Statistical method for testing the neutral mutation hypothesis by DNA polymorphism.

▪ Abstract We hypothesize that the evolutionary potential of a pathogen population is reflected in its population genetic structure. Pathogen populations with a high evolutionary potential are more likely to overcome genetic resistance than pathogen populations with a low evolutionary potential. We propose a flexible framework to predict the evolutionary potential of pathogen populations based on analysis of their genetic structure. According to this framework, pathogens that pose the greatest risk of breaking down resistance genes have a mixed reproduction system, a high potential for genotype flow, large effective population sizes, and high mutation rates. The lowest risk pathogens are those with strict asexual reproduction, low potential for gene flow, small effective population sizes, and low mutation rates. We present examples of high-risk and low-risk pathogens. We propose general guidelines for a rational approach to breed durable resistance according to the evolutionary potential of the pathogen.

Pathogen population genetics, evolutionary potential, and durable resistance

Biocontrol fungi (BCF) are agents that control plant diseases. These include the well-known Trichoderma spp. and the recently described Sebacinales spp. They have the ability to control numerous foliar, root, and fruit pathogens and even invertebrates such as nematodes. However, this is only a subset of their abilities. We now know that they also have the ability to ameliorate a wide range of abiotic stresses, and some of them can also alleviate physiological stresses such as seed aging. They can also enhance nutrient uptake in plants and can substantially increase nitrogen use efficiency in crops. These abilities may be more important to agriculture than disease control. Some strains also have abilities to improve photosynthetic efficiency and probably respiratory activities of plants. All of these capabilities are a consequence of their abilities to reprogram plant gene expression, probably through activation of a limited number of general plant pathways.

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Induced Systemic Resistance and Plant Responses to Fungal Biocontrol Agents

This review summarizes recent research in Australia on: (i) climate and geophysical trends over the last few decades; (ii) projections for climate change in the 21st century; (iii) predicted impacts from modelling studies on particular ecosystems and native species; and (iv) ecological effects that have apparently occurred as a response to recent warming. Consistent with global trends, Australia has warmed ~ 0.8 � C over the last century with minimum temperatures warming faster than maxima. There have been significant regional trends in rainfall with the northern, eastern and southern parts of the continent receiving greater rainfall and the western region receiving less. Higher rainfall has been associated with an increase in the number of rain days and heavy rainfall events. Sea surface temperatures on the Great Barrier Reef have increased and are associated with an increase in the frequency and severity of coral bleaching and mortality. Sea level rises in Australia have been regionally variable, and considerably less than the global average. Snow cover and duration have declined significantly at some sites in the Snowy Mountains. CSIRO projections for future climatic changes indicate increases in annual average temperatures of 0.4-2.0 � C by 2030 (relative to 1990) and 1.0-6.0 � C by 2070. Considerable uncertainty remains as to future changes in rainfall, El Nino Southern Oscillation events and tropical cyclone activity. Overall increases in potential evaporation over much of the continent are predicted as well as continued reductions in the extent and duration of snow cover. Future changes in temperature and rainfall are predicted to have significant impacts on most vegetation types that have been modelled to date, although the interactive effect of continuing increases in atmospheric CO 2 has not been incorporated into most modelling studies. Elevated CO 2 will most likely mitigate some of the impacts of climate change by reducing water stress. Future impacts on particular ecosystems include increased forest growth, alterations in competitive regimes between C3 and C4 grasses, increasing encroachment of woody shrubs into arid and semiarid rangelands, continued incursion of mangrove communities into freshwater wetlands, increasing frequency of coral bleaching, and establishment of woody species at increasingly higher elevations in the alpine zone. Modelling of potential impacts on specific Australian taxa using bioclimatic analysis programs such as BIOCLIM consistently predicts contraction and/or fragmentation of species' current ranges. The bioclimates of some species of plants and vertebrates are predicted to disappear entirely with as little as 0.5-1.0 � C of warming. Australia lacks the long-term datasets and tradition of phenological monitoring that have allowed the detection of climate-change-related trends in the Northern Hemisphere. Long-term changes in Australian vegetation can be mostly attributed to alterations in fire regimes, clearing and grazing, but some trends, such as encroachment of rainforest into eucalypt woodlands, and establishment of trees in subalpine meadows probably have a climatic component. Shifts in species distributions toward the south (bats, birds), upward in elevation (alpine mammals) or along changing rainfall contours (birds, semiarid reptiles), have recently been documented and offer circumstantial evidence that temperature and rainfall trends are already affecting geographic ranges. Future research directions suggested include giving more emphasis to the study of climatic impacts and understanding the factors that control species distributions, incorporating the effects of elevated CO 2 into climatic modelling for vegetation and selecting suitable species as indicators of climate-induced change.

