Biological Control of Phytopathogenic Fungi by Kluyveromyces marxianus and Torulaspora delbrueckii Isolated from Iraqi Date Vinegar

Dissecting an ancient stress resistance trait syndrome in the compost yeast Kluyveromyces marxianus

In the search to understand how evolution builds new traits, ancient events are often the hardest to dissect. Species-unique traits pose a particular challenge for geneticists-cases in which a character arose long ago and, in the modern day, is conserved within a species, distinguishing it from reproductively isolated relatives. In this work, we have developed the budding yeast genus Kluyveromyces as a model for mechanistic dissection of trait variation across species boundaries. Phenotypic profiling revealed robust heat and chemical-stress tolerance phenotypes that distinguished the compost yeast K. marxianus from the rest of the clade. We used culture-based, transcriptomic, and genetic approaches to characterize the metabolic requirements of the K. marxianus trait syndrome. We then generated a population-genomic resource for K. marxianus and harnessed it in molecular-evolution analyses, which found hundreds of housekeeping genes with evidence for adaptive protein variation unique to this species. Our data support a model in which, in the distant past, K. marxianus underwent a vastly complex remodeling of its proteome to achieve stress resistance. Such a polygenic architecture, involving nucleotide-level allelic variation on a massive scale, is consistent with theoretical models of the mechanisms of long-term adaptation, and suggests principles of broad relevance for interspecies trait genetics.

Inhibitory effects of epiphytic Kluyveromyces marxianus from Indian senna (Cassia angustifolia Vahl.) on growth and aflatoxin production of Aspergillus flavus

Aspergillus flavus infection and subsequent aflatoxin contamination are considered the major constraints in senna (Cassia angustifolia Vahl.) export. Using native epiphytic yeast to control phytopathogens is a successful strategy for managing plant diseases. In the present investigation, we exploited the antagonistic potential of epiphytic yeast isolates obtained from senna against A. flavus growth and aflatoxin B1 (AFB1) production. Four Kluyveromyces marxianus strains (YSL3, YSL16, YSP12, and YSF9) exhibited vigorous antagonistic activity with a maximum inhibition of 64 %. In vivo evaluation of senna pods showed that K. marxianus strains effectively reduced A. flavus colonization with a population range of 5.87 to 7.08 log10 CFU/g. In contrast, the untreated senna pods were found to have severe fungal colonization with a population of 7.84 log10 CFU/g. In addition, HPLC analysis showed that aflatoxin B1 in senna pods was drastically reduced upon yeast treatment up to 14 DAI. Furthermore, we demonstrated the antifungal action mechanisms of K. marxianus, such as surface colonizing ability on pods, production of antifungal volatiles (VOCs), siderophores, extracellular lytic enzymes, and cell wall binding ability to AFB1. All four strains of K. marxianus showed rapid colonization on the senna pod, and YSP12 reached the maximum population of 7.18 log10 CFU/pod at 9 days after inoculation (DAI). The exposure of A. flavus to K. marxianus VOCs significantly reduced the growth by up to 99 and 93.2 % at 7 and 14 DAI, respectively. Scanning electron microscopic images demonstrated severe mycelial damage and hyphal deformities of A. flavus. In addition, yeast VOCs can reduce aflatoxin biosynthesis in A. flavus by up to 99 and 93.2 % at 7 and 14 DAI, respectively. Gas chromatography-mass spectrometry analysis confirmed the presence of antimicrobial compounds such as dimethyl trisulfide, ethyl acetate, ethanol, 3-methyl butanal, 2-methyl-1-butanol, and 3-methyl-1-butanol in the volatiles. K. marxianus strains produced siderophores and hydrolytic enzymes such as chitinase and β-1,3-glucanase. A higher AFB1 binding ability was observed in the heat-killed cells (47.5 to 70.65 %) than in the viable cells (43.16 to 60.98 %) of K. marxianus. The current study demonstrated that epiphytic K. marxianus isolated from senna could be a successful biocontrol source to reduce aflatoxin contamination in senna pods.

Biotechnological Importance of Torulaspora delbrueckii: From the Obscurity to the Spotlight

Torulaspora delbrueckii has attracted interest in recent years, especially due to its biotechnological potential, arising from its flavor- and aroma-enhancing properties when used in wine, beer or bread dough fermentation, as well as from its remarkable resistance to osmotic and freezing stresses. In the present review, genomic, biochemical, and phenotypic features of T. delbrueckii are described, comparing them with other species, particularly with the biotechnologically well-established yeast, Saccharomyces cerevisiae. We conclude about the aspects that make this yeast a promising biotechnological model to be exploited in a wide range of industries, particularly in wine and bakery. A phylogenetic analysis was also performed, using the core proteome of T. delbrueckii, to compare the number of homologous proteins relative to the most closely related species, understanding the phylogenetic placement of this species with robust support. Lastly, the genetic tools available for T. delbrueckii improvement are discussed, focusing on adaptive laboratorial evolution and its potential.