Fungal and bacterial maize kernel interactions with the vertically transmitted endophytic state of fusarium verticillioides

Abstract

The genus Fusarium consists of 100 or more valid species, although it is anticipated that additional cryptic species will be defined. We are concerned with one species F. verticillioides (Saccardo) Nirenberg, the sexual stage of which is Gibberella moniliformis Wineland. Historically, this fungus was based on the broad concept of Fusarium nomenclature, and the common synonyms F. moniliforme or Gibberella fujikuroi, mating population A, were used. In this review, both synonyms will be used depending upon the author's earlier citations. F. verticillioides under its various synonyms is associated with well over 32 plant families and is known to be seed-borne on at least 10 of these, where it causes seedling blights, root rots, stem rots, and pre-and post-harvest kernel rots. Thus, this species and other Fusarium species are nonhost-specific or host generalists. F. verticillioides, however, is a noted parasite of maize and is found throughout the world wherever maize is grown. Of considerable importance is that this fungus infects maize as a symptomless endophyte, where it produces several homologues of the fumonisin mycotoxins, primarily fumonisin B1 and B2. The fumonisins produce toxicity syndromes in livestock, poultry, and is implicated in several human diseases. Surveys indicate that approximately 95% of the world's isolates are capable of producing this group of mycotoxins apparently resulting from the endophytic infection. In addition to the fumonisins, F. verticillioides produces several other metabolites of varying toxicities to plants and animals. Fusaric acid is one such toxin although it is produced by this and all other fusaria examined. The role of fusaric acid within the association now includes activity as a quorum sensing inhibitor. Quorum sensing is crucial to the proper performance and ecological fitness of most microbial populations, including fungi. Inhibiting this mechanism dooms most organisms to low competitive interspecific and intraspecific competition, while allowing the fusaria to dominate the intercellular spaces of maize. Thus, importance of quorum sensing is briefly reviewed along with the emphasis that it might play in maintaining dominant interactions among other F. verticillioides as a means of control or regulating competing organisms.