Relationship between secondary metabolism and fungal development

Abstract

Filamentous fungi are unique organisms-rivaled only by actinomycetes and plants-in producing a wide range of natural products called secondary metabolites. These compounds are very diverse in structure and perform functions that are not always known. However, most secondary metabolites are produced after the fungus has completed its initial growth phase and is beginning a stage of development represented by the formation of spores. In this review, we describe secondary metabolites produced by fungi that act as sporogenic factors to influence fungal development, are required for spore viability, or are produced at a time in the life cycle that coincides with development. We describe environmental and genetic factors that can influence the production of secondary metabolites. In the case of the filamentous fungus Aspergillus nidulans, we review the only described work that genetically links the sporulation of this fungus to the production of the mycotoxin sterigmatocystin through a shared G-protein signaling pathway.

FIG. 1.

Sporogenic effect of linoleic acid-derived compounds on Aspergillus. Conidia of A. flavus were spread evenly over YGT medium (26) at a concentration of 10⁶ spores/ml. Discs containing 1 mg of 13S-hydroperoxylinoleic acid (L) or solvent control (R) were placed on the surface, and the plate was incubated at 29°C for 2 days. Conidium production is induced by 13S-hydroperoxylinoleic acid (24).

FIG. 2.

Structures of linoleic acid and linoleic acid-derived molecules from seed-, 13S-hydroperoxylinoleic acid (13S-HPODE)- and 9S-HPODE-, and linoleic acid-derived sporogenic molecules from A. nidulans, psiA, psiB, and psiC.

FIG. 3.

(A) Phenotype effect of signaling mutants. Strains shown are wild-type A. nidulans (a), ΔfluG (b), ΔflbA (c), fadA^G42R (d), and ΔpkaA (e). (B) Proposed model of G-protein signal transduction regulating sterigmatocystin production and sporulation. Arrows indicate positive regulation, and blocked arrows indicate negative regulation. Solid lines indicate known pathways. Dashed lines indicate postulated pathways. Abbreviations: FluG, early acting developmental regulator; FlbA, regulator of G-protein signaling; FadA, α subunit of G-protein; PkaA, catalytic subunit of PKA; LaeA, AflR regulator; AflR, sterigmatocystin-aflatoxin-specific transcription factor; BrlA, conidiation-specific transcription factor (55, 106).

FIG. 4.

Polyamine biosynthetic pathway.