The Sclerotinia sclerotiorum pac1 gene is required for sclerotial development and virulence

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.

The path in fungal plant pathogenicity: many opportunities to outwit the intruders?

The number of genes implicated in the infection and disease processes of phytopathogenic fungi is increasing rapidly. Forward genetic approaches have identified mutated genes that affect pathogenicity, host range, virulence and general fitness. Likewise, candidate gene approaches have been used to identify genes of interest based on homology and recently through 'comparative genomic approaches' through analysis of large EST databases and whole genome sequences. It is becoming clear that many genes of the fungal genome will be involved in the pathogen-host interaction in its broadest sense, affecting pathogenicity and the disease process in planta. By utilizing the information obtained through these studies, plants may be bred or engineered for effective disease resistance. That is, by trying to disable pathogens by hitting them where it counts.

Pathogenic fungi: leading or led by ambient pH?

SUMMARY Pathogenic fungi have successfully attacked a wide range of hosts, which has forced them into ambient-adaptation. pH is one of the major ambient traits affecting the activity of pathogenicity factors secreted by the pathogen, hence, a pH sensing-response system was developed to enable the pathogen to tailor its arsenal to best fit its host. The pacC palA, B, C, F, H and I apparatus was first identified in Aspergillus nidulans and later found in other fungi. Secreted pathogenicity factors, such as cell wall degrading enzymes, were recognized to be controlled by environmental pH and later shown to be regulated by the pH regulatory system, either directly or by harbouring the pacC consensus sequence. The ability of the pathogen to actively increase or decrease its surrounding pH allows it to select the specific virulence factor, out of its vast arsenal, to best fit a particular host.