Gα proteins Gvm2 and Gvm3 regulate vegetative growth, asexual development, and pathogenicityon apple in Valsa mali

Mechanisms of Trichoderma longibrachiatum T6 Fermentation against Valsa mali through Inhibiting Its Growth and Reproduction, Pathogenicity and Gene Expression

Apple Valsa canker is one of the most serious diseases, having caused significant apple yield and economic loss in China. However, there is still no effective biological methods for controlling this disease. Our present study focused on the inhibitory activity and mechanisms of Trichoderma longibrachiatum (T6) fermentation on Valsa mali that causes apple Valsa canker (AVC). Our results showed that the T6 fermentation exhibited effective antifungal activity on the mycelial growth and conidia germination of V. mali, causing mycelium malformation and the hyphal disintegrating in comparison to the control. The activity of pathogenically related enzymes that are secreted from V. mali and the expression level of gene of V. mali were significantly inhibited and downregulated by treatment with T6 fermentation. In addition, the lesion area and number of pycnidia of V. mali formed on the branches were significantly reduced after treatment with the T6 fermentation through the pathogenicity test on the detached branches. Our results indicate that the possible mechanism of T6 fermentation against V. mali occurs through inhibiting its growth and reproduction, the pathogenic enzyme activity, and its related gene expression.

Signal transduction by plant heterotrimeric G-protein

Heterotrimeric G-proteins are complexes that regulate important signalling pathways essential for growth and development in both plants and animals. Although plant cells are composed of the core components (Gα, Gβ and Gγ subunits) found in animal G-proteins, the complexities of the architecture, function and signalling mechanisms of those in animals are dissimilar to those identified in some plants. Current studies on plant G-proteins have improved knowledge of the essential physiological and agronomic properties, which when harnessed, could potentially impact global food security. Extensive studies on the molecular mechanisms underlying these properties in diverse plant species will be imperative in improving our current understanding of G-protein signalling pathways involved in plant growth and development. The advancement of G-protein signalling networks in distinct plant species could significantly aid in better crop development. This review summarizes current progress, novel discoveries and future prospects for this area in potential crop improvement.

Gα3 subunit Thga3 positively regulates conidiation, mycoparasitism, chitinase activity, and hydrophobicity of Trichoderma harzianum

Heterotrimeric G-proteins are key elements of signal transduction pathways, which participate in regulating multiple biological processes in fungi including growth, conidiation, antagonism, and mycoparasitism. Among G protein subunits, Gα3 showed diverse regulatory functions in different fungi. In this study, we cloned a Gα3 subunit coding gene thga3 from T. harzianum Th33 that can antagonize Rhizoctonia solani and some other plant pathogenic fungi. A thga3 deletion strain Δthga3 was generated using the double-crossover homologous recombination strategy, and Rthga3 was generated by transforming thga3-expressing vector into the protoplasts of Δthga3 by the PEG/CaCl₂-mediated method. The biological characteristics of wild-type Th33, Δthga3 and Rthga3 were evaluated. Compared with wild-type Th33, Δthga3 showed 15%, 94%, and 23% decrease in hyphal growth, conidia yield, and chitinase activity, respectively, and Δthga3 showed lower antagonistic and mycoparasitism abilities, while there were no significant differences between wild-type Th33 and Rthga3. The hyphal surface hydrophobicity of Δthga3 significantly decreased compared with those of the wild-type Th33 and Rthga3. qRT-PCR analysis revealed that transcript abundance of the hydrophobin gene (tha_09745) of Δthga3 decreased by 80% compared with that of wild-type Th33 and Rthga3. The results showed that thga3 positively regulates the growth, conidiation, hydrophobicity, chitinase activities, and mycoparasitism of Th33 towards R. solani. We hence deduced that the expression level of Tha_09745 is correlated to the hyphal hydrophobicity of Th33 and therefore affects the other biological characteristics of Th33. The findings of this report provide a foundation for elucidating the G-protein signal regulatory mechanisms of fungi.

