Identification of differentially expressed genes associated with changes in the morphology of Pichia fermentans on apple and peach fruit

Yeast-Mycelial Dimorphism in Pichia pastoris SMD1168 Is Triggered by Nutritional and Environmental Factors

This study reports, for the first time, morphological transition from yeast-like to filamentous form, normally associated with pathogenicity/increased protein secretion, in Pichia pastoris SMD1168 strain. The response was recorded in response to nutritional and environmental cues. The factors affecting this switch were extracellular pH (under nitrogen starvation conditions), carbon and nitrogen source under nitrogen- and carbon-limiting conditions respectively. Under nitrogen-limiting conditions, addition of fructose and sucrose in the culture medium induced filamentous morphology in a segregated form whereas addition of galactose led to a mixture of yeast and the filamentous form of the cells. Under carbon-limiting conditions, isoleucine and proline forced a filamentous form whereas glycine, valine, alanine and phenylalanine promoted yeast-like morphology. Similar dimorphic shift was also displayed by a recombinant methanol slow utilizing (Mut^s) strain (SMD-GCSF Mut^s) producing human granulocyte colony-stimulating factor in response to change in the initial inoculum level. Analysis of the extracellular metabolome by GC-MS indicated that several amino acids (leucine, proline, tyrosine), carboxylic acids (phenylacetic-, propanoic acid), alcohols and butylamine were present at different levels in the culture broth of the two morphological forms. High accumulation of proline and butylamine was seen in the extracellular culture filtrate of the filamentous form of the yeast. Presence of quorum-sensing molecules (phenylethyl alcohol, dodecanol) suggested complex network of pathways involved in this morphological transition.

Soil Fungal Community Composition and Diversity of Culturable Endophytic Fungi from Plant Roots in the Reclaimed Area of the Eastern Coast of China

As an important resource for screening microbial strains capable of conferring stress tolerance in plants, the fungal community associated with the plants grown in stressful environments has received great attention. In this study, high-throughput sequencing was employed to study the rhizosphere fungal community in the reclaimed area (i.e., sites F, H, and T) of the eastern coast of China. Moreover, endophytic fungi from the root of six plant species colonizing the investigated sites were isolated and identified. The differences in soil physicochemical parameters, fungal diversity, and community structure were detected among the sampling sites and between the seasons. Ectomycorrhizal (ECM) fungi (e.g., genera Tuber and Geopora) were dominant at site F, which was characterized by high soil total carbon (SC) and total nitrogen (SN) contents and low soil electrical conductivity (EC) value. Arbuscular mycorrhizal (AM) fungi, including genera Glomus, Rhizophagus, and Entrophospora were dominant at sites H (winter), H (summer), and T (summer), respectively. The positive relationship between the EC value and the abundance of genus Glomus indicated the ability of this AM fungus to protect plants against the salt stress. Endophytic fungi at sites F (Aspergillus and Tetracladium), H (Nigrospora), and T (Nigrospora, Coniochaeta and Zopfiella) were recognized as the biomarkers or keystone taxa, among which only genus Aspergillus was isolated from the plant roots. The aforementioned AM fungi and endophytic fungi could contribute to the promotion of plant growth in the newly reclaimed land.

Postharvest Treatments with Three Yeast Strains and Their Combinations to Control Botrytis cinerea of Snap Beans

Three yeast strains, namely Cryptococcus albidus (Ca63), Cryptococcus albidus (Ca64), and Candida parapsilosis (Yett1006), and their combinations, including single yeast agent, two combined yeast strains, single yeast agent + NaHCO₃, single yeast agent + chitosan, single yeast agent + ascorbic acid, and single yeast agent + konjac powder, were evaluated for their activity against Botrytis cinerea, the most economically important fungal pathogens causing postharvest disease of snap beans. In in vitro tests, no inhibition zone was observed in dual cultures of three yeast strains and B. cinerea. The mycelial growth inhibition rates of B. cinerea for Ca63, Ca64, and Yett1006 were 97%, 95%, and 97%, respectively. In in vivo tests, the optimal combination of the lowest disease index of snap beans with B. cinerea was Ca63 + Ca64, with a preventing effect of 75%. The decay rate and rust spots index of Ca64 + ascorbic acid combination were 25% and 20%, respectively, which were the lowest. The activities of defense-related enzymes increased, while malondialdehyde (MDA) content was suppressed in snap beans after different treatments. Our results highlight the potential of the three yeast strains and their combinations as new nonpolluting agents for the integrated control of B. cinerea on snap beans.

Biocontrol yeasts: mechanisms and applications

Yeasts occur in all environments and have been described as potent antagonists of various plant pathogens. Due to their antagonistic ability, undemanding cultivation requirements, and limited biosafety concerns, many of these unicellular fungi have been considered for biocontrol applications. Here, we review the fundamental research on the mechanisms (e.g., competition, enzyme secretion, toxin production, volatiles, mycoparasitism, induction of resistance) by which biocontrol yeasts exert their activity as plant protection agents. In a second part, we focus on five yeast species (Candida oleophila, Aureobasidium pullulans, Metschnikowia fructicola, Cryptococcus albidus, Saccharomyces cerevisiae) that are or have been registered for the application as biocontrol products. These examples demonstrate the potential of yeasts for commercial biocontrol usage, but this review also highlights the scarcity of fundamental studies on yeast biocontrol mechanisms and of registered yeast-based biocontrol products. Yeast biocontrol mechanisms thus represent a largely unexplored field of research and plentiful opportunities for the development of commercial, yeast-based applications for plant protection exist.

