1,365 research outputs found
Arthur James McComb 1936-2017
No full textProfessor Arthur McComb conducted pioneer research on the occurrence and mode of action of the plant growth hormones gibberellins for fifteen years. He then applied his experimental skills and physiological knowledge to develop a whole ecosystem approach to the study of aquatic systems. He was passionate in wanting to improve the state of environmental management, based on rational, logical and well-founded biological principles. He and his team focused primarily on the mechanisms controlling plant growth and productivity in aquatic environments, and especially the effects of nutrient enrichment and its consequences, eutrophication. He became a leader in nutrient analysis of water systems, with innovations in how to determine nutrient pathways into waterways and strategies for fixing these issues. This important research has informed the long-term management of several important aquatic systems in Western Australia: the Blackwood River Estuary, the Peel Harvey Estuary, and the protection of seagrasses in Shark Bay, the Swan River and Cockburn Sound. Arthur McComb had a seminal influence on a generation of researchers. Thirty-nine students completed their higher degrees under his supervision and they are spread internationally and throughout Australia in universities, state government departments and consulting firms, confirming his influence on driving our understanding and management of marine, estuarine and freshwater systems
Interview with Emeritus Professor Jennifer (Jen) A. McComb
No full textOral history interview with Emeritus Professor Jennifer (Jen) A. McComb, former staff member at Murdoch University.
This sound recording is part of the History of Murdoch University Collection
In vitro soil-less (IVS) rooting medium
No full textThe principle hypothesis of this thesis is that hypoxia, in agar-based media, compromises rooting in vitro. From a practical point of view this is important because most plant tissue culture activities require the material to be successfully acclimatised in a nursery environment. Compromised rooting often results in excessive losses at this stage which are costly and inconvenient. In addition, many plants with commercial and/or scientific interest remain unavailable as they are not able to be rooted and acclimatised reliably. The use of agar as a rooting medium has limited the capacity of plant tissue culture to clonally propagate many plants.
The thesis begins by demonstrating how poorly some plants respond to agar rooting media. Juvenile Chamelaucium hybrid microcuttings were pulsed with IBA 40 mcg M and then placed for 3 weeks on either M1 (1/2 MS) or aerated in vitro soil-less substrate (IVS) (Chapter 2). IVS had 42-82% rooting at the end of Stage 3 compared with 0-1% in agar. Shoot survival for IVS-rooted microcuttings was significantly greater than M1-rooted shoots. Pulsed shoots placed in IVS showed root primordia after 7 days. In contrast, shoots placed in agar showed no root primordia after 21 days and formed callus but did not root when subsequently placed in IVS for a further 4 weeks. The agar medium almost totally and permanently inhibited the capacity of competent shoots to form root primordia and roots.
The effectiveness of different types of aerated and non-aerated media, including IVS, were tested to validate the hypothesis (Chapter 3). Microcuttings from shoot cultures of two Australian plants Grevillea thelemanniana and Verticordia plumosa x Chamelaucium uncinatum were pulsed for 7 days on a high auxin (40 mcg M IBA), agar-solidified medium in the dark. Rooting of the microcuttings was then compared on five experimental substrates: a) standard agar M1 medium (1/2 MS, no hormones, 8 g agar L-1), b) porous-agar medium (1/2 MS, no hormones, 30 g agar L-1, solidified then blended to provide aeration), c) white sand wet with liquid M1, d) white sand with M1 medium containing agar, and e) IVS. A separate experiment involved flushing the IVS soil profile with low or normal oxygen. Low and variable rooting percentages were recorded on the controls on M1 medium. Root induction and average total root length per microcutting at final harvest were significantly higher using the porous media including IVS, blended agar or white sand. The M1 medium and the addition of M1 medium to sand suppressed the percentage rooting and elongation. Flushing the IVS rooting medium with low oxygen also suppressed rooting. The experiments showed that increasing the air-filled porosity of the rooting medium has a positive effect on rooting and this is most likely due to the increased oxygen at the base of the microcutting. The role of ethylene, and the sugar and nutrients in M1 were not investigated.
