Identification of Candidate Ice Nucleation Activity (INA) Genes in Fusarium avenaceum by Combining Phenotypic Characterization with Comparative Genomics and Transcriptomics

Ice nucleation activity (INA) is the capacity of certain particles to catalyze ice formation at temperatures higher than the temperature at which pure water freezes. INA impacts the ratio of liquid to frozen cloud droplets and, therefore, the formation of precipitation and Earth's radiative balance. Some Fusarium strains secrete ice-nucleating particles (INPs); they travel through the atmosphere and may thus contribute to these atmospheric processes. Fusarium INPs were previously found to consist of proteinaceous aggregates. Here, we determined that in F. avenaceum, the proteins forming these aggregates are smaller than 5 nm and INA is higher after growth at low temperatures and varies among strains. Leveraging these findings, we used comparative genomics and transcriptomics to identify candidate INA genes. Ten candidate INA genes that were predicted to encode secreted proteins were present only in the strains that produced the highest number of INPs. In total, 203 candidate INA genes coding for secreted proteins were induced at low temperatures. Among them, two genes predicted to encode hydrophobins stood out because hydrophobins are small, secreted proteins that form aggregates with amphipathic properties. We discuss the potential of the candidate genes to encode INA proteins and the next steps necessary to identify the molecular basis of INA in F. avenaceum.

Automated summary

Ice nucleation activity (INA) is the ability of certain particles to catalyze ice formation at temperatures higher than the freezing point of pure water. This study found that Fusarium strains secrete ice-nucleating particles (INPs) which consist of proteinaceous aggregates smaller than 5 nm. INA in these strains is higher after growth at low temperatures and varies among different strains. Comparative genomics and transcriptomics were used to identify candidate INA genes, with ten predicted to encode secreted proteins only present in strains that produce the highest number of INPs. A total of 203 candidate INA genes coding for secreted proteins were induced at low temperatures, including two predicted to encode hydrophobins, small secreted proteins with amphipathic properties. The potential of these candidate genes to encode INA proteins and the next steps for identifying the molecular basis of INA in F. avenaceum are discussed.