Elicitation of phytoalexin production in cultured carrot cells

A 2-component system is involved in the early stages of the Pisolithus tinctorius-Pinus greggii symbiosis

Ectomycorrhizal symbiosis results in profound morphological and physiological modifications in both plant and fungus. This in turn is the product of differential gene expression in both co-symbionts, giving rise to specialized cell types capable of performing novel functions. During the precolonization stage, chemical signals from root exudates are sensed by the ectomycorrizal fungus, and vice versa, which are in principle responsible for the observed change in the developmental symbionts program. Little is known about the molecular mechanisms involved in the signaling and recognition between ectomycorrhizal fungi and their host plants. In the present work, we characterized a novel lactone, termed pinelactone, and identified a gene encoding for a histidine kinase in Pisolithus tictorius, which function is proposed to be the perception of the aforementioned metabolites. In this study, the use of closantel, a specific inhibitor of histidine kinase phosphorylation, affected the capacity for fungal colonization in the symbiosis between Pisolithus tinctorius and Pinus greggii, indicating that a 2-component system (TCS) may operate in the early events of plant-fungus interaction. Indeed, the metabolites induced the accumulation of Pisolithus tinctorius mRNA for a putative histidine kinase (termed Pthik1). Of note, Pthik1 was able to partially complement a S. cerevisiae histidine kinase mutant, demonstrating its role in the response to the presence of the aforementioned metabolites. Our results indicate a role of a 2-component pathway in the early stages of ectomycorrhizal symbiosis before colonization. Furthermore, a novel lactone from Pinus greggii root exudates may activate a signal transduction pathway that contributes to the establishment of the ectomycorrhizal symbiosis.

The survival of E. coli O157:H7, S. Typhimurium and L. monocytogenes in black carrot (Daucus carota) juice

In this study the survival and growth patterns of Escherichia coli O157:H7, Salmonella Typhimurium and Listeria monocytogenes in various concentrations of black carrot juice were investigated during incubation period at 4 °C and 37 °C for 7 days. Several parameters, such as juice concentration (%), pH, incubation temperature (°C) and time (days) were found effective on the survival of pathogens tested. Although L. monocytogenes has been found to be the less resistant microorganism to the variable conditions, there were only ca. 1 and 2 log reductions in the number of the cells in the juice samples incubated at 4 °C for 2 and 7 days, respectively. Incubating at low temperature (4 °C) enhanced the survival of test microorganisms.

Infection of barley roots by Chaetomium globosum: evidence for a protective role of the exodermis

Plant pathogenesis by fungi is known to be dependent on the host genotype, the virulence of the pathogen and certain environmental conditions influencing fungal establishment. Previously, it has been shown that Chaetomium globosum, a fungus well-characterized for its biocontrol potential, causes necrosis on barley roots grown in Murashige and Skoog (MS)-agar. Using MS-agar and aeroponic culture as axenic plant growth systems, C. globosum pathogenesis was analyzed with serological and histological methods. Irrespective of the growth system, C. globosum infected the root epidermis. Roots grown in MS-agar were extensively colonized intercellularly and intracellularly up to the inner cortex and the tissue underwent necrosis. In contrast, roots grown in aeroponic culture were not colonized beyond the epidermis and the roots appeared healthy. Histochemical analyses revealed that hypodermal suberization stopped fungal invasion. In root tips known to lack suberization, epidermal papilla formation reduced overall infection frequency. The results indicate that specific environmental conditions are important for infection and disease expression in barley roots. Infection is restricted by two spatial and temporal distinct defence mechanisms: (1) papillae in root tips retarding fungal invasion; and (2) suberization of hypodermal cells blocking fungal radial growth.

Derailment product in NADPH-dependent synthesis of a dihydroisocoumarin 6-hydroxymellein by elicitor-treated carrot cell extracts

Synthetic activity of 6-hydroxymellein, the immediate precursor of carrot phytoalexin 6-methoxymellein, from acetyl-CoA and malonyl-CoA was induced in carrot cell extracts when the root disks were treated with CuCl2 or oligogalacturonide elicitor. These elicitors showed specific inducing activity of phytoalexin production and did not affect fatty acid synthesis in carrot tissues which may share some common properties with 6-hydroxymellein biosynthesis. 6-Hydroxymellein production was an NADPH-dependent process and, in the absence of the reagent, triacetic acid lactone was produced as a derailment product of the reaction process. This finding suggested that the reduction of the double bond at the 3,4-position of the phytoalexin takes place during the elongation of the poly(oxomethylene) chain. This NADPH-dependent reduction seems to occur at the triacetate stage before the condensation of the third malonyl-CoA as the conversion of carbonyl to hydroxyl group.