A Race for Survival: Can Bromus tectorum Seeds Escape Pyrenophora semeniperda-caused Mortality by Germinating Quickly?

BACKGROUND AND AIMS: Pathogen-seed interactions may involve a race for seed resources, so that seeds that germinate more quickly, mobilizing reserves, will be more likely to escape seed death than slow-germinating seeds. This race-for-survival hypothesis was tested for the North American seed pathogen Pyrenophora semeniperda on seeds of the annual grass Bromus tectorum, an invasive plant in North America. In this species, the seed germination rate varies as a function of dormancy status; dormant seeds germinate slowly if at all, whereas non-dormant seeds germinate quickly. METHODS: Three experimental approaches were utilized: (a) artificial inoculations of mature seeds that varied in primary dormancy status and wounding treatment; (b) naturally inoculated undispersed seeds that varied in primary dormancy status; and (c) naturally inoculated seeds from the carry-over seed bank that varied in degree of secondary dormancy, habitat of origin and seed age. KEY RESULTS: In all three approaches, seeds that germinated slowly were usually killed by the pathogen, whereas seeds that germinated quickly frequently escaped. Pyrenophora semeniperda reduced B. tectorum seed banks. Populations in drier habitats sustained 50 times more seed mortality than a population in a mesic habitat. Older carry-over seeds experienced 30 % more mortality than younger seeds. CONCLUSIONS: Given the dramatic levels of seed death and the ability of this pathogen to reduce seed carry-over, it is intriguing to consider whether P. semeniperda could be used to control B. tectorum through direct reduction of its seed bank.     

Evaluation of the desiccation tolerance of blastospores of Paecilomyces fumosoroseus (Deuteromycotina: Hyphomycetes) using a lab-scale, air-drying chamber with controlled relative humidity

The stabilization of living microbial agents for use as biological control agents is often accomplished through desiccation. Our air-drying studies with the entomopathogenic fungus Paecilomyces fumosoroseus have shown that the relative humidity (RH) of the drying air significantly affects the desiccation tolerance and the storage stability of blastospores. Drying air with a RH of more than 40% supported significantly higher rates of initial blastospore survival (68-82%) after drying compared to drying with lower relative humidity air. Drying air with a RH above 50% improved the shelf-life of the air-dried blastospore preparations. Adjustment of the pH or replacement of the spent medium with deionized water (d-H₂O) in the blastospore suspension had no significant impact on blastospore desiccation tolerance or storage stability. We have developed and describe a lab-scale, air-drying chamber that delivers air flow over the sample and that can be operated at controlled relative humidity.     

Effects of fungal volatile organic compounds on Arabidopsis thaliana growth and gene expression

Many microorganisms produce volatile organic compounds (VOCs) with biological effects on plants. In this study, Arabidopsis seeds or 14-day-old vegetative plants were exposed to 0.5 μg/l of chemical standards of 26 VOCs previously identified from the biocontrol fungus Trichoderma. Seven compounds (1-decene, 2-heptylfuran, 2-methyl-1-propanol, 2-methyl-1-butanol, 3-methyl-1- butanol, 2-heptanone, and 1-octen-3-ol) were further tested at the physiological concentration (10 ng/l) and 3-methyl-1-butanol, 1-decene, and 2-heptylfuran induced significant increases in fresh weight and total chlorophyll content. Plants exposed to 1-decene had the greatest increase in plant fresh shoot weight (38.9%) and chlorophyll content (67.8%). An RNA sequencing analysis was performed on plants treated with vapors of 1-decene. The expression of 123 genes was differentially affected, encompassing genes involved in cell wall modification, auxin induction, stress, and defense responses, with several major classes of stress-related genes showing down-regulation. To our knowledge, this is the first report of the effect of a plant growth promoting VOC on gene expression in Arabidopsis thaliana. As the role of fungal VOCs in biocontrol moves from correlative studies to more hypothesis driven approaches, our findings can guide both basic and applied studies in agricultural research.     