The efficacy of the IVS protocol on a range of Australian herbaceous and woody species was investigated to determine whether the observed benefits were generic or plant specific (Chapter 4). Improved rooting in IVS compared to agar was shown for 28 Australian species and genotypes from the families Liliaceae, Haemodoraceae, Myrtaceae, Thymelaeaceae, Proteaceae, Goodeniaceae and Rutaceae. Twenty-seven of the 28 species rooted in IVS medium at equal or better rates than in M1. In three cases - Actinodium cunninghamii, one of the Pimelea physodes genotypes and one of the Eriostemon australasius genotypes - shoots did not root in M1 but showed good root development in IVS medium. With few exceptions average root length and number in microcuttings rooted in IVS was superior to those in agar medium.
To further test the resilience of the hypothesis, it was tested on nodal microcuttings of lentil which are recalcitrant to root in vitro (Chapter 5). The veracity of a published conclusion that inverted lentil microcuttings (with their base in the air) root better because of their altered polarity was also examined. It was found that, as is the case for many species, roots initiated and grew only at the proximal end of the microcutting regardless of its orientation. When the proximal end was in agar (a hypoxic environment) the rooting percentage was low (9-25%) even when the orientation of the microcutting was altered by inverting the culture tube. In contrast, when the proximal end of the microcutting was in an aerobic environment (from the shoot being placed upside down in agar medium or placed normally or upside down in an aerated medium) rooting percentages were higher (62-100%).
Given that Stage 2 microcuttings are prepared with the objective to root and acclimatise them to nursery conditions, the duration of this activity becomes important as it can impact on plant quality and costs. The pulsing protocol and the length of time that Stage 3 cultures remain in the culture room during the rooting phase is a component of the unit cost of production of each rooted microcutting. Initially a 7-day IBA pulse was used after which the pulsed microcuttings were transferred to IVS to root. Chapter 6 shows that the pulsing period can be shortened to one day or replaced with a single auxin dip while still achieving high rooting percentages and maintaining plant quality. These materials handling improvements go some way to realising the logistical benefits of ex vitro rooting but without compromising the positive influences of hygiene and a stable environment of the in vitro environment
Interactions between Phytophthora cinnamomiand Acacia pulchella: consequences on ecology and epidemiology of the pathogen
No full textPhytophthora cinnamomi is an important pathogen of many plant species in natural ecosystems and horticulture industries around the world. In Western Australia, a high proportion of native plant species are susceptible to P. cinnamomi attack. Acacia pulchella, a resistant legume species native to Western Australia has been considered as a potential biological control tool against P. cinnamomi. To develop effective control methods, it is important to understand the interactions between the control agent and the different life forms of the pathogen. In this thesis the interactions are investigated between P. cinnamomi and varieties of A. pulchella which occur in jarrah (Eucalyptus marginata) forest and sand plain ecosystems.
The soil inoculum of P. cinnamomi was compared under the potted plants of the three common varieties of A. pulchella, var. pulchella, var. glaberrima and var. goadbyi. These were grown in infected jarrah forest soil in the glasshouse and in vitro in a sterilised soil-less mix aseptically. Acacia urophylla (a species non suppressive towards P. cinnamomi) was also included as a control. An isolate of the most commonly found clonal lineage of P. cinnamomi in the jarrah forest, A2 type 1 was selected for use in experiments after testing showed it reliably produced zoospores and chlamydospores both axenically and in non-sterile conditions, in comparison to several other isolates. The lowest survival of P. cinnamomi inoculum was found under A. pulchella var. goadbyi plants grown both in non sterile soil and in aseptic soil-less mix.