Effects of Different G-Protein α-Subunits on Growth, Development and Secondary Metabolism of Monascus ruber M7

Strains of Monascus filamentous fungal species have been used to produce fermented foods in Asian countries, such as China, Japan, and The Korean Peninsula, for nearly 2,000 years. At present, their fermented products are widely used as food additives and nutraceutical supplements worldwide owing to their production of beneficial secondary metabolites. Heterotrimeric G-protein signaling pathways participate in regulating multiple biological processes in fungi. Previously, we identified three Monascus ruber M7 G-protein α subunits (Mga1–3) and demonstrated that Mga1 can regulate growth, reproduction and some secondary metabolites’ production. Here, we systematically analyzed and compared the roles of mga1–3 by combining single- and double-gene(s) knockouts and their transcriptomic data. First, mga2 and mga3 knock-out mutants and pairwise combinations of mga1–3 deletion strains were generated. Then the changes in growth, development and the main secondary metabolites, Monascus pigments and citrinin, in these mutants were systematically compared with M. ruber M7. Moreover, RNA-Seq analyses of these mutants were performed. All three Gα subunits worked together to regulate biological processes in M. ruber M7, with Mga1 playing a major role, while Mga2 and Mga3 playing supplemental roles. According to the existing literatures which we can find, gene knock-out mutants of the pairwise combination of mga1–3 and their transcriptome analysis are first reported in this study. The current results have clearly demonstrated the functional division of Mga1–3 in M. ruber M7, and could provide a deeper understanding of the effects of different Gα subunits on growth, development and secondary metabolism in other filamentous fungi.

Secreted peroxidases VmPODs play critical roles in the conidiation, H2O2 sensitivity and pathogenicity of Valsa mali

Apple Valsa canker, caused by the necrotrophic pathogen Valsa mali, is a devastating disease of apples and causes great financial loss in East Asia. Improving the understanding of apple - V. mali interactions will contribute to disease management. In this study, three predicted secreted peroxidases (VmPOD1, VmPOD2 and VmPOD3) were uncovered based on the secretome and genome information of V. mali. Phylogenetic analysis showed that VmPOD1 is a catalase peroxidase, VmPOD2 is a chloroperoxidase, and VmPOD3 is a plant peroxidase-like peroxidase. The secretion function of the corresponding genes was confirmed using the yeast invertase secretion system. The deletion of VmPODs did not affect the vegetative growth when the mutants (ΔVmPOD1, ΔVmPOD2 and ΔVmPOD3) and the wild-type strain 03-8 were grown on PDA medium at 25 °C in the dark. However, the respective mutants showed impaired conidiation ability with fewer pycnidia, and all gene deletion mutants grew more slowly than 03-8 on PDA supplemented with H₂O₂ (Final concentration: 0.06 mol/L H₂O₂)_. In addition, VmPOD1 and VmPOD2 were found to be significantly up-regulated at an early infection stage, and VmPOD3 showed sustained high expression during the whole infection progress of V. mali. In addition, the virulence of ΔVmPOD3 was significantly reduced, implying that VmPOD3 plays a critical role during the interaction between V. mali and apple. All of the defective phenotypes could be nearly restored by re-introducing the wild-type VmPOD1, VmPOD2 or VmPOD3 allele. The results enhanced our understanding of the secreted peroxidase, which could also act as a type of virulence factor from the necrotrophic pathogen V. mali and provided new insight into the role of the pathogen-secreted peroxidase.

Two members of the velvet family, VmVeA and VmVelB, affect conidiation, virulence and pectinase expression in Valsa mali

Velvet protein family members are important fungal-specific regulators which are involved in conidial development, secondary metabolism and virulence. To gain a broader insight into the physiological functions of the velvet protein family of Valsa mali, which causes a highly destructive canker disease on apple, we conducted a functional analysis of two velvet protein family members (VmVeA and VmVelB) via a gene replacement strategy. Deletion mutants of VmVeA and VmVelB showed increased melanin production, conidiation and sensitivity to abiotic stresses, but exhibited reduced virulence on detached apple leaves and twigs. Further studies demonstrated that the regulation of conidiation by VmVeA and VmVelB was positively correlated with the melanin synthesis transcription factor VmCmr1. More importantly, transcript levels of pectinase genes were shown to be decreased in deletion mutants compared with those of the wild-type during infection. However, the expression of other cell wall-degrading enzyme genes, including cellulase, hemi-cellulase and ligninase genes, was not affected in the deletion mutants. Furthermore, the determination of pectinase activity and immunogold labelling of pectin demonstrated that the capacity for pectin degradation was attenuated as a result of deletions of VmVeA and VmVelB. Finally, the interaction of VmVeA with VmVelB was identified through co-immunoprecipitation assays. VmVeA and VmVelB play critical roles in conidiation and virulence, probably via the regulation of the melanin synthesis transcription factor VmCmr1 and their effect on pectinase gene expression in V. mali, respectively.