Climate change and Australia: Trends, projections and impacts

It is accepted that most fungal avirulence genes encode virulence factors that are called effectors. Most fungal effectors are secreted, cysteine-rich proteins, and a role in virulence has been shown for a few of them, including Avr2 and Avr4 of Cladosporium fulvum, which inhibit plant cysteine proteases and protect chitin in fungal cell walls against plant chitinases, respectively. In resistant plants, effectors are directly or indirectly recognized by cognate resistance proteins that reside either inside the plant cell or on plasma membranes. Several secreted effectors function inside the host cell, but the uptake mechanism is not yet known. Variation observed among fungal effectors shows two types of selection that appear to relate to whether they interact directly or indirectly with their cognate resistance proteins. Direct interactions seem to favor point mutations in effector genes, leading to amino acid substitutions, whereas indirect interactions seem to favor jettison of effector genes.

Fungal Effector Proteins

One protectant (mancozeb) and four systemic (mefenoxam, fosetyl-Al, azoxystrobin and dimethomorph) fungicides provided varying levels of activity against Plasmopara viticola at different stages during the pre- and postinfection processes of the pathogen on grapevine (Vitis vinifera) leaves. Azoxystrobin and fosetyl-Al produced the strongest inhibition, with inhibitory effects (compared with the control) evident at all stages of the asexual phase of the life cycle assessed. Mancozeb had the strongest protectant activity and mefenoxam was the most inhibitive as a curative [applied at 4 days after inoculation (dai)] and as an antisporulant (applied at 4 dai and 7 dai). Zoospore motility was the most sensitive of the stages to fungicides in in vitro tests, while the production of sporangia following treatments at 4 dai was the most sensitive during the in vivo phases. The performance of fosetyl-Al and azoxystrobin on the parasitic and reproductive phases of the pathogen was outstanding, especially in the treatment involving application at 7 dai. Fosetyl-Al and azoxystrobin reduced viability of sporangia, with 93% for the two higher rates of these fungicides. We believe our study to be the first to identify that the specific germination and infection phases of P. viticola, such as encysted zoospores, zoospore germination and germ tube growth, are differentially responsive to the major classes of fungicides available for use against this pathogen.Asa relatively inexpensive and non-toxic chemical, fosetyl-Al, a phosphonate, has significant potential to become an effective management tool against grapevine downy mildew.

Inhibition of the pre- and postinfection processes of Plasmopara viticola on Vitis vinifera leaves by one protectant and four systemic fungicides

New subterranean clovers with reduced cotyledon susceptibility to redlegged earth mites

Genetic transformation of tomato was first accomplished around 30 years ago. However, variability in transformation efficiency of distinct cultivars exists and to some extent remains a bottleneck for transgenic research. This study reports strategies to improve transformation efficiency in tomato and investigates regeneration capacity of transgenic plants under different selection regimes and hormonal applications. Tomato cv. Rio Grande was used as plant material and hygromycin and phosphinothricin (PPT) were used as selection agents. We found that cv. Rio Grande inherently produced a significant number of abnormal (“blind”) shoots lacking an apical meristem. Replacing cytokinin zeatin riboside with 6-benzylaminopurine (BAP) reduced the number of blind shoots although it slightly prolonged regeneration time. Survival rate of calli and shoots was very low using PPT as selection, whereas regeneration was achieved using hygromycin. Delayed application of hygromycin selection following co-cultivation with Agrobacterium tumefaciens improved the overall callus and shoot production. In vivo GFP fluorescence was detected to investigate the development of transgenic tissues using different hygromycin selection regimes. Higher transformation frequency was achieved when explants were continuously exposed to selection agents immediately following the pre-selection stage. Reducing the selection period followed by a non-selection stage increased the number of shoots, but these shoots were mostly non-transgenic. Thus, although less stringent selection, as expected, encouraged regeneration of shoots from calli, it did not improve transformation efficiency. Omitting selection altogether greatly reduced the efficiency of transformation. It was concluded that BAP is more suitable for normal shoot development, and that delayed selection followed by continuous selection results in higher transformation frequency. A significant improvement in regeneration of transgenic tomato through identification of key regeneration factors and robust screening is described, enabling high throughput genetic engineering in tomato.

