| All organisms appear capable of undergoing some form of intrinsically programmed cell suicide. This programmed cell death (PCD), or “apoptosis”, is well characterized in metazoans (animals) and is associated with numerous morphological and biochemical hallmarks. Little is known about PCD in fungi. Thus, numerous fungi were examined for hallmarks of PCD after heterokaryon incompatibility (HI) and treatments with antifungal compounds. HI occurs when conspecific fungi, attempting plasmogamous heterokaryosis, differ at one or more genetic het loci, resulting in the fused cells' compartmentalization and death. The cytology of HI in the fungal model system, Neurospora crassa, was examined using fluorescent DNA stains and terminal deoxynucleotidyl transferase-mediated dU&barbelow;TP-X n&barbelow;ick e&barbelow;nd l&barbelow;abeling (TUNEL). Transformants carrying incompatible combinations of het alleles and hyphal fusions between incompatible vector control transformants all contained heavily degraded nuclear DNA in cells also displaying vacuolated, then shrunken cytoplasms. Transformants carrying compatible het transgenes and self-fusions between control transformants contained little or no degraded DNA and few cells with shrunken morphology. DNA degradation during heterokaryon incompatibility or after various chemical treatments did not occur through oligonucleosomal fragmentation, a critical hallmark of metazoan apoptosis. However, fungi did possess caspase-like protease activity as detected with fluorimetric tetrapeptide substrates and specific inhibitors. In the systems examined, only circumstantial evidence was found for homologous apoptotic pathways in fungi.; In a second project, the etiology and epidemiology a new canker disease of cold stored almond and fruit trees were determined. During storage, trees became covered with molds near and above the graft unions and the inner bark, cambium, and sapwood became necrotic, which in severe cases girdled the trees. Diseased dormant trees planted by growers then collapsed in the field. Facultative fungi, residing endophytically in the bark, could attack almond stems only when predisposed by desiccation stress. Koch's postulates implicated Fusarium avenaceum, F. acuminatum, F. solani and an undescribed Cylindrocarpon sp. as the causal agents. These fungi were marginally pathogenic on healthy, turgid almond stems but highly virulent on almond stems predisposed by desiccation. Also, most isolates of F.avenaceum and F. acuminatum nucleated ice at temperatures greater than −5°C, an ability with possible relevance to disease establishment. |