All the life forms of P. cinnamomi were affected by A. pulchella (Chapters 2, 3, 4 and 5). The soil leachates from potted plants of A. pulchella var. goadbyi reduced sporangial production (Chapter 2) and caused cytoplasm collapse of chlamydospores (Chapter 3). The confirmation was obtained that soil under A. pulchella was inhibitory to sporangial stage of P. cinnamomi and new evidence was obtained on chlamydospore inactivation. Cytoplasm collapse in the chlamydospores was observed both for chlamydospores on mycelial discs on Mira cloth exposed to the soil leachate and within infected roots buried in soils under the three varieties of A. pulchella plants. The effect was strongest under the plants of A. pulchella var. goadbyi and indicated that the chlamydospores of P. cinnamomi are unlikely to act as persistent structures under A. pulchella var. goadbyi plants.
In Chapter 4, bioassays were conducted with axenically produced mycelia, chlamydospores and zoospores to test the inhibitory effect of the root exudates collected from aseptically grown A. pulchella var. goadbyi plants. The zoospores of the same isolate used in the soil leachate tests were immobilised (became sluggish and encysted) within one to two minutes. When incubated for 24 h, zoospores predominantly clumped and germ tubes were observed only from the clumped ones. Chlamydospores produced by four isolates of the common A2 type 1 strain and the only one A2 type 2 strain available at the time were tested. A higher percentage of chlamydospores collapsed and a very low percentage germinated after 24 h. Chlamydospores of all the A2 type 1 isolates were inhibited by the root exudates whilst the A2 type 2 isolate remained viable. The findings showed that the suppressive effect must be due at least in part to substances exuded by the A. pulchella plants. However, it appeared that the A2 type 1 isolates were more vulnerable to this effect than the single A2 type 2 isolate.
In Chapter 5, the effect of season on sporangial suppression of P. cinnamomi was shown using field soils collected from three jarrah forest soil vegetation types and a Banksia woodland on Bassendean sand, collected in winter and summer. The effect of age of A. pulchella plants was demonstrated using the soils collected from rehabilitated bauxite mine pits. In all the locations soils were collected under A. pulchella plants and 5 m away from the nearest A. pulchella. An effect of soil type was evident as whilst the soil leachates made from the three lateritic jarrah forest soil types where A. pulchella is common in the understorey were suppressive to the sporangial stage of P. cinnamomi, this effect was not evident in the Bassendean sand under A. pulchella. A. pulchella soils collected in winter were less suppressive towards sporangial production than soils collected in summer. An effect of plant age was demonstrated as soil leachates from four year-old A. pulchella stands in rehabilitated bauxite mine sites were more suppressive for sporangia than leachates from one year-old stands.
Further information on the behaviour of the pathogen in soil and in potting mix with and without A. pulchella was obtained by infecting lupin radicles with an isolate of each A2 type, 1 and 2 strains of P. cinnamomi and burying them in the soil under the three varieties of A. pulchella plants. After a week, the chlamydospores were mostly collapsed and hyphae deteriorated. Oospores were observed and in significant numbers under the potted plants of A. pulchella var. glaberrima.
Isolates of all three clonal lineages of P. cinnamomi found in Australian soil were tested for the ability to produce oospores. Two isolates of the A1 and A2 type 2 and three isolates of the common A2 type 1 were screened. The two isozyme types of the A2 clonal lineage isolated in Australia varied in ability to self and produce oospores in planta in several soils from the jarrah forest. The isozyme type 2 of the A2 clonal lineage of P. cinnamomi produced oospores under these experimental conditions. This stimulation was not effective for most of the tested isolates of the A2 type 1 and the A1 clonal lineage. The in planta oospores were viable but dormant and the oogonial-antheridial associations were amphigynous both in vitro and in vivo. For the first time it was established that, the stimulus for selfing and oospore formation in the A2 type 2 of P. cinnamomi is available in some jarrah forest soils, with and without A. pulchella and also in the potting mix used. This raises important questions for the management of the pathogen.
Several factors were identified as potential stimuli for selfing. Among them, soil nutrient levels and essentially enhanced sulphur presence were found important. Temperature also played a key role. Oospores were produced abundantly at 21 - 25 degrees C but not over 28 degrees C.
