The phytohormones, abscisic acid and cytokinin, once were thought to be present uniquely in plants, but increasing evidence suggests that these hormones are present in a wide variety of organisms. Few studies have examined fungi for the presence of these "plant" hormones or addressed whether their levels differ based on the nutrition mode of the fungus. This study examined 20 temperate forest fungi of differing nutritional modes (ectomycorrhizal, wood-rotting, saprotrophic). Abscisic acid and cytokinin were present in all fungi sampled; this indicated that the sampled fungi have the capacity to synthesize these two classes of phytohormones. Of the 27 cytokinins analyzed by HPLC-ESI MS/MS, seven were present in all fungi sampled. This suggested the existence of a common cytokinin metabolic pathway in fungi that does not vary among different nutritional modes. Predictions regarding the source of isopentenyl, cis-zeatin and methylthiol CK production stemming from the tRNA degradation pathway among fungi are discussed.

Ustilago maydis is the causative agent of common smut of corn. Early studies noted its ability to synthesize phytohormones and, more recently these growth promoting substances were confirmed as cytokinins (CKs). Cytokinins comprise a group of phytohormones commonly associated with actively dividing tissues. Lab analyses identified variation in virulence between U. maydis dikaryon and solopathogen infections of corn cob tissue. Samples from infected cob tissue were taken at sequential time points post infection and biochemical profiling was performed using high performance liquid chromatography-electrospray ionization tandem mass spectrometry (HPLC-ESI MS/MS). This hormone profiling revealed that there were altered levels of ABA and major CKs, with a marked reduction in CK glucosides, increases in methylthiol CKs and a particularly dramatic increase in cisZ CK forms, in U. maydis infected tissue. These changes were more pronounced in the more virulent dikaryon relative to the solopathogenic strain suggesting a role for cytokinins in moderating virulence during biotrophic infection. These findings highlight the fact that U. maydis does not simply mimic a fertilized seed but instead reprograms the host tissue. Results underscore the suitability of the Ustilago maydis– Zea mays model as a basis for investigating the control of phytohormone dynamics during biotrophic infection of plants.

The released genome sequence of Ustilago maydis provided immense insight into this pathogen's genetic structure; however, thorough annotation of the genome requires data from many sources. Information from 4425 expressed sequence tags from the filamentous dikaryon provided an excellent resource for genome annotation. This allowed confirmation and correction of gene models, as well as the documentation of transcript structural features. The depth of coverage provided by the normalized dikaryon cDNA library contributed to the discovery of new candidate pathogenesis genes and enabled the identification of U. maydis antisense and noncoding RNAs. Candidate pathogenesis genes were identified based on their representation in the dikaryon library only or in the dikaryon and diploid cDNA libraries, followed by comparative analysis with the genome sequences of other plant pathogens. Six genes that were conserved only among pathogenic fungi and six genes unique to U. maydis were confirmed to be expressed in planta using reverse-transcriptase PCR. The transcript level of three of these genes varied during the transition from filamentous dikaryotic growth to the formation of the teliospore. This discovery provides representative genes that can be used to begin dissecting the control of gene expression during this transition in pathogenic development. Many of the newly identified U. maydis noncoding RNAs were differentially represented among cDNA libraries, suggesting potential functional roles in different U. maydis cell types. The deletion of one such ncRNA in a solopathogenic strain reduced virulence, revealing a new type of pathogenesis gene in fungi.