultivars when exposed to Foc, P. eumusae, and P. coffeae. Cloning of these genes was successfully performed from both resistant and susceptible cultivars of Foc race 1 and TR4, P. eumusae, and P. coffeae. Distinct characteristics were observed among the PR-1 genes, with groups 1 and 2 being acidic with signal peptides, and group 3 being basic without signal peptides. All cloned PR-1 proteins belonged to the CAP superfamily (PF00188). Phylogenetic analysis revealed clustering patterns for acidic PR-1 proteins, and KEGG orthology showed associations with vital pathways, including MAPK signaling, plant hormone signal transduction, and plant-pathogen interaction. Secondary and tertiary structure analyses confirmed sequence conservation across studied species. Docking studies explored interactions between the cerato-platanin (CP) gene from Foc TR4 and Ma02_g15060 from banana, suggesting the potential hindrance of PR-1 antifungal activity through direct interaction.

CONCLUSIONS

The findings underscore the crucial role of cloned PR-1 genes in banana plant defense mechanisms against a broad spectrum of biotic stresses. These genes, especially those in groups 1 and 2, hold promise as candidates for developing stress-tolerant banana cultivars. The study provides valuable insights into the molecular aspects of banana defense strategies, emphasizing the potential applications of PR-1 genes in enhancing banana resilience.

The growth of marine fungi on seaweed polysaccharides produces cerato-platanin and hydrophobin self-assembling proteins

The marine fungi Paradendryphiela salina and Talaromyces pinophilus degrade and assimilate complex substrates from plants and seaweed. Additionally, these fungi secrete surface-active proteins, identified as cerato-platanins and hydrophobins. These hydrophobic proteins have the ability to self-assemble forming amyloid-like aggregates and play an essential role in the growth and development of the filamentous fungi. It is the first time that one cerato-platanin (CP) is identified and isolated from P. salina (PsCP) and two Class I hydrophobins (HFBs) from T. pinophilus (TpHYD1 and TpHYD2). Furthermore, it is possible to extract cerato-platanins and hydrophobins using marine fungi that can feed on seaweed biomass, and through a submerged liquid fermentation process. The propensity to aggregate of these proteins has been analyzed using different techniques such as Thioflavin T fluorescence assay, Fourier-transform Infrared Spectroscopy, and Atomic Force Microscopy. Here, we show that the formation of aggregates of PsCP and TpHYD, was influenced by the carbon source from seaweed. This study highlighted the potential of these self-assembling proteins generated from a fermentation process with marine fungi and with promising properties such as conformational plasticity with extensive applications in biotechnology, pharmacy, nanotechnology, and biomedicine.

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.

Partitioning the structural features that underlie expansin-like and elicitor activities of cerato-platanin

Cerato-platanin family (CPF) proteins are produced by fungi and elicit defences when applied to plants, behaving as PAMPs/MAMPs. CPF proteins share structural similarity to plant and bacterial expansins, and have been demonstrated, in some cases, to possess expansin-like loosening activity on cellulose. This is the case of cerato-platanin (CP), the founder of the CPF, which shows both eliciting and cellulose-loosening activities, raising the question as to whether the expansin-like activity may be responsible for defence activation. To pinpoint structural and thermodynamic features underlying eliciting and expansin-like activity of CP, we carried out site-directed mutagenesis targeting separately net charge (N84D mutation), conformational stability (V63A mutation), or conserved position previously shown to affect expansin-like activity in CP (D77A mutation), and characterized wild-type protein and its variants. Removing or adding negative charges on the protein surface led to reducing or increasing, respectively, the expansin-like activity. The activity was instead not affected by mutations affecting protein fold and stability. In contrast, all the mutants showed reduced capacity to elicit defences in plants. We conclude that the expansin-like activity of CP depends on net charge and ability to (weakly) bind cellulose, whereas the eliciting activity on plants does not depend on the cellulose-loosening activity.