The biology of P. cinnamomi has been studied for several decades but some important aspects remain un-researched. This thesis pioneers research into the in planta selfing aspect of the pathogen in soil. It also improved the understanding of the interactions between P. cinnamomi and A. pulchella which to some extent supports use of A. pulchella as a biological control tool against P. cinnamomi. However, attention is drawn to the natural mechanisms of this complex pathogen to survive in planta by producing oospores, the most persistent form of its life cycle
Floral biology and propagation of blue-flowered Conospermum spp
No full textBlue-flowered Conospermum are endemic to Western Australia, and show great potential as cut flowers. Propagation from cuttings or seed proved difficult, and root initiation in vitro is problematic. This thesis examines the floral biology of the species and the possibility of using somatic embryogenesis to overcome propagation problems.
A survey of explant tissue types for C. eatoniae and C. caeruleum was carried out to identify tissue that could be induced into embryogenic pathways. Vegetative, semi-floral and floral buds were initiated into culture from February to June, but were found unsuitable for embryogenesis, producing shoots, callus or dying in culture. Leaves from in vitro leaf cultures formed callus in the presence of 2,4-D and BAP, but were unable to differentiate into embryos in the presence of a variety of growth regulator combinations and concentrations. Immature zygotes died in culture. Direct embryogenesis and/or embryogenic callus was observed on mature zygotes of the species C. caeruleum, C. spectabile, C. dorrienii and C. brownii, and somatic embryos were maintained in culture for up to 18 months for C. caeruleum.
Maturation and germination of somatic embryos proved difficult; treatments of cold, ABA, desiccation or mannitol did not induce maturation. It appears that developmental pathways in Conospermum are well defined and are difficult to alter in vitro. It was concluded that somatic embryogenesis has limited commercial potential in these species.
Conospermum species have an active pollination mechanism where the style is held in a state of tension when the flower opens. When pressure is applied at the base of the style by an insect, the style flicks downwards, striking the insect pollinator and releasing pollen from the anther in a single dusty mass. However, the breeding systems of blue-flowered Conospermum have not previously been well explored.
Flowers on a C. eatoniae inflorescence opened from the basal end upwards acropetally, with the terminal two or three buds never opening. Fruit and seed set occurred only from the basal one to three buds. Isolation of C. eatoniae and C. amoenum flowers showed they were unable to self-pollinate in the absence of insect pollinators.
Experiments to determine the timing of the peak of stigmatic receptiveness were inconclusive. Pollen germinated and penetrated the stigma 0 - 6 days after anther dehiscence. Pollen loads on the stigma did not relate to the number of pollen tubes observed down the style. Controlled pollinations of cultivated C. eatoniae at a field station using self and cross pollen, revealed compatibility with a range of pollen genotypes, as pollen tubes were observed extending down the style. However, late-acting incompatibility could not be ruled out as controlled crosses failed to set any seed as flowers were shed from the bush.
DNA analysis of open pollinated C. eatoniae seed progeny from two plants from a field station and two plants in natural bushland revealed very different pollination habits. Plants from the field station showed no outcrossing, with progeny closely resembling the maternal parent, whereas plants from the wild population showed outcrossing with several different paternal parents. These results suggest self-pollinated seed can be reliably obtained in a plantation situation using stands of ramets of the same clone. Alternatively, assuming that the required insect pollinators are present in a cultivated stand, it should be possible to obtain cross pollinated seed by surrounding the maternal plant with the desired paternal parent.
Unusual pollen behaviour was observed for many blue-flowered species, a white-flowered species of Conospermum, and close relative, Synaphea petiolaris. Up to three pollen tubes emerged from the triporate pollen in vitro, and at rates of up to 55 mcgms-1. This rate was maintained for only 2 s but is greater than 20 times faster than reported in the literature for any species, in vitro or in vivo. Pollen with multiple tubes was also observed on the stigma in vivo in C. amoenum flowers.
Changing the osmotic pressure of the germination medium by altering sucrose concentration influenced the number of tubes to emerge from the pollen grain; generally the number of tubes decreased as sucrose increased. However, the rate of tube growth was unaffected. The addition of calcium channel blockers to the germination medium had no effect on Conospermum growth rate, nor did they eliminate pulses of tube growth.
