The Role of Plant Latex in Virus Biology.

At least 20,000 plant species produce latex, a capacity that appears to have evolved independently on numerous occasions. With a few exceptions, latex is stored under pressure in specialized cells known as laticifers and is exuded upon injury, leading to the assumption that it has a role in securing the plant after mechanical injury. In addition, a defensive effect against insect herbivores and fungal infections has been well established. Latex also appears to have effects on viruses, and laticifers are a hostile environment for virus colonization. Only one example of successful colonization has been reported: papaya meleira virus (PMeV) and papaya meleira virus 2 (PMeV2) in Carica papaya. In this review, a summary of studies that support both the pro- and anti-viral effects of plant latex compounds is provided. The latex components represent a promising natural source for the discovery of new pro- and anti-viral molecules in the fields of agriculture and medicine.

Carica papaya microRNAs are responsive to Papaya meleira virus infection.

MicroRNAs are implicated in the response to biotic stresses. Papaya meleira virus (PMeV) is the causal agent of sticky disease, a commercially important pathology in papaya for which there are currently no resistant varieties. PMeV has a number of unusual features, such as residence in the laticifers of infected plants, and the response of the papaya to PMeV infection is not well understood. The protein levels of 20S proteasome subunits increase during PMeV infection, suggesting that proteolysis could be an important aspect of the plant defense response mechanism. To date, 10,598 plant microRNAs have been identified in the Plant miRNAs Database, but only two, miR162 and miR403, are from papaya. In this study, known plant microRNA sequences were used to search for potential microRNAs in the papaya genome. A total of 462 microRNAs, representing 72 microRNA families, were identified. The expression of 11 microRNAs, whose targets are involved in 20S and 26S proteasomal degradation and in other stress response pathways, was compared by real-time PCR in healthy and infected papaya leaf tissue. We found that the expression of miRNAs involved in proteasomal degradation increased in response to very low levels of PMeV titre and decreased as the viral titre increased. In contrast, miRNAs implicated in the plant response to biotic stress decreased their expression at very low level of PMeV and increased at high PMeV levels. Corroborating with this results, analysed target genes for this miRNAs had their expression modulated in a dependent manner. This study represents a comprehensive identification of conserved miRNAs inpapaya. The data presented here might help to complement the available molecular and genomic tools for the study of papaya. The differential expression of some miRNAs and identifying their target genes will be helpful for understanding the regulation and interaction of PMeV and papaya.

Comparison of biofilm and attachment mechanisms of a phytopathological and clinical isolate of Klebsiella pneumoniae Subsp. pneumoniae.

Some bacterial species can colonize humans and plants. It is almost impossible to prevent the contact of clinically pathogenic bacteria with food crops, and if they can persist there, they can reenter the human food chain and cause disease. On the leaf surface, microorganisms are exposed to a number of stress factors. It is unclear how they survive in such different environments. By increasing adhesion to diverse substrates, minimizing environmental differences, and providing protection against defence mechanisms, biofilms could provide part of the answer. Klebsiella pneumoniae subsp. pneumoniae is clinically important and also associated with fruit diseases, such as "pineapple fruit collapse." We aimed to characterize biofilm formation and adhesion mechanisms of this species isolated from pineapple in comparison with a clinical isolate. No differences were found between the two isolates quantitatively or qualitatively. Both tested positive for capsule formation and were hydrophobic, but neither produced adherence fibres, which might account for their relatively weak adhesion compared to the positive control Staphylococcus epidermidis ATCC 35984. Both produced biofilms on glass and polystyrene, more consistently at 40°C than 35°C, confirmed by atomic force and high-vacuum scanning electron microscopy. Biofilm formation was maintained in an acidic environment, which may be relevant phytopathologically.

Influence of cellular fatty acid composition on the response of Saccharomyces cerevisiae to hydrostatic pressure stress.

High hydrostatic pressure (HHP) interferes with cellular membrane structure. The orientation of lipid molecules is changed, especially in the vicinity of proteins, leading to decreased membrane fluidity. Adaptation to HHP requires increased membrane fluidity, often achieved through a rise in the proportion of unsaturated fatty acids. In this work, a desaturase-deficient Saccharomyces cerevisiae mutant strain (OLE1 gene deletion) was grown in media supplemented with fatty acids differing in size and number of unsaturations and submitted to pressure up to 200 MPa for 30 min. Desaturase-deficient yeast supplemented with palmitoleic acid demonstrated increased sensitivity to pressure compared to cells supplemented with oleic acid or a proportionate mixture of both acids. In contrast, yeast cells grown with linoleic and linolenic acids were more piezoresistant than cells treated with oleic acid. Furthermore, growth with palmitoleic acid led to higher levels of lipid peroxidation. Intracellular trehalose during HHP treatment increased cell tolerance to pressure. However, when trehalose remained extracellular cells were sensitised to pressure. Therefore, fatty acid composition and trehalose content might play a role in the protection of the cell membrane from oxidative damage produced by HHP, confirming that alteration in cell membrane fluidity is correlated with pressure resistance in yeast.

Molecular diagnosis of Papaya meleira virus (PMeV) from leaf samples of Carica papaya L. using conventional and real-time RT-PCR.

Papaya meleira virus (PMeV) is the causal agent of papaya sticky disease. This study describes two methods for molecular diagnosis of PMeV using conventional and real-time PCR. These methods were shown to be more efficient than current methods of viral detection using extraction of PMeV dsRNA and observation of symptoms in the field. The methods described here were used to evaluate the effect of inoculation of papaya plants with purified PMeV dsRNA on the progress of PMeV infection. A single inoculation with PMeV dsRNA was observed to delay the progress of the virus infection by several weeks. The possibility of vertical transmission of PMeV was also investigated. No evidence was found for PMeV transmission through seeds collected from diseased fruit. The implications of these results for the epidemiology of PMeV and the management of papaya sticky disease are discussed.

Candida krusei and Kloeckera apis inhibit the causal agent of pineapple fusariosis, Fusarium guttiforme.

Studies based on microbial ecology and antagonistic interactions play an important role in the development of new alternative strategies in controlling plant pathogens and are relevant to further biotechnological applications. Antagonistic interactions between the yeasts Candida krusei and Kloeckera apis isolated from rotten pineapple fruits, and two isolates of the pathogenic filamentous fungus Fusarium guttiforme (Syn.: Fusarium subglutinans f. sp. ananas) resistant and susceptible to fungicide benzimidazole were studied in broth culture, and on plate assays. The yeasts significantly reduced Fusarium conidial germination after 24h of cocultivation in broth culture, and also mycelial growth on plate assays. Slide coculture appeared to show attachment of yeasts to the hyphal surface and also slight morphological abnormalities caused by C. krusei. Filtrates of cocultures of fungi and yeasts inhibited fungal growth, but filtrates of the yeast cultures alone did not, suggesting that the antagonistic action of the yeasts is inducible. The F. guttiforme isolate sensitive to benzimidazole was most affected by both yeasts in pineapple juice, reaching a maximum of 36.5 % germ tube inhibition. This isolate was also inhibited by yeasts in mycocinogenic plate assay. These results demonstrated that C. krusei and K. apis are effective in inhibiting F. guttiforme growth and that the mode of action is associated with hyperparasitism and mycocinogenic activity.