The mycelial response of the white-rot fungus, Schizophyllum commune to the biocontrol agent, Trichoderma viride

In this study, agar plate interaction between Schizophyllum commune and Trichoderma viride was investigated to characterise the physiological responses occurring during interspecific mycelial combat. The metabolite profiles and morphological changes in both fungi paired on agar were studied relative to the modulation of phenoloxidase activity in S. commune. The calcium ionophore A23187 was incorporated in self-paired cultures of S. commune to explore possible involvement of calcium influx in the response of S. commune to T. viride. The levels of lipid peroxides and protein carbonyls in the confronted mycelia of S. commune were also measured. Contact with T. viride induced pigmentation and cell wall hydrolysis in S. commune with concomitant increase in phenoloxidase activity, rise in the levels of oxidative stress indicators and increased levels of phenolic compounds, antioxidant γ-amino butyric acid, and pyridoxine and osmo-protective sugar alcohols. Calcium ionophore mimicked the pigmentation in the T. viride-confronted mycelia of S. commune, implicating calcium influx in the response to T. viride. The changes in S. commune are indicative of targeted responses to osmotic and oxidative stresses and phenoloxidase-mediated detoxification of noxious compounds in the contact interface with T. viride, which may confer resistance in natural environments.     

Rhizotron studies on Zea mays L. to evaluate biocontrol activity of Bacillus subtilis

The effect of Bacillus as a biocontrol agent against some root-rot fungi was tested using maize (Zea mays L.) in rhizotrons placed in a growth chamber with relative humidity 60% with a 12 h photoperiod and day and night temperatures of 24 and 18°C respectively. Rhizoctonia solani Kühn caused pre-emergence damping-off in the untreated maize seeds showing weak and soft roots as observed through the perspex rhizotrons. Image analysis was used to quantify the effects of Bacillus treatment on seedlings infected with Pythium sp. Bacillus B77 and B81 were most effective in the control of the pathogen, R. solani which achieved a biocontrol activity of 24 and 35% respectively with regard to shoot dry biomass while B81 achieved 48% biocontrol with reference to root dry biomass. There was no effect on the root area. For root dry biomass, B81, B69, B11 and B77 showed higher biocontrol activity in comparison to the control. Pythium sp. caused pre- and post emergence damping- off in the untreated seeds. Root rot of the maize seedlings caused by Pythium sp. was slightly controlled by Bacillus B69 and B81 which achieved biocontrol activity of 18 and 11% respectively. For the biocontrol of Fusarium solani, Bacillus B77, B69, B81 achieved biocontrol activity of 50, 48 and 33% respectively with reference to root dry biomass.     

Recent advances and future prospects in peptaibiotics, hydrophobin, and mycotoxin research, and their importance for chemotaxonomy of Trichoderma and Hypocrea

Fungi of the genus Trichoderma with teleomorphs in Hypocrea are abundant producers of a group of amphiphilic, non-ribosomal peptide antibiotics, which are rich in the non-proteinogenic amino acid Aib (alpha-aminoisobutyric acid). They are referred to as peptaibiotics, or peptaibols, if a 1,2-amino alcohol is present at the C-terminus. Trichoderma/Hypocrea, like other ascomycetous fungi, also produce hydrophobins, a class of small, cysteine-rich proteins. Advanced soft ionization mass spectrometric techniques such as LC-CID-MS, LC-ESI-MS(n), and IC-MALDI-TOF-MS enabled the high-throughput analysis, simultaneous detection and sequence determination of peptaibiotics and hydrophobins from minute quantities of fungal materials. Some Trichoderma species have been recognized to produce peptaibiotics as well as simple mycotoxins of the trichothecene group. The combination of sequence data of both groups of peptides with the pattern of low-molecular-weight secondary metabolites, including trichothecene-type mycotoxins, independently confirmed the results of morphological, molecular, and phylogenetic analyses. This approach established a new lineage in Trichoderma/Hypocrea, the Brevicompactum clade, comprising four new and one redescribed species. Notably, commercial preparations of single or mixed cultures of Trichoderma species, in particular T. harzianum, and T. koningii, are registered as biocontrol agents for soil and plant pathogens. In this context, it is emphasized that the four mycotoxin-producing species of the recently established Brevicompactum clade (T. brevicompactum, T. arundinaceum, T. turrialbense, and T. protrudens) are not closely related to any of the Trichoderma species currently used as biocontrol agents. Furthermore, possible health concerns about release of peptaibiotics in the biosphere are discussed with respect to their bioactivities and their use as drugs in human and veterinary medicine. Finally, future prospects regarding novel bioactivities and further research needs, including interdisciplinary taxonomic approaches, are outlined.     

