The Streptomyces scabiei Pathogenicity Factor Thaxtomin A Induces the Production of Phenolic Compounds in Potato Tubers.

The phytotoxin thaxtomin A (TA) is the key pathogenicity factor synthesized by the bacteria Streptomyces scabiei, the main causal agent of common scab of potato (Solanum tuberosum L.). TA treatment of potato tuber flesh produces a brown color that was attributed to necrosis. The intensity of TA-induced browning was generally thought to correlate with potato sensitivity to the disease. In this study, we found that TA-induced browning was much more intense in the potato tuber flesh of the common scab moderately resistant variety Russet Burbank (RB) than that observed in tubers of the disease-susceptible variety Yukon Gold (YG). However, there was no significant difference in the level of TA-induced cell death detected in both varieties, suggesting that tubers response to TA does not correlate with the level of sensitivity to common scab. TA-treated potato tuber tissues accumulated significantly higher levels of phenolic compounds than untreated controls, with a higher phenol content detected in RB TA-treated tissues than in those of YG. Browning was associated with a significant induction of the expression of genes of the phenylpropanoid pathway in RB tubers, indicating that TA activated this metabolic pathway. These results suggest that tuber flesh browning induced by TA is due to the accumulation of phenolic compounds. These phenolics may play a role in the protection of potato tubers against S. scabiei.

Habituation to thaxtomin A increases resistance to common scab in 'Russet Burbank' potato.

Common scab is a potato disease characterized by the formation of scab-like lesions on the surface of potato tubers. The actinobacterium Streptomyces scabiei is the main causal agent of common scab. During infection, this bacterium synthesizes the phytotoxin thaxtomin A which is essential for the production of disease symptoms. While thaxtomin A can activate an atypical programmed cell death in plant cell suspensions, it is possible to gradually habituate plant cells to thaxtomin A to provide resistance to lethal phytotoxin concentrations. Potato 'Russet Burbank' calli were habituated to thaxtomin A to regenerate the somaclone RB9 that produced tubers more resistant to common scab than those obtained from the original cultivar. Compared to the Russet Burbank cultivar, somaclone RB9 generated up to 22% more marketable tubers with an infected tuber area below the 5% threshold. Enhanced resistance was maintained over at least two years of cultivation in the field. However, average size of tubers was significantly reduced in somaclone RB9 compared to the parent cultivar. Small RB9 tubers had a thicker phellem than Russet Burbank tubers, which may contribute to improving resistance to common scab. These results show that thaxtomin A-habituation in potato is efficient to produce somaclones with increased and durable resistance to common scab.

Transfection efficiencies of PAMAM dendrimers correlate inversely with their hydrophobicity.

Dendriplexes were characterized by ethidium bromide intercalation assay and their transfection efficiency was studied using HEK 293 cells and human mesenchymal stem cells. PAMAM G4 showed a higher transfection efficiency than PAMAM G3-G6, G4-OH, G4-25% or G4-50% dendrimers. Substitution of OH groups for the NH(2) surface groups rendered the dendrimer unable to form dendriplexes and to transfect cells. Partial (25%) substitution of CH(3) groups for the NH(2) groups markedly impaired transfection; 50% substitution decreased the ability of PAMAM G4 to transfect threefold. It was concluded that increased hydrophobicity decreased the ability of dendrimers to transfect. PAMAM G4-50% is highly hydrophobic and forms micelles in solution, which can transfect pGFP. The results of ethidium bromide intercalation assays, ANS fluorescence studies and transfection efficiencies of PAMAM dendrimers were correlated. Subsequently, we constructed a neurotrophin-encoding plasmid and studied its delivery to mesenchymal stem cells using PAMAM G4 dendrimer and Lipofectamine 2000. Lipofectamine 2000 was a more effective carrier (18.5%) than PAMAM G4 dendrimer (1.2%).