Molecular genetic analysis of germination and sporulation in two obligate phytopathogens
Baker, Lorina Gale
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Two obligate fungal phytopathogens, Cronartium quercuum f. sp. fusiforme and Ustilago maydis were used to study how obligate pathogens sense and respond to host stimuli. Depending upon the surface where C. q. fusiforme basidiospores alight on, they germinate directly by sending out a long thin germ tube or indirectly by sending out a short thick germ tube and producing a secondary basidiospore. I altered surface hydrophilicities to test if this characteristic affected the germination response of C. q. fusiforme basidiospores. I found that altered surface hydrophilicity did have an affect on the basidiospore germination fate and that there was a critical threshold between 42% and 54% surface wettability where C. q. fusiforme basidiospores switched germination type. To ascertain what genes are involved in the different germination types, two expression libraries enriched for genes expressed during direct or indirect germination were made. The combined expression libraries contained one hundred and eighty unique cDNAs of which fourteen were differentially expressed between the two germination states. Proteins represented by the genes within the expression libraries may be determinants of the basidiospores’ response to a favorable substrate or environment. The second part of my dissertation focuses on how obligate phytopathogens, such as Ustilago maydis, switch from in planta vegetative growth to reproductive growth. I identified three genes encoding putative regulators of G-protein signaling rgs1, rgs2, and rgs3 that could be involved in the switch from vegetative growth to sporulation. Deletion of rgs1 resulted in hyphal growth on minimal medium agar, reduced pathogenicity, and reduced in planta sporulation. I determined that the hyphal growth phenotype was due to the rgs1 strain’s inability to deactivate the G± subunits, Gpa1 and Gpa4. This suggested that both Gpa1 or Gpa4 are negative regulators of in planta sporulation. This was verified using a sporulation time course experiment where I found that the deletion of gpa1 and gpa4 caused hyper-sporulation in planta. The combined data supports a model where multiple pathways control of in planta sporulation and that under the appropriate conditions Rgs1 regulates these pathways.