Endosymbiont-Dependent Host Reproduction Maintains Bacterial-Fungal Mutualism

Abstract

Bacterial endosymbionts play essential roles for many organisms, and thus specialized mechanisms have evolved during evolution that guarantee the persistence of the symbiosis during or after host reproduction. The rice seedling blight fungus Rhizopus microsporus represents a unique example of a mutualistic life form in which a fungus harbors endobacteria (Burkholderia sp.) for the production of a phytotoxin. Here we report the unexpected observation that in the absence of endosymbionts, the host is not capable of vegetative reproduction. Formation of sporangia and spores is restored only upon reintroduction of endobacteria. To monitor this process, we succeeded in GFP labeling cultured endosymbionts. We also established a laserbeam transformation technique for the first controlled introduction of bacteria into fungi to observe their migration to the tips of the aseptate hyphae. The persistence of this fungal-bacterial mutualism through symbiont-dependent sporulation is intriguing from an evolutionary point of view and implies that the symbiont produces factors that are essential for the fungal life cycle. Reproduction of the host has become totally dependent on endofungal bacteria, which in return provide a highly potent toxin for defending the habitat and accessing nutrients from decaying plants. This scenario clearly highlights the significance for a controlled maintenance of this fungal-bacterial symbiotic relationship.

Endofungal bacterium controls its host by an hrp type III secretion system

“…This means that external selecting pressures acting on the symbiosis rather than intrinsic control mechanisms of the host have prevented B. rhizoxinica from turning into a parasitic symbiont by losing its biosynthetic capability. Considering that diffusible low-molecular-weight metabolites do not seem to mediate sporulation (Partida-Martinez et al, 2007b), our results are in agreement with a model where the sporulation trigger itself is secreted by the type III apparatus. We propose that the endosymbionts have acquired or mimic a host factor that is necessary to launch its vegetative reproduction program.…”

“…Quite unexpectedly, we observed that the host is not capable of vegetative reproduction in the absence of endosymbionts. However, after reinfection of the fungal host, we observed endobacteria migrate into sporangia, enter vegetative spores of Rhizopus and survive therein until germination (Partida-Martinez et al, 2007b). This unique symbiont-dependent sporulation is an elegant way to prevent formation of symbiont-free spores, thus securing the persistence of the symbiosis.…”

“…Pure cultures of B. rhizoxinica were grown in MGY medium (M9 minimal medium supplemented with 1.25 g l À1 yeast extract and 10 g l À1 gycerol) at 30 1C. Isolation of endobacteria (Partida-Martinez and Hertweck, 2005), preparation of electrocompenent cells and curation of the fungus from endosymbionts (Partida-Martinez et al, 2007b) were conducted as described before. Wild type and cured strains of R. microsporus were grown on potato dextrose agar at 30 1C.…”

Bacterial-Fungal Interactions: Hyphens between Agricultural, Clinical, Environmental, and Food Microbiologists

“…However, the bacterial biosynthesis of the toxin rhizoxin is crucial for fungal pathogenicity toward rice seedlings and therefore to the fungal exploitation of plant-derived carbon and nitrogen (298). R. microcarpus also requires B. rhizoxinica for vegetative reproduction (299).…”

“…Bacteria stimulate spore germination in several fungi, including the plant-pathogenic oomycete Phytophthora alni (68), the saprophytic cheese-associated fungus Penicillium roqueforti (152), several bark beetle fungal symbionts (1), and the arbuscular mycorrhizal fungus Glomus intraradices Sy167 (161,162). Interestingly, antibiotic treatment to "cure" Rhizopus of the Burkholderia endobacteria living within its hyphae results in a fungus that no longer produces reproductive sporangia or spores (299). This is believed to be a mechanism that ensures the presence of bacteria within fungal spores during vegetative reproduction, guaranteeing the vertical transmission of the bacteria (299), and thus is likely to have been essential for spreading the symbiosis globally (205).…”