The Fusarium graminearum cerato-platanins loosen cellulose substrates enhancing fungal cellulase activity as expansin-like proteins

Cerato-platanin proteins (CPPs) are small non-catalytic, cysteine-rich hydrophobic proteins produced by filamentous fungi. The genome of Fusarium graminearum, the causal agent of Fusarium head blight disease of wheat and other cereal grains, contains two genes putatively encoding for CPPs. To better characterize their features, the two FgCPPs were heterologously expressed in Pichia pastoris. The recombinant FgCPPs reduced the viscosity of a cellulose soluble derivate (carboxymethyl cellulose, CMC). The same effect was not observed on other polysaccharide substrates such as chitin, 1,3-β-glucan, xylan and pectin. Indeed, differently from other fungal CPPs and similarly to expansins, FgCPPs are trapped by cellulose and not by chitin, thus suggesting that these proteins interact with cellulose. A double knock-out mutant deleted of both FgCPPs encoding genes produces much more cellulase activity than the corresponding wild type strain when grown on CMC, likely compensating the absence of FgCPPs. This result prompted us to investigate a possible synergistic effect of these proteins with fungal cellulases. The incubation of FgCPPs in the presence of a fungal cellulase (EC 3.2.1.4) determines an increased enzymatic activity on CMC, filter paper and wheat cell walls. The observation that FgCPPs act with a non-hydrolytic mechanism indicates that these proteins favor fungal cellulase activity in an expansin-like manner. Though the disruption of the FgCPP genes had no demonstrable impact on fungal virulence, our experimental data suggest their probable involvement in virulence, thus we refer to them as accessory virulence genes. Our results suggest also that the FgCPPs could be exploited for future biotechnological application in second-generation biofuels production on lignocellulosic biomasses rich in cellulose. Finally, we demonstrate that FgCPPs act as elicitors of defense responses on Arabidopsis leaves, increasing resistance to Botrytis cinerea infections.

SsSm1, a Cerato-platanin family protein, is involved in the hyphal development and pathogenic process of Sclerotinia sclerotiorum

The filamentous fungus Sclerotinia sclerotiorum is an important plant pathogen with a worldwide distribution. It can infect a wide variety of plants, causing serious disease in many types of crops, such as rapeseed, sunflower and soybean. Sclerotinia stem rot caused by this fungus affects main crops and has led to great economic loss. Elicitors are a group of compounds that inspire the host plant to produce an immune response against invading pathogens. This study describes a protein that has high homology with the Trichoderma elicitor Sm1 and was found in the genome of S. sclerotiorum. We named this protein SsSm1. To determine whether this protein has an elicitor function like its homology protein, we constructed a heterologous expression vector for SsSm1 and expressed it in Escherichia coli. The protein of heterologous expression led to the formation of lesions in tobacco that closely resemble hypersensitive response lesions. Transient expression of the encoding gene of SsSm1 in tobacco leaves also caused hypersensitive response. Then, RNA silencing was used to identify the function of SsSm1. The hyphal growth and pathogenicity of silenced transformants were shown to be obviously lagging and branched abnormally. Transformants produced less infection cushions and deformed sclerotiorum. In addition, SsSm1 silencing caused weak tolerance to NaCl, sorbitol and SDS, and the sensitivity of mutants to carbendazim was also significantly decreased. Based on the above results, we speculate that this protein may be related to the development of hyphae, infection cushions and sclerotiorum, but the specific molecular mechanism needs to be studied further.

