Triggers and targets of cAMP signalling

Transcriptome analysis reveals new insight into appressorium formation and function in the rice blast fungus Magnaporthe oryzae

BACKGROUND

Rice blast disease is caused by the filamentous Ascomycetous fungus Magnaporthe oryzae and results in significant annual rice yield losses worldwide. Infection by this and many other fungal plant pathogens requires the development of a specialized infection cell called an appressorium. The molecular processes regulating appressorium formation are incompletely understood.

RESULTS

We analyzed genome-wide gene expression changes during spore germination and appressorium formation on a hydrophobic surface compared to induction by cAMP. During spore germination, 2,154 (approximately 21%) genes showed differential expression, with the majority being up-regulated. During appressorium formation, 357 genes were differentially expressed in response to both stimuli. These genes, which we refer to as appressorium consensus genes, were functionally grouped into Gene Ontology categories. Overall, we found a significant decrease in expression of genes involved in protein synthesis. Conversely, expression of genes associated with protein and amino acid degradation, lipid metabolism, secondary metabolism and cellular transportation exhibited a dramatic increase. We functionally characterized several differentially regulated genes, including a subtilisin protease (SPM1) and a NAD specific glutamate dehydrogenase (Mgd1), by targeted gene disruption. These studies revealed hitherto unknown findings that protein degradation and amino acid metabolism are essential for appressorium formation and subsequent infection.

CONCLUSION

We present the first comprehensive genome-wide transcript profile study and functional analysis of infection structure formation by a fungal plant pathogen. Our data provide novel insight into the underlying molecular mechanisms that will directly benefit efforts to identify fungal pathogenicity factors and aid the development of new disease management strategies.

cAMP blocks MAPK activation and sclerotial development via Rap-1 in a PKA-independent manner in Sclerotinia sclerotiorum

Sclerotinia sclerotiorum is a filamentous ascomycete phytopathogen able to infect an extremely wide range of cultivated plants. Our previous studies have shown that increases in cAMP levels result in the impairment of the development of the sclerotium, a highly differentiated structure important in the disease cycle of this fungus. cAMP also inhibits the activation of a S. sclerotiorum mitogen-activated protein kinase (MAPK), which we have previously shown to be required for sclerotial maturation; thus cAMP-mediated sclerotial inhibition is modulated through MAPK. However, the mechanism(s) by which cAMP inhibits MAPK remains unclear. Here we demonstrate that a protein kinase A (PKA)-independent signalling pathway probably mediates MAPK inhibition by cAMP. Expression of a dominant negative form of Ras, an upstream activator of the MAPK pathway, also inhibited sclerotial development and MAPK activation, suggesting that a conserved Ras/MAPK pathway is required for sclerotial development. Evidence from bacterial toxins that specifically inhibit the activity of small GTPases, suggested that Rap-1 or Ras is involved in cAMP action. The Rap-1 inhibitor, GGTI-298, restored MAPK activation in the presence of cAMP, further suggesting that Rap-1 is responsible for cAMP-dependent MAPK inhibition. Importantly, inhibition of Rap-1 is able to restore sclerotial development blocked by cAMP. Our results suggest a novel mechanism involving the requirement of Ras/MAPK pathway for sclerotial development that is negatively regulated by a PKA-independent cAMP signalling pathway. Cross-talk between these two pathways is mediated by Rap-1.

MAPK regulation of sclerotial development in Sclerotinia sclerotiorum is linked with pH and cAMP sensing

Sclerotial development is fundamental to the disease cycle of the omnivorous broad host range fungal phytopathogen Sclerotinia sclerotiorum. We have isolated a highly conserved homolog of ERK-type mitogen-activated protein kinases (MAPKs) from S. sclerotiorum (Smk1) and have demonstrated that Smk1 is required for sclerotial development. The smk1 transcription and MAPK enzyme activity are induced dramatically during sclerotiogenesis, especially during the production of sclerotial initials. When PD98059 (a specific inhibitor of the activation of MAPK by MAPK kinase) was applied to differentiating cultures or when antisense expression of smk1 was induced, sclerotial maturation was impaired. The smk1 transcript levels were highest under acidic pH conditions, suggesting that Smk1 regulates sclerotial development via a pH-dependent signaling pathway, involving the accumulation of oxalic acid, a previously identified pathogenicity factor that functions at least in part by reducing pH. Addition of cyclic AMP (cAMP) inhibited smk1 transcription, MAPK activation, and sclerotial development. Thus, S. sclerotiorum can coordinate environmental signals (such as pH) to trigger a signaling pathway mediated by Smk1 to induce sclerotia formation, and this pathway is negatively regulated by cAMP.

Of smuts, blasts, mildews, and blights: cAMP signaling in phytopathogenic fungi

cAMP regulates morphogenesis and virulence in a wide variety of fungi including the plant pathogens. In saprophytic yeasts such as Saccharomyces cerevisiae, cAMP signaling plays an important role in nutrient sensing. In filamentous saprophytes, the cAMP pathway appears to play an integral role in vegetative growth and sporulation, with possible connections to mating. Infection-related morphogenesis includes sporulation (conidia and teliospores), formation of appressoria, infection hyphae, and sclerotia. Here, we review studies of cAMP signaling in a variety of plant fungal pathogens. The primary fungi to be considered include Ustilago maydis, Magnaporthe grisea, Cryphonectria parasitica, Colletotrichum and Fusarium species, and Erisyphe graminis. We also include related information on Trichoderma species that act as mycoparasites and biocontrol agents of phytopathogenic fungi. We point out similarities in infection mechanisms, conservation of signaling components, as well as instances of cross-talk with other signaling pathways.