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ABSTRACT It is widely recognized that plant-symbiotic fungi are supported by photosynthates; however, little is known about the molecular mechanisms underlying the utilization of plant-derived sugars by rhizospheric fungi. In the insect-pathogenic and plant-symbiotic fungus Metarhizium robertsii, we previously showed that the utilization of oligosaccharides by the transporter MRT (Metarhizium raffinose transporter) is important for rhizosphere competency. In this study, we identified a novel monosaccharide transporter (MST1) that is involved in the colonization of the rhizoplane and acts additively with MRT to colonize the rhizosphere. MST1 is not involved in infection of insects by M. robertsii. MST1 is an H+ symporter and is able to transport a broad spectrum of monosaccharides, including glucose, sorbose, mannose, rhamnose, and fructose. Deletion of the Mst1 gene impaired germination and mycelial growth in medium containing the sugars that it can transport. Homologs of MST1 were widely found in many fungi, including plant symbionts such as Trichoderma spp. and mycorrhizal fungi and plant pathogens such as Fusarium spp. This work significantly advances insights into the development of symbiotic relationships between plants and fungi. IMPORTANCE Over 90% of all vascular plant species develop an intimate symbiosis with fungi, which has an enormous impact on terrestrial ecosystems. It is widely recognized that plant-symbiotic fungi are supported by photosynthates, but little is known about the mechanisms for fungi to utilize plant-derived carbon sources. In the fungus Metarhizium robertsii, we identified a novel monosaccharide transporter (MST1) that is an H+ symporter and can transport a broad spectrum of monosaccharides, including glucose, sorbose, mannose, rhamnose, and fructose. MST1 is involved in the colonization of the rhizoplane and acts additively with the previously characterized oligosaccharide transporter MRT to colonize the rhizosphere. Homologs of MST1 were found in many fungi, including plant symbionts and plant pathogens, suggesting that the utilization of plant-derived sugars by MST1 homologs could also be important for other fungi to develop a symbiotic or parasitic relationship with their respective plant hosts.

Cerato-platanins (CPs) form a family of fungal small-secreted cysteine-rich proteins (SSCPs) and are of particular interest not only because of their surface activity but also their abundant secretion by fungi. We performed an evolutionary analysis for 283 CPs from 157 fungal genomes with the focus on the opportunistic plant-beneficial and mycoparasitic fungus Trichoderma. Our results revealed the long evolutionary history of CPs in Dikarya fungi that have undergone several events of lateral gene transfer and gene duplication. Three genes were maintained in the core genome of Trichoderma, while some species have up to four CP encoding genes. All Trichoderma CPs evolve under stabilizing natural selection pressure. The functional analysis of CPs in T. guizhouense and T. harzianum revealed that only EPL1 is active at all stages of the development but plays a minor role in interactions with other fungi and bacteria. The deletion of this gene results in increased colonization of tomato roots by Trichoderma spp. Similarly, the biochemical tests of the heterologously produced EPL1 by Pichia pastoris support the above claims. Based on the obtained results, we conclude that the function of CPs is probably linked to their surfactant properties and the ability to modify the hyphosphere of submerged mycelium and thus facilitate the nutritional versatility of fungi. The effector-like properties do not sufficiently describe the diversity and evolution of these proteins in fungi as they are also maintained, duplicated, or laterally transferred in the genomes of non-herbivore fungi. [ABSTRACT FROM AUTHOR]

The rhizobacterium Pseudomonas pseudoalcaligenes AVO110, isolated by enrichment of competitive avocado root tip colonizers, controls avocado white root rot disease caused by Rosellinia necatrix. Here, we applied signature-tagged mutagenesis (STM) during the growth and survival of AVO110 in fungal exudate-containing medium with the goal of identifying molecular mechanisms linked to the interaction of this bacterium with R. necatrix. A total of 26 STM mutants out-competed by the parental strain in fungal exudate, but not in rich medium, were selected and named growth-attenuated mutants (GAM). Twenty-one genes were identified as required for this bacterial-fungal interaction, including membrane transporters, transcriptional regulators and genes related to the metabolism of hydrocarbons, amino acids, fatty acids and aromatic compounds. The bacterial traits identified here that are involved in the colonization of fungal hyphae include proteins involved in membrane maintenance (a dynamin-like protein and ColS) or cyclic-di-GMP signaling and chemotaxis. In addition, genes encoding a DNA helicase (recB) and a regulator of alginate production (algQ) were identified as required for efficient colonization of the avocado rhizosphere. [ABSTRACT FROM AUTHOR]