Oxylipins act as quorum sensing signals and cell density regulators in Aspergillus spp

Aspergillus flavus differentiates to produce asexual conidia or overwintering survival structures called sclerotia. Results here show both processes are oppositely regulated by density dependent mechanisms where increasing cell density (from 10 super(1) to 10 super(7) cells) yields lowest sclerotial and highest conidial numbers. Production of most secondary metabolites, including aflatoxin, follows the same trend as sclerotial production. These density dependent phenomena can be affected by lipid availability. Addition of linoleic acid increased sclerotial formation in intermediate cell densities (10 super(4) to 10 super(5)). Furthermore, disruption of a putative oxylipin generating gene, Aflox encoding a lipoxygenase, greatly diminished density dependent development of both sclerotia and conidia with an overall increase in sclerotia and decrease in conidia at high cell densities (>10 super(5) cells). This is reminiscent of studies in tne model fungus A. nidulans where cleistothecial-conidial balance is governed by oxylipins (oxygenated unsaturated fatty acids). In addition to Aflox, analysis of the A. flavus genome has led to the identification four other oxylipin generating oxygenases (ppoA-D). Transcriptional profiling of ppo and Aflox shows each has a distinct expression pattern during A. flavus development as might be predicted for proteins generating signaling molecules. These accumulating data supported a hypothesis that lipid pools, particularly oxylipins, may be involved in quorum driven cell density transitions in Aspergillus.