Establishment potential of the predatory mirid <Emphasis Type="Italic">Dicyphus hesperus</Emphasis> in northern Europe

The predatory mirid Dicyphus hesperus Knight (Hemiptera: Miridae) is native to North America. The species has been used for the control of glasshouse whitefly on aubergine in the Netherlands, and is currently being evaluated for continued and wider release in Europe. Field and laboratory studies were conducted on a population collected from southern California, USA, to assess the cold tolerance and potential for outdoor establishment under prevailing northern European climates. The supercooling points (whole animal freezing temperatures) of nymphal and adult insects were around −20°C. The lethal temperatures (LTemp₅₀) of non-diapausing nymphs and adults and diapausing adults were close to their respective freezing temperatures at −17.6, −17.6 and −19.2°C. At 5°C, the LTime₅₀ was 54, 101.7 and 117.5 days for fed nymphs, non-diapausing and diapausing adults respectively. When first instar nymphs were placed in the field in winter, starved samples died out after 70 days, but 5% of the fed nymphs survived until the end of winter (140 days) and developed to adult on return to the laboratory. After a similar 5-month field exposure, 50% of fed diapausing adults and 15% of fed non-diapausing adults were still alive at the end of winter, whereas starved diapausing adults died after 140 days. On return to the laboratory after 5 months in the field, both diapausing and non-diapausing adults mated and laid eggs, forming viable populations. Overall, the field and laboratory experiments indicate that this population of D. hesperus is able to enter diapause and that winter temperatures are not a barrier to establishment in northern Europe.     

Antagonistic effects of volatiles generated by <Emphasis Type="Italic">Bacillus subtilis</Emphasis> on spore germination and hyphal growth of the plant pathogen, <Emphasis Type="Italic">Botrytis cinerea</Emphasis>

Botrytis cinerea is one of the most serious post-harvest pathogens of fruits and vegetables. Volatiles generated by Bacillus subtilis JA significantly inhibited both spore germination and elongation of germ tubes in Botrytis cinerea using a two-compartment agar-plate assay. The volatiles caused protoplasm retraction from the hyphal tips to the spores. Hua Chen and Xiang Xiao have contributed equally to this work.     

<i>Streptomyces</i> PRIO41 as plant growth promoter of jalapeño pepper plants and as biocontrol agent of <i>Fusarium</i>

Chili pepper is one of the main crops of economic importance in Mexico, and Fusarium wilting is a disease that limits its production. In addition, the inappropriate use of agrochemicals in farming activities generate environmental and health problems. Therefore, in this study the effectiveness of Streptomyces sp PRIO41 was evaluated as a (1) biocontrol agent of Fusarium spp and (2) plant growth promoter bacteria. Assays of pathogenicity and virulence of Fusarium spp. in jalapeño pepper seeds, and interactions of these pathogens with Streptomyces PRIO41 were evaluated under two nutritional conditions. In the greenhouse, the effectiveness of Streptomyces sp. PRIO41 was determined as a (1) biocontrol of Fusarium, and (2) plant growth promoter of wilt of pepper plants. The results showed that all fungal isolates caused symptoms in pepper seeds and seedlings with different degrees of virulence. Interactions in vitro showed that Streptomyces showed the most effective range of virulence against Fusarium isolates in the poor medium (37.6%-100%), with fungicidal effects in some cases. In the greenhouse, Streptomyces PRIO41 reduced Fusarium wilting up to a 40%, and positively affected all vegetative growth parameters, particularly plant height, leaf area, root length, and leaf and root dry biomasses. This study showed the potential of Streptomyces PRIO41 as a biocontrol agent of Fusarium spp., and as a biofertilizer of pepper plants.