Comparison of cerato-platanin family protein BcSpl1 produced in Pichia pastoris and Escherichia coli

The Botrytis cinerea BcSpl1 protein is a member of the cerato-platanin family, and consists of 137 amino acids and two disulfide bridges. This protein induces the onset of necrosis in infiltrated plant hosts. Recombinant BcSpl1 proteins produced in Pichia pastoris (pBcSpl1) and Escherichia coli (eBcSpl1) were initially compared regarding their abilities to induce necrosis and systemic acquired resistance (SAR). The pBcSpl1 and eBcSpl1 treatments led to the development of necrotic lesions on tomato leaves, and provided tomato plants with SAR to B. cinerea. The lesion area of leaves infiltrated with the BcSpl1 proteins decreased by 22.7% (pBcSpl1) and 21.8% (eBcSpl1). Additionally, eBcSpl1 up-regulated the expression levels of some defense-related genes, including PR-1a, prosystemin, PI I, and PI II, as well as SIPK and TPK1b, which encode two protein kinases. Furthermore, eBcSpl1 exhibited chitin-binding properties. Our data revealed that the E. coli expression system produces higher BcSpl1 yields than the P. pastoris system. This high-yield expression of BcSpl1 may be relevant for future large-scale applications of this elicitor to improve crop production.

Involvement of the Fusarium graminearum cerato-platanin proteins in fungal growth and plant infection

The genome of Fusarium graminearum, a necrotrophic fungal pathogen causing Fusarium head blight (FHB) disease of wheat, barley and other cereal grains, contains five genes putatively encoding for proteins with a cerato-platanin domain. Cerato-platanins are small secreted cysteine-rich proteins possibly localized in the fungal cell walls and also contributing to the virulence. Two of these F. graminearum proteins (FgCPP1 and FgCPP2) belong to the class of SnodProt proteins which exhibit phytotoxic activity in the fungal pathogens Botrytis cinerea and Magnaporthe grisea. In order to verify their contribution during plant infection and fungal growth, single and double gene knock-out mutants were produced and no reduction in symptoms severity was observed compared to the wild type strain on both soybean and wheat spikes. Histological analysis performed by fluorescence microscopy on wheat spikelets infected with mutants constitutively expressing the dsRed confirmed that FgCPPs do not contribute to fungal virulence. In particular, the formation of compound appressoria on wheat paleas was unchanged. Looking for other functions of these proteins, the double mutant was characterized by in vitro experiments. The mutant was inhibited by salt and H2O2 stress similarly to wild type. Though no growth difference was observed on glucose, the mutant grew better than wild type on carboxymethyl cellulose. Additionally, the mutant's mycelium was more affected by treatments with chitinase and β-1,3-glucanase, thus indicating that FgCPPs could protect fungal cell wall polysaccharides from enzymatic degradation.

A paralog of the proteinaceous elicitor SM1 is involved in colonization of maize roots by Trichoderma virens

The biocontrol agent, Trichoderma virens, has the ability to protect plants from pathogens by eliciting plant defense responses, involvement in mycoparasitism, or secreting antagonistic secondary metabolites. SM1, an elicitor of induced systemic resistance (ISR), was found to have three paralogs within the T. virens genome. The paralog sm2 is highly expressed in the presence of plant roots. Gene deletion mutants of sm2 were generated and the mutants were found to overproduce SM1. The ability to elicit ISR in maize against Colletotrichum graminicola was not compromised for the mutants compared to that of wild type isolate. However, the deletion strains had a significantly lowered ability to colonize maize roots. This appears to be the first report on the involvement of an effector-like protein in colonization of roots by Trichoderma.

Cerato-platanins: elicitors and effectors

Cerato-platanins are an interesting group of small, secreted, cysteine-rich proteins that have been implicated in virulence of certain plant pathogenic fungi. The relatively recent discovery of these proteins in plant beneficial fungi like Trichoderma spp., and their positive role in induction of defense in plants against invading pathogens has raised the question as to whether these proteins are effectors or elicitor molecules. Here we present a comprehensive review on the occurrence of these conserved proteins across the fungal kingdom, their structure-function relationships, and their physiological roles in plant pathogenic and symbiotic fungi. We also discuss the usefulness of these proteins in evolving strategies for crop protection through a transgenic approach or direct application as elicitors.