Notes on Rotuman Grammar.

n/

A role for jasmonates in the release of dormancy by cold stratification in wheat

Hydration at low temperatures, commonly referred to as cold stratification, is widely used for releasing dormancy and triggering germination in a wide range of species including wheat. However, the molecular mechanism that underlies its effect on germination has largely remained unknown. Our previous studies showed that methyl-jasmonate, a derivative of jasmonic acid (JA), promotes dormancy release in wheat. In this study, we found that cold-stimulated germination of dormant grains correlated with a transient increase in JA content and expression of JA biosynthesis genes in the dormant embryos after transfer to 20 (o)C. The induction of JA production was dependent on the extent of cold imbibition and precedes germination. Blocking JA biosynthesis with acetylsalicylic acid (ASA) inhibited the cold-stimulated germination in a dose-dependent manner. In addition, we have explored the relationship between JA and abscisic acid (ABA), a well-known dormancy promoter, in cold regulation of dormancy. We found an inverse relationship between JA and ABA content in dormant wheat embryos following stratification. ABA content decreased rapidly in response to stratification, and the decrease was reversed by addition of ASA. Our results indicate that the action of JA on cold-stratified grains is mediated by suppression of two key ABA biosynthesis genes, TaNCED1 and TaNCED2.This project was funded by a CSIRO Office of the Chief Executive PDF scheme

Fungicide resistance and parasexual recombination in pseudocercosporella herpotrichoides

A detailed investigation of benzimidazole resistance in field isolates and laboratory mutants of Pseudocercosporella herpotrichoides resulted in the description of four major resistance catagories, based on the sensitivity of isolates to carbendazim: high-level (MIC - >1000 pM), intermediate-level (MIC - 50 - 200 pM), low-level (MIC - 10 - 20 pM) and sensitive (MIC - 0.6 - 2.5 pM). Cross-resistance to other benzimidazole compounds, including thiabendazole, was observed in all cases, although the level of resistance to thiabendazole was not directly related to the level of resistance to carbendazim. Strains from each resistance catagory were tested for sensitivity to two experimental phenylcarbamate fungicides, MOPC and S32165. Wild-type carbendazim-sensitive isolates were insensitive to both compounds, as were most low-level and intermediate-level carbendazim-resistant mutants. Many high-level carbendazim-resistant strains, including most of the carbendazimresistant field isolates showed increased sensitivity to one or both phenylcarbamates. In addition, a fifth resistance phenotype was identified, showing increased sensitivity to MOPC and S32165 but associated with only slightly reduced benzimidazole sensitivity. Spontaneous mutants from a high-level carbendazim-resistant strain, selected for reversion to insensitivity to Mope were readily obtained. Analysis of their fungicide response showed some to be back-mutations in which insensitivity to MOPC was associated with the loss of resistance to carbendazim, While others appeared to carry suppressor or modifier mutations producing a range of resistance phenotypes. A study of sensitivity to a range of ergosterol biosynthesis inhibiting (EBI) fungicides revealed considerable variation between different isolates. In general, BWR-type isolates were much less sensitive to the triazole compounds tested than BWtypes, while the reverse relationship was found for the morpholine fungicide, fenpropimorph. Sensitivity to prochloraz showed much less variation. No correlation was found between benzimidazole resistance and reduced sensitivity to the EBI fungicides. Two pathotypes of ~ herpotrichoides, BW and BWR, usually distinguished on the basis of cultural morphology, are recognised. The morphological character, however, was found to be unstable, particularly in BWR-types, which readily sectored to give colonies typical of the other pathotype. Parasexual recombination in P. herpotrichoides was demonstrated using auxotrophic mutants isolated after UVirradiation of conidia. Unstable heterokaryons were obtained following hyphal anastomosis between complementary strains. From these heterokaryons stable prototrophs were recovered both from spores and isolated protop1asts. Genetic evidence and DNA estimations indicated that these stable prototrophs were dip1oids. Recombinant types were isolated from diploids after treatment with 5-fluorouracil. It was also shown that viable heterokaryons could be recovered following PEG-mediated fusion of protop1asts isolated from complementary strains. Heterokaryon incompatibility between unrelated field isolates was tested using auxotrophic mutants obtained by selection for resistance to chlorate and selenate. Vegetative incompatibility appeared to be widespread, few positive interactions being observed, suggesting that the potential for parasexual gene exchange in the field is low. Protoplast fusion may provide a method whereby this incompatibility can be overcome in the laboratory

Freeze avoidance: a dehydrating moss gathers no ice

Using cryo-SEM with EDX fundamental structural and mechanical properties of the moss Ceratodon purpureus (Hedw.) Brid. were studied in relation to tolerance of freezing temperatures. In contrast to more complex plants, no ice accumulated within the moss during the freezing event. External ice induced desiccation with the response being a function of cell type; water-filled hydroid cells cavitated and were embolized at -4 ºC while parenchyma cells of the inner cortex exhibited cytorrhysis, decreasing to ~20% of their original volume at a nadir temperature of -20 ºC. Chlorophyll fluorescence showed that these winter acclimated mosses displayed no evidence of damage after thawing from -20 ºC while GCMS showed that sugar concentrations were not sufficient to confer this level of freezing tolerance. In addition, differential scanning calorimetry showed internal ice nucleation occurred in hydrated moss at ~ -12 ºC while desiccated moss showed no evidence of freezing with lowering of nadir temperature to -20 ºC. Therefore the rapid dehydration of the moss provides an elegantly simple solution to the problem of freezing; remove that which freezes

Fungicide resistance and parasexual recombination in pseudocercosporella herpotrichoides