Optimisation of regeneration parameters improves transformation efficiency of recalcitrant tomato

Tedera (Bituminaria bituminosa (L.) C.H. Stirton var. albomarginata) has been successfully established across the mixed-farming (wheat-sheep) region of Western Australia because this species has remarkable drought tolerance and can survive the dry-summer period with strong retention of green leaf. A leaf spot symptom involving pale brown lesions with distinct dark brown margins had been observed in genetic evaluation plots of tedera at Medina and Mount Barker, Western Australia, and a Phoma sp. was isolated. Single-spore isolations of a typical Phoma sp. isolate were made onto potato dextrose agar and maintained at 20°C, and a representative culture has been lodged in the Western Australian Culture Collection Herbarium maintained at the Department of Agriculture and Food Western Australia (Accession No. WAC13435). Amplification of the internal transcribed spacer (ITS) 1 and ITS2 regions flanking the 5.8S rRNA gene were carried out with universal primers ITS1 and ITS4 according to published protocol (3). T...

First Report of Phoma herbarum on Tedera (Bituminaria bituminosa var. albomarginata) in Australia

Summary. Thirty-three subterranean clover varieties, comprising 16 commercial cultivars and 17 promising midseason breeding and introduced lines, were screened for resistance to powdery mildew (caused by Erysiphe polygonii) under controlled environment conditions, and where possible, comparisons were made with their performance in field plots. There were large differences between varieties in the incidence and severity of powdery mildew on plants. Under controlled environment conditions, Tallarook, CPI 47308C, CPI 89860D, 75S13-12, CPI 6.53284 CPI 89820D and Clare had the lowest levels of leaf infection, the lowest mildew sporulation scores and the least amount of leaf scorching from mildew infection. Karridale had the highest level of leaf infection, followed by 69837-1, 76841-1, Junee, Bacchus Marsh and 69S37-3. Under field conditions, Tallarook, CPI 47308C, CPI 65328A and CPI 65328F were mildew-free, while CPI 89777C, CPI 89860D, CPI 89830F, CPI 89841E, Clare and Rosedale had good resistance. Junee was clearly the most susceptible under field conditions, followed by 76S41-1, 69837-3, Karridale, Bacchus Marsh and 69S37-1. There was generally excellent correlation between the different powdery mildew disease parameters, namely leaf infection, mildew sporulation and leaf scorch, measured under controlled environment conditions, and there was also good overall correlation between controlled environment and field data. Bacchus Marsh, Junee, 69337-1, 69837-3 and 76341-1 were highly susceptible under both controlled environment and field conditions, indicating that either environment could be used to identify highly susceptible varieties. Under controlled environment conditions, a high degree of resistance was observed in Clare, CPI 47308C, CPI 65328A, CPI 89820D, CPI 89860D and 75S13-12, while field plots of the varieties Tallarook, CPI 47308C and CPI 65328A showed a complete absence of powdery mildew. These varieties may have value as parents in breeding programs for powdery mildew resistance.

Martin J. Barbetti

Papers

Inhibition of the pre- and postinfection processes of Plasmopara viticola on Vitis vinifera leaves by one protectant and four systemic fungicides

New subterranean clovers with reduced cotyledon susceptibility to redlegged earth mites

Optimisation of regeneration parameters improves transformation efficiency of recalcitrant tomato

First Report of Phoma herbarum on Tedera (Bituminaria bituminosa var. albomarginata) in Australia

Susceptibility of subterranean clover varieties to powdery mildew under controlled environment and field conditions