Observation of Conospermum pollen ultrastructure revealed similarities to Gramineae pollen. The tube cytoplasm was packed with vesicles filled with material of similar electron density to the cell wall. Few golgi were identified, and the apical end of the tube contained these vesicles, smaller secretory vesicles and mitochondria. This is atypical of the tip, which is normally free of large vesicles. Distinct zones in the cytoplasm were not identified, which is similar to Gramineae. Like the grasses, Conospermum appears to pre-manufacture cell wall material and store it in vesicles ready for rapid germination and extension.
A biological function of multiple pollen tube emergence with such rapid growth was not elucidated.
This research has shown Conospermum to be a complex and very interesting genus. Further investigation into the remarkable growth of multiple pollen tubes would enhance our knowledge of the biological processes involved in tube growth and the process of fast wall formation. The potential benefits to the cut flower industry of commercialising some of these species warrants further effort to find an efficient method of propagation. Introduction into horticulture may be the only means by which these threatened species will survive
Mycorrhizal specificity in endemic Western Australian terrestrial orchids (tribe Diurideae): implications for conservation
No full textThe specificity of fungal isolates from endemic Western Australian orchid species and hybrids in the tribe Diurideae was investigated using symbiotic seed germination and analysis of the fungal DNA by amplified fragment length polymorphism (AFLP). The distribution of the fungal isolates in the field was also assessed using two different seed baiting techniques. The information from these investigations is essential for developing protocols for reintroduction and translocation of orchid species.
Two groups of orchids in the tribe Diurideae were studied. Firstly, a number of Caladenia species, their natural hybrids and close relatives from the southwest of Western Australia were selected because orchid species from the genus Caladenia are considered to have among the most specific mycorrhizal relationships known in the orchid family ? an ideal situation for the investigation of mycorrhizal specificity. Secondly, species of Drakaea and close relatives, from the southwest of Western Australia and elsewhere in Australia, which are never common in nature and occur in highly specialised habitats, were selected to investigate the influence of habitat on specificity.
Seed from the common species Caladenia arenicola germinated on fungal isolates from adult plants of both C. arenicola and its rare and endangered relative C. huegelii, while seed from C. huegelii only germinated on its own fungal isolates. The AFLP analysis grouped the fungal isolates into three categories: nonefficaceous fungi, C. huegelii type fungi, and C. arenicola type fungi. The group of C. huegelii type fungi included some fungal isolates from C. arenicola. An analysis of the AFLP fingerprints of C. arenicola fungal isolates from different collection locations showed that some, but not all, populations were genetically distinct, and that one population in particular was very variable.
Despite being thought to have very specific mycorrhizal relationships, Caladenia species hybridise frequently and prolifically in nature, often forming self-perpetuating hybrid lineages. Five natural hybrids within Caladenia and its closest relatives were investigated. Symbiotic cross-germination studies of parental and hybrid seed on fungi from the species and the naturally occurring hybrids were compared with AFLP analyses of the fungal isolates to answer the question of which fungi the hybrids use. The germination study found that, while hybrid seeds can utilise the fungi from either parental species under laboratory conditions, it is likely that the natural hybrids in situ utilise the fungus of only one parental species. Supporting these observations, the AFLP analyses indicated that while the parental species always possessed genetically distinct fungal strains, the hybrids may share the mycorrhizal fungus of one parental species or possess a genetically distinct fungal strain which is more closely related to the fungus of one parental species than the other.
The work on Caladenia hybrids revealed that C. falcata has a broadly compatible fungus that germinated seeds of C. falcata, the hybrid C. falcata x longicauda, and species with different degrees of taxonomic affinity to C. falcata. In general, germination was greater from species that were more closely related to C. falcata: seeds from Caladenia species generally germinated well on most C. falcata isolates; species from same subtribe (Caladeniinae) germinated well to the stage of trichome development on only some of the fungal isolates and rarely developed further; and seeds from species from different subtribes (Diuridinae, Prasophyllinae, Thelymitrinae) or tribes (Orchideae, Cranichideae) either germinated well to the stage of trichome development but did not develop further, or did not germinate at all. The AFLP analysis of the fungal isolates revealed that the fungi from each location were genetically distinct.
