Minimization of between-well sample variance of antifungal activity using a high-throughput screening microplate bioassay

Engineering a Non-Natural Photoenzyme for Improved Photon Efficiency*

Photoenzymes are biological catalysts that use light to convert starting materials into products. These catalysts require photon absorption for each turnover, making quantum efficiency an important optimization parameter. Flavin-dependent "ene"-reductases (EREDs) display latent photoenzymatic activity for synthetically valuable hydroalkylations; however, protein engineering has not been used to optimize this non-natural function. We describe a protein engineering platform for the high throughput optimization of photoenzymes. A single round of engineering results in improved catalytic function toward the synthesis of γ, δ, ϵ-lactams, and acyclic amides. Mechanistic studies show that key mutations can alter the enzyme's excited state dynamics, enhance its photon efficiency, and ultimately increase catalyst performance. Transient absorption spectroscopy reveals that engineered variants display dramatically decreased radical lifetimes, indicating an evolution toward a concerted mechanism.

SnTox1, a Parastagonospora nodorum necrotrophic effector, is a dual-function protein that facilitates infection while protecting from wheat-produced chitinases

SnTox1 induces programmed cell death and the up-regulation of pathogenesis-related genes including chitinases. Additionally, SnTox1 has structural homology to several plant chitin-binding proteins. Therefore, we evaluated SnTox1 for chitin binding and localization. We transformed an avirulent strain of Parastagonospora nodorum as well as three nonpathogens of wheat (Triticum aestivum), including a necrotrophic pathogen of barley, a hemibiotrophic pathogen of sugar beet and a saprotroph, to evaluate the role of SnTox1 in infection and in protection from wheat chitinases. SnTox1 bound chitin and an SnTox1-green fluorescent fusion protein localized to the mycelial cell wall. Purified SnTox1 induced necrosis in the absence of the pathogen when sprayed on the leaf surface and appeared to remain on the leaf surface while inducing both epidermal and mesophyll cell death. SnTox1 protected the different fungi from chitinase degradation. SnTox1 was sufficient to change the host range of a necrotrophic pathogen but not a hemibiotroph or saprotroph. Collectively, this work shows that SnTox1 probably interacts with a receptor on the outside of the cell to induce cell death to acquire nutrients, but SnTox1 accomplishes a second role in that it protects against one aspect of the defense response, namely the effects of wheat chitinases.

Postinfection Activity of Synthetic Antimicrobial Peptides Against Stemphylium vesicarium in Pear

Brown spot of pear is a fungal disease of economic importance caused by Stemphylium vesicarium that affects the pear crops in Europe. Due to the characteristics of this disease and the moderate efficacy of available fungicides, the effectiveness of control measures is very limited; however, synthetic antimicrobial peptides (AMPs) may be a complement to these fungicides. In the present study, 12 AMPs of the CECMEL11 library were screened for fungicidal activity against S. vesicarium. In vitro experiments showed that eight AMPs significantly reduced the germination of conidia. The most effective peptides, BP15, BP22, and BP25, reduced fungal growth and sporulation at concentrations below 50 μM. Leaf assays showed that preventive application of BP15 and BP22 did not reduce infection; however, when the peptides were applied curatively, infection was significantly reduced. The use of a BP15 fluorescein 5-isothiocyanate conjugate revealed that the peptide binds to hyphae and germ tubes and produces malformations that irreversibly stop their development.

Optimisation of an in vitro antifungal protein assay for the screening of potential antifungal proteins against Leptosphaeria maculans

Canola is second only to soybean as the most important oilseed crop in the world. The global production of canola is forecast to continue to increase and as a result the canola industry will continue to flourish. However, it is threatened by several fungal diseases that affect canola and cost producers hundreds of millions of dollars a year in reduced yield and quality. Blackleg is the most common and devastating disease of canola and is caused by the fungus Leptosphaeria maculans. The fungus can infect any part of the plant at all growth stages and is a serious threat to the canola industry. Novel and more efficient antifungal agents which interfere with fungal growth and development are clearly needed to control this pathogen. This paper reports the establishment of a simple functional assay system for the screening of antifungal proteins against a virulent strain of L. maculans.

Optimization of spore and antifungal lipopeptide production during the solid-state fermentation of Bacillus subtilis

Bacillus subtilis strain TrigoCor 1448 was grown on wheat middlings in 0.5-l solid-state fermentation (SSF) bioreactors for the production of an antifungal biological control agent. Total antifungal activity was quantified using a 96-well microplate bioassay against the plant pathogen Fusarium oxysporum f. sp. melonis. The experimental design for process optimization consisted of a 2(6-1) fractional factorial design followed by a central composite face-centered design. Initial SSF parameters included in the optimization were aeration, fermentation length, pH buffering, peptone addition, nitrate addition, and incubator temperature. Central composite face-centered design parameters included incubator temperature, aeration rate, and initial moisture content (MC). Optimized fermentation conditions were determined with response surface models fitted for both spore concentration and activity of biological control product extracts. Models showed that activity measurements and spore production were most sensitive to substrate MC with highest levels of each response variable occurring at maximum moisture levels. Whereas maximum antifungal activity was seen in a limited area of the design space, spore production was fairly robust with near maximum levels occurring over a wider range of fermentation conditions. Optimization resulted in a 55% increase in inhibition and a 40% increase in spore production over nonoptimized conditions.

Comparative study of antimicrobial peptides to control citrus postharvest decay caused by Penicillium digitatum

The objective of this study was to investigate and compare the in vitro efficacy and in vivo potential of eight distinct short antimicrobial peptides to control the postharvest green mold disease of oranges caused by the fungus Penicillium digitatum. The L-amino acid versions of the four peptides PAF26, PAF38, PAF40, and BM0, previously obtained by combinatorial approaches, were examined. The study included two antibacterial peptides formerly identified by rational design, BP15 and BP76, and it is demonstrated that they also have in vitro antifungal properties. The natural antimicrobial peptides melittin and indolicidin were also selected for comparison, due to their well-known properties and modes of action. In vitro and in vivo results indicated differential behaviors among peptides, regarding the inhibitory potency in growth media, selectivity against distinct microorganisms, fungicidal activity towards nongerminated conidia of P. digitatum, and efficacy in fruit inoculation assays. Interestingly, a high in vitro inhibitory activity did not necessarily associate with an effective control of fruit infection by P. digitatum. The short tryptophan-rich cationic peptides PAF26, PAF38, PAF40, and BM0 were lethal to conidia of P. digitatum, and this property is correlated with better protection in the decay control test.