Role of autotrophic versus heterotrophic endobacteria in promoting fungal nutrition. Role of autotrophic versus heterotrophic endobacteria in promoting fungal nutrition. (Left) Cartoon illustrating the nutritional relationship between Nostoc cyanobacteria and the lichenous fungus Geosiphon pyriforme during their BFI. Fungal cell structures such as vacuoles, nuclei, lipid bodies, and mitochondria are omitted for clarity. Carbon fixation occurs in photosynthetic cyanobacterial cells, while Nostoc heterocysts are able to fix atmospheric nitrogen (197, 198). Bacteria benefit from micronutrients supplied by the fungus, such as phosphate (356). Other nonphotosynthetic endobacteria living within the bladder may also supply fixed nitrogen to the fungus (355). (Right) Cartoon illustrating the role of Burkholderia rhizoxinica in the nutrition of Rhizopus microcarpus. In contrast to the cyanolichen example, the bacterium is not a primary producer of organic carbon or nitrogen for the fungus (208). 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). P. Frey-Klett et al. Microbiol. Mol. Biol. Rev. 2011; doi:10.1128/MMBR.00020-11