Baseline sensitivity and biochemical responses of Valsa mali to propamidine

In the current study, baseline sensitivity of Valsa mali to propamidine was determined using 80 strains collected from apple orchards in Shaanxi Province, China. The median effective concentration (EC₅₀) values for propamidine inhibiting mycelial growth ranged from 0.086 to 0.852 μg/mL, with a mean of 0.405 ± 0.137 μg/mL. After treated with propamidine, mycelia were contorted with an increased number of branches, loss of fruiting body production, and decreased cell membrane permeability. Moreover, the enzyme activities of the complexes I, II, IV and ATPase in the mitochondrial respiratory chain were increased significantly, while the enzyme activities of complexes III decreased. Importantly, both on detached leaves and branches of apple trees, propamidine applied at 100 μg/mL exhibited over 75% protective and curative efficacies, which were even better than the efficacies obtained by carbendazim at the same concentration. These results indicated that propamidine could be used as an alternative compound in controlling Valsa canker and mitochondrial respiratory chains might be correlated with the action mode of propamidine. This study encourages further investigation for the action mechanism of propamidine against plant pathogens and the information could be valuable for synthesis of new antifungal drugs with novel modes of action.

G Protein α Subunit GpaB is Required for Asexual Development, Aflatoxin Biosynthesis and Pathogenicity by Regulating cAMP Signaling in Aspergillus flavus

The heterotrimeric G proteins are critical for signal transduction and function in numerous biological processes including vegetative growth, asexual development and fungal virulence in fungi. Here, we identified four G protein alpha subunits (GanA, GpaB, FadA and GaoC) in the notorious Aflatoxin-producing fungus Aspergillus flavus. GanA, GpaB and FadA have homologues in other fungal species, while GaoC is a novel one. Here, we showed that the loss function of gpaB displayed a defect in conidiophore formation and considerably reduced expression levels of conidia-specific genes brlA and abaA. A decreased viability of cell wall integrity stress and oxidative stress were also found in the ∆gpaB mutant. More importantly, aflatoxin (AF) biosynthesis and infection on crop seeds were severely impaired in the gpaB-deficient mutant. Further analyses demonstrated that the intracellular cAMP levels significantly reduced in the gpaB-deficient mutant compared to wildtype strains. Additionally, an alteration of PKA activities in the ∆gpaB mutant was also found. Overall, our results indicated that GpaB played diverse roles in asexual sporulation, AF biosynthesis and virulence by regulating cAMP signaling in Aspergillus flavus.

A mitogen-activated protein kinase gene (VmPmk1) regulates virulence and cell wall degrading enzyme expression in Valsa mali

Mitogen-activated protein kinases (MAPKs) play critical roles in the regulation of different developmental processes and hydrolytic enzyme production in many fungal plant pathogens. In this study, an FUS3/KSS1-related MAPK gene, VmPmk1, was identified and characterized in Valsa mali, which causes a highly destructive canker disease on apple. VmPmk1 deletion mutant showed a significant reduction in growth rate in vitro, and could not produce pycnidium, indicating that the MAPK gene is important for growth and asexual development. Also, VmPmk1 played a significant role in response to oxidative stress and in the maintenance of cell wall integrity. More importantly, when deletion mutant was inoculated onto detached apple leaves and twigs, an obvious decrease in lesion size was observed. Furthermore, expression of many cell wall degrading enzyme (CWDE) genes declined in the VmPmk1 deletion mutant during infection. VmPmk1 deletion mutant also showed a significant reduction in activities of CWDEs in both induced media and infection process. Finally, the determination of immunogold labeling of pectin demonstrated that the capacity of degradation pectin was attenuated due to the deletion of VmPmk1. These results indicated that VmPmk1 plays important roles in growth, asexual development, response to oxidative stress, and maintenance of cell wall integrity. More importantly, VmPmk1 is involved in pathogenicity of V. mali mainly by regulating CWDE genes expression.