Analysis of extracellular vesicles produced in the biofilm by the dimorphic yeast Pichia fermentans

The yeast Pichia fermentans DISAABA 726 strain (P. fermentans) is a dimorphic yeast that under different environmental conditions may switch from a yeast-like to pseudohyphal morphology. We hypothesize that exosomes-like vesicles (EV) could mediate this rapid modification. EV are membrane-derived vesicles carrying lipids, proteins, mRNAs and microRNAs and have been recognized as important mediators of intercellular communication. Although it has been assumed for a long time that fungi release EV, knowledge of their functions is still limited. In this work we analyze P. fermentans EV production during growth in two different media containing urea (YCU) or methionine (YCM) where yeast-like or pseudohyphal morphology are produced. We developed a procedure to extract EV from the neighboring biofilm which is faster and more efficient as compared to the widely used ultracentrifugation method. Differences in morphology and RNA content of EV suggest that they might have an active role during dimorphic transition as response to the growth conditions. Our findings are coherent with a general state of hypoxic stress of the pseudohyphal cells.

Stress tolerance and biocontrol performance of the yeast antagonist, Candida diversa, change with morphology transition

As an eco-friendly management method, biological control of postharvest diseases, utilizing antagonistic yeasts, is a research topic receiving considerable attention. Detailed knowledge on the biology of yeast antagonists is crucial when considering their potential application and development as biocontrol products. Changes in the growth form, such as single-cell to pseudohyphae, have been associated with the mode of action in postharvest biocontrol yeasts. In this study, the antagonistic yeast, Candida diversa, reversibly shifted from a single-cell morphology on yeast peptone dextrose (YPD) medium with 2 % agar to a pseudohyphal morphology on YPD with 0.3 % agar. The tolerance of the pseudohyphal form to heat and oxidative stresses, as well as the biocontrol efficacy against Botrytis cinerea on apple and kiwifruit stored at 25 and 4 °C, was significantly higher as compared to the single-cell form. This study provides new information on the ability of C. diversa to change its morphology and the impact of the morphology shift on stress tolerance and biocontrol performance.

Proteome changes during yeast-like and pseudohyphal growth in the biofilm-forming yeast Pichia fermentans

The Pichia fermentans strain DISAABA 726 is a biofilm-forming yeast that has been proposed as biocontrol agent to control brown rot on apple. How ever, when inoculated on peach, strain 726 shows yeast-like to pseudohyphal transition coupled to a pathogenic behaviour. To identify the proteins potentially involved in such transition process, a comparative proteome analysis of P. fermentans 726 developed on peach (filamentous growth) vs apple (yeast-like growth) was carried out using two-dimensional gel electrophoresis coupled with mass spectrometry analysis. The proteome comparison was also performed between the two different cell morphologies induced in a liquid medium amended with urea (yeast-like cells) or methionine (filamentous cells) to exclude fruit tissue impact on the transition. Seventy-three protein spots showed significant variations in abundance (±twofold, p < 0.01, confidence intervals 99 %) between pseudohyphal vs yeast-like morphology produced on fruits. Among them, 30 proteins changed their levels when the two morphologies were developed in liquid medium. The identified proteins belong to several pathways and functions, such as glycolysis, amino acid synthesis, chaperones, and signalling transduction. The possible role of a group of proteins belonging to the carbohydrate pathway in the metabolic re-organisation during P. fermentans dimorphic transition is discussed.

A putative phospholipase C is involved in Pichia fermentans dimorphic transition

BACKGROUND

Pichia fermentans DiSAABA 726 is a dimorphic yeast that reversibly shifts from yeast-like to pseudohyphal morphology. This yeast behaves as a promising antagonist of Monilia spp. in the yeast-like form, but becomes a destructive plant pathogen in the pseudohyphal form thus raising the problem of the biological risk associated with the use of dimorphic yeasts as microbial antagonists in the biocontrol of phytopathogenic fungi.

METHODS

Pichia fermentans DiSAABA 726 was grown in urea- and methionine-containing media in order to induce and separate yeast-like and pseudohyphal morphologies. Total RNA was extracted from yeast-like cells and pseudohyphae and retro-transcribed into cDNA. A rapid subtraction hybridization approach was utilized to obtain the cDNA sequences putatively over-expressed during growth on methionine-containing medium and involved in pseudohyphal transition.

RESULTS

Five genes that are over-expressed during yeast-like/pseudohyphal dimorphic transition were isolated. One of these, encoding a putative phospholipase C, is involved in P. fermentans filamentation. In fact, while the inhibition of phospholipase C, by means of 1-O-octadecyl-2-O-methyl-rac-glycero-3-phosphorylcholine (Et-18), is accompanied by a significant reduction of pseudohyphae formation in P. fermentans, the addition of exogenous cAMP fully restores pseudohyphal growth also in the presence of Et-18.

CONCLUSION

Phospholipase C is part of a putative "methionine sensing machinery" that activates cAMP-PKA signal transduction pathway and controls P. fermentans yeast-like/pseudohyphal dimorphic transition.

GENERAL SIGNIFICANCE

Phospholipase C is a promising molecular target for further investigations into the link between pseudohyphae formation and pathogenicity in P. fermentans.