Molecular Genetics of Pichia anomala Killer Strains Isolated From Naturally Fermented Olive Brine

Towards unlocking the biocontrol potential of Pichia kudriavzevii for plant fungal diseases: in vitro and in vivo assessments with candidate secreted protein prediction

BACKGROUND

Plant fungal pathogens cause substantial economic losses through crop yield reduction and post-harvest storage losses. The utilization of biocontrol agents presents a sustainable strategy to manage plant diseases, reducing the reliance on hazardous chemical. Recently, Pichia kudriavzevii has emerged as a promising biocontrol agent because of its capacity to inhibit fungal growth, offering a potential solution for plant disease management.

RESULTS

Two novel Pichia kudriavzevii strains, Pk_EgyACGEB_O1 and Pk_EgyACGEB_O2, were isolated from olive brine samples. The microscopic characterization of the strains revealed similar structures. However, there were noticeable differences in their visual morphology. Based on their internal transcribed spacer (ITS) DNA sequences, Pk_EgyACGEB_O1 and Pk_EgyACGEB_O2 strains assigned by GenBank IDs MZ507552.1 and MZ507554.1 shared high sequence similarity (~ 99.8% and 99.5%) with P. kudriavzevii, respectively. Both strains were evaluated in vitro against plant pathogenic fungi. The strains revealed the ability to consistently inhibit fungal growth, with Pk_EgyACGEB_O2 showing higher effectiveness. In addition, both P. kudriavzevii strains effectively controlled grey mold disease caused by B. cinerea in golden delicious apples, suggesting their potential as sustainable and eco-friendly biocontrol agents for post-harvest diseases. Based on a comprehensive bioinformatics pipeline, candidate-secreted proteins responsible for the potent antifungal activity of P. kudriavzevii were identified. A total of 59 proteins were identified as common among the P. kudriavzevii CBS573, SD108, and SD129 strains. Approximately 23% of the secreted proteins in the P. kudriavzevii predicted secretome are hydrolases with various activities, including proteases, lipases, glycosidases, phosphatases, esterases, carboxypeptidases, or peptidases. In addition, a set of cell-wall-related proteins was identified, which might enhance the biocontrol activity of P. kudriavzevii by preserving the structure and integrity of the cell wall. A papain inhibitor was also identified and could potentially offer a supplementary defense against plant pathogens.

CONCLUSION

Our results revealed the biocontrol capabilities of P. kudriavzevii against plant pathogenic fungi. The research focused on screening novel strains for their ability to inhibit the growth of common pathogens, both in vitro and in vivo. This study shed light on how P. kudriavzevii interacts with fungal pathogens. The findings can help develop effective strategies for managing plant diseases.

Killer Yeasts for the Biological Control of Postharvest Fungal Crop Diseases

Every year and all over the world the fungal decay of fresh fruit and vegetables frequently generates substantial economic losses. Synthetic fungicides, traditionally used to efficiently combat the putrefactive agents, emerged, however, as the cause of environmental and human health issues. Given the need to seek for alternatives, several biological approaches were followed, among which those with killer yeasts stand out. Here, after the elaboration of the complex of problems, we explain the hitherto known yeast killer mechanisms and present the implementation of yeasts displaying such phenotype in biocontrol strategies for pre- or postharvest treatments to be aimed at combating postharvest fungal decay in numerous agricultural products.

Bioprotective Role of Yeasts

The yeasts constitute a large group of microorganisms characterized by the ability to grow and survive in different and stressful conditions and then to colonize a wide range of environmental and human ecosystems. The competitive traits against other microorganisms have attracted increasing attention from scientists, who proposed their successful application as bioprotective agents in the agricultural, food and medical sectors. These antagonistic activities rely on the competition for nutrients, production and tolerance of high concentrations of ethanol, as well as the synthesis of a large class of antimicrobial compounds, known as killer toxins, which showed clearly a large spectrum of activity against food spoilage microorganisms, but also against plant, animal and human pathogens. This review describes the antimicrobial mechanisms involved in the antagonistic activity, their applications in the processed and unprocessed food sectors, as well as the future perspectives in the development of new bio-drugs, which may overcome the limitations connected to conventional antimicrobial and drug resistance.

Exoglucanase-encoding genes from three Wickerhamomyces anomalus killer strains isolated from olive brine

Wickerhamomyces anomalus killer strains are important for fighting pathogenic yeasts and for controlling harmful yeasts and bacteria in the food industry. Targeted disruption of key genes in β-glucan synthesis of a sensitive Saccharomyces cerevisiae strain conferred resistance to the toxins of W. anomalus strains BS91, BCA15 and BCU24 isolated from olive brine. Competitive inhibition of the killing activities by laminarin and pustulan refer to β-1,3- and β-1,6-glucans as the main primary toxin targets. The extracellular exoglucanase-encoding genes WaEXG1 and WaEXG2 from the three strains were sequenced and were found to display noticeable similarities to those from known potent W. anomalus killer strains.

Efficacy of killer yeasts in the biological control of Penicillium digitatum on Tarocco orange fruits (Citrus sinensis)

Killer Saccharomyces cerevisiae and Wickerhamomyces anomalus yeast strains were tested as biocontrol agents against Penicillium digitatum, one the most important causes of postharvest decay in orange fruits. W. anomalus, grown on acidified medium, demonstrated micocinogenic activity against P. digitatum, as indicated by large inhibition halos and hyphal damage resulting from β-glucanase activity. Oranges that had been deliberately inoculated with pathogens were protected from decay by W. anomalus. Inoculation of oranges with W. anomalus strains BS 91 and BS 92 reduced disease severity to 1 and 4%, respectively, for up to 10 days in storage.