A detailed investigation of benzimidazole resistance in field isolates and laboratory mutants of Pseudocercosporella herpotrichoides resulted in the description of four major resistance catagories, based on the sensitivity of isolates to carbendazim: high-level (MIC - >1000 pM), intermediate-level (MIC - 50 - 200 pM), low-level (MIC - 10 - 20 pM) and sensitive (MIC - 0.6 - 2.5 pM). Cross-resistance to other benzimidazole compounds, including thiabendazole, was observed in all cases, although the level of resistance to thiabendazole was not directly related to the level of resistance to carbendazim. Strains from each resistance catagory were tested for sensitivity to two experimental phenylcarbamate fungicides, MOPC and S32165. Wild-type carbendazim-sensitive isolates were insensitive to both compounds, as were most low-level and intermediate-level carbendazim-resistant mutants. Many high-level carbendazim-resistant strains, including most of the carbendazimresistant field isolates showed increased sensitivity to one or both phenylcarbamates. In addition, a fifth resistance phenotype was identified, showing increased sensitivity to MOPC and S32165 but associated with only slightly reduced benzimidazole sensitivity. Spontaneous mutants from a high-level carbendazim-resistant strain, selected for reversion to insensitivity to Mope were readily obtained. Analysis of their fungicide response showed some to be back-mutations in which insensitivity to MOPC was associated with the loss of resistance to carbendazim, While others appeared to carry suppressor or modifier mutations producing a range of resistance phenotypes. A study of sensitivity to a range of ergosterol biosynthesis inhibiting (EBI) fungicides revealed considerable variation between different isolates. In general, BWR-type isolates were much less sensitive to the triazole compounds tested than BWtypes, while the reverse relationship was found for the morpholine fungicide, fenpropimorph. Sensitivity to prochloraz showed much less variation. No correlation was found between benzimidazole resistance and reduced sensitivity to the EBI fungicides. Two pathotypes of ~ herpotrichoides, BW and BWR, usually distinguished on the basis of cultural morphology, are recognised. The morphological character, however, was found to be unstable, particularly in BWR-types, which readily sectored to give colonies typical of the other pathotype. Parasexual recombination in P. herpotrichoides was demonstrated using auxotrophic mutants isolated after UVirradiation of conidia. Unstable heterokaryons were obtained following hyphal anastomosis between complementary strains. From these heterokaryons stable prototrophs were recovered both from spores and isolated protop1asts. Genetic evidence and DNA estimations indicated that these stable prototrophs were dip1oids. Recombinant types were isolated from diploids after treatment with 5-fluorouracil. It was also shown that viable heterokaryons could be recovered following PEG-mediated fusion of protop1asts isolated from complementary strains. Heterokaryon incompatibility between unrelated field isolates was tested using auxotrophic mutants obtained by selection for resistance to chlorate and selenate. Vegetative incompatibility appeared to be widespread, few positive interactions being observed, suggesting that the potential for parasexual gene exchange in the field is low. Protoplast fusion may provide a method whereby this incompatibility can be overcome in the laboratory

Detection and assessment of chemical hormesis on the radial growth in vitro of oomycetes and fungal plant pathogens

Although plant diseases can be caused by bacteria, viruses, and protists, most are caused by fungi and fungus-like oomycetes. Intensive use of fungicides with the same mode of action can lead to selection of resistant strains increasing the risk of unmanageable epidemics. In spite of the integrated use of nonchemical plant disease management strategies, agricultural productivity relies heavily on the use of chemical pesticides and biocides for disease prevention and treatment and sanitation of tools and substrates. Despite the prominent use of fungi in early hormesis studies and the continuous use of yeast as a research model, the relevance of hormesis in agricultural systems has not been investigated by plant pathologists, until recently. A protocol was standardized for detection and assessment of chemical hormesis in fungi and oomycetes using radial growth as endpoint. Biphasic dose-responses were observed in Pythium aphanidermatum exposed to sub-inhibitory doses of ethanol, cyazofamid, and propamocarb, and in Rhizoctonia zeae exposed to ethanol. This report provides an update on chemical hormesis in fungal plant pathogens and a perspective on the potential risks it poses to crop productivity and global food supply.Peer reviewedEntomology and Plant Patholog

Plasticity of photosynthetic heat tolerance in plants adapted to thermally contrasting biomes

In many biomes, plants are subject to heatwaves, potentially causing irreversible damage to the photosynthetic apparatus. Field surveys have documented global, temperature-dependent patterns in photosynthetic heat tolerance (PHT); however, it remains unclear if these patterns reflect acclimation in PHT or inherent differences among species adapted to contrasting habitats. To address these unknowns, we quantified seasonal variations in Tcrit (high temperature where minimal chlorophyll-a fluorescence rises rapidly, reflecting disruption to photosystem II) in 62 species native to 6 sites from 5 thermally contrasting biomes across Australia. Tcrit and leaf fatty acid (FA) composition (important for membrane stability) were quantified in three temperature-controlled glasshouses in 20 of those species. Tcrit was greatest at hot field sites and acclimated seasonally (summer > winter, increasing on average 0.34 °C per °C increase in growth temperature). The glasshouse study showed that Tcrit was inherently higher in species from warmer habitats (increasing 0.16 °C per °C increase in origin annual mean maximum temperature) and acclimated to increasing growth temperature (0.24 °C °C−1). Variations in Tcrit were positively correlated with the relative abundance of saturated FAs, with FAs accounting for 40% of Tcrit variation. These results highlight the importance of both plastic adjustments and inherent differences determining contemporary continent-wide patterns in PHT.This work was funded by grants from the Australian Research Council (Grants DP0986823, DP130101252, CE140100008) to O. K. A. We also acknowledge the support of the Australian SuperSite Network, part of the Australian Government's Terrestrial Ecosystem Research Network (www.tern.org.au)