In situ seed baiting was used to study the introduction, growth and persistence of orchid mycorrhizal fungi. A mycorrhizal fungus from Caladenia arenicola was introduced to sites within an area from which the orchid and fungus were absent, adjacent to a natural population of C. arenicola. In the first growing season, the fungus grew up to 50 cm from its introduction point, usually persisted over the summer drought into the second season and even into the third season, stimulating germination and growth to tuber formation of the seeds in the baits. Watering the inoculated areas significantly increased seed germination.
Mycorrhizal relationships in Drakaeinae were less specific than in Caladeniinae. A study of the species Spiculaea ciliata revealed that this species, when germinated symbiotically, develops very rapidly and has photosynthetic protocorms, unlike all other members of the Drakaeinae. An AFLP analysis of the fungal isolates of this species grouped the isolates according to whether they had been isolated from adult plants or reisolated from protocorms produced in vitro. Isolates were genetically distinct when compared before germination and after reisolation. A cross-species symbiotic germination study of seeds of three Drakaea species and one Paracaleana species against fungal isolates from the same species and several other Drakaeinae species revealed lower specificity in this group than previously thought. A number of fungal isolates from Drakaea and Paracaleana species germinated two or more seed types, while all seed types germinated on fungal isolates from other species and the seed of Drakaea thynniphila germinated to some extent on every fungal isolate tested. An AFLP analysis of the Drakaeinae fungal isolates supported this information, revealing little genetic differentiation between the fungi of different orchid species.
An ex situ seed baiting technique was used to examine the role of mycorrhizal fungi in microniche specialisation in the narrow endemic Drakaea. Soil samples from within and outside two Drakaea populations were tested for germination of the relevant seed types. In both cases, germination was significantly higher on soil samples from within than outside the populations, suggesting that the relevant mycorrhizal fungi may be restricted to the same microniches as the Drakaea species. The presence of similar fungi at distant, disjunct locations may be related to the extreme age and geological stability of the Western Australian landscape.
The information from these investigations is essential for developing protocols for reintroduction and translocation of orchid species. It appears that the mycorrhizal relationships in these groups of orchids are not as specific as was previously thought. For reintroduction work, a broad sampling strategy is necessary, as it cannot be assumed that the same orchid species has the same fungus at different locations. A broadly compatible fungus may be of considerable utility in conservation work, such as in situations where a specific fungus appears to have poor saprophytic competence or where soil conditions have been altered. Seed baiting studies provide additional data on fungal distribution in situ. In general, molecular data do not provide information about efficacy or fungal distribution, so research programs that combine symbiotic germination studies with seed baiting investigations and genetic analyses of the fungi will provide the maximum benefit for designing more effective conservation programs
Jen Delos Reyes
No full textProjects in this collection: Open Engagement
From http://www.jendelosreyes.com/about: Jen Delos Reyes was born in the city of Winnipeg, Manitoba, Canada, and educated first in its local music scene of the mid-90’s infused with the energy of Riot grrrl and DIY, and then in its university. [1] How she works today is rooted in what she learned in her formative years as a show organizer, listener, creator of zines, and band member. Graduate work at the University of Regina made the space possible for her to see her work as an organizer as a key component of her continued creative work.
Jen Delos Reyes is a \u27farmer of sorts and an artist of sorts\u27[2], educator, writer, and radical community arts organizer. She is defiantly optimistic, a friend to all birds, and proponent that our institutions can become tender and vulnerable. Her practice is as much about working with institutions as it is about creating and supporting sustainable artist-led culture.
Delos Reyes worked within Portland State University from 2008-2014 to create the first flexible residency Art and Social Practice MFA program in the United States and devised the curriculum that focused on place, engagement, and dialogue. The flexible residency program allowed for artists embedded in their communities to remain on site throughout their course of study.
She worked with the Portland Art Museum from 2009-14 on a series of programs and integrated systems that allowed artists to rethink what can happen in a museum, and reinvigorate the idea of the museum as a public space.
From 2015-2022 Delos Reyes was the Associate Director of the School of Art & Art History of the University of Illinois, Chicago’s only public research university, where she taught in the departments of Art and Museum and Exhibition Studies.
She was the Director and founder of Open Engagement, an international annual conference on socially engaged art that was active between 2007-2019 and hosted ten conferences in two countries at locations including the Queens Museum in New York. After over a decade of large scale organizing she is now focused on work on the scale of her life.
She is the author of I’m Going to Live the Life I Sing About in My Song: How Artists Make and Live Lives of Meaning, Everything You Ever Wanted to Know About Public Engagement But Were Afraid to Ask, and Defiantly Optimistic: Turning Up in a World on Fire.
Delos Reyes divides her time between Chicago, IL where she is the founder of Garbage Hill Farm, and Ithaca, NY where she is an Associate Professor of Art at Cornell University.
[1] Credit to Saul Alinsky in form, and for the reminder that often the most formative educational experiences happen outside of the classroom.
[2] Grateful to Wendell Berry in general, and for this descriptor I am using.https://pdxscholar.library.pdx.edu/artandsocialpractice_creators/1030/thumbnail.jp
A Personal Journey with Gish Jen, Author
No full textBorn and raised in the United States, Gish Jen has become a leading literary voice of the Chinese-American experience. In this program, Bill Moyers talks with the critically acclaimed writer, whose novels and short stories are known for their humorous and incisive edge. (14 minutes, color
Biochemical effects of phosphite on the phytopathogenicity of Phytophthora cinnamomi Rands
No full textPhosphite, a chemical analogue of orthophosphate, controls disease symptoms and spread of Oomycete plant pathogens, particularly those caused by Phythophthora spp. Phosphite can be applied to horticultural and native plant species as a foliar spray or trunk injection and results in in planta phosphite concentrations of between 25 - 425 μg g-1 dry weight (equivalent to 0.3 – 6.0 mM). However, despite its extensive use it is not known why phosphite is biostatic towards oomycetes, although several mechanisms have been proposed.
This thesis aims to devise and test a biochemical model of phosphite action that could account for the observed effects of phosphite on the interaction between Phytophthora cinnamomi and a susceptible plant host. However, prior to this it was necessary to devise a test to assess the concentration of phosphite in planta and to establish that phosphite needs to be present at the plant /pathogen interface in order to have an effect. A silver nitrate staining method was developed and its ability to detect phosphite was assessed in a variety of native Australian and horticultural plants. The method demonstrated that phosphite concentrations of between 1 and 3 mM were present in the tips of the roots of lupins that had been foliar sprayed with 0.5 % phosphite (equivalent to 62 mM) and that in most instances, these concentrations were sufficient to completely control the development of disease symptoms.
As phosphite is chemically similar to orthophosphate its presence in a cell is likely to interfere with many aspects phosphate metabolism in both plant and pathogen. In order to discern the mechanism of action of phosphite it was important to separate the antipathogenic effects from the general/ pleotropic effects. A bioassay was devised whereby the roots of lupin seedlings were inoculated with filter paper discs that had been colonised with P. cinnamomi isolate MP94-48 and then treated with phosphite or other chemicals that would be expected to reduce its pathogenicity. The extent of lesion development and the root growth below the point of inoculation were the two parameters by which the effect of the chemicals on pathogenicity was assessed.
Increasing either the concentration of orthophosphate (0 – 100 mM) or phosphite (0 – 10 mM) in the growth medium of P. cinnamomi colonised discs reduced lesion development on inoculated lupin seedling roots. Orthophosphate concentrations of between 3 – 10 mM, in combination with 1 mM phosphite did not reduce the extent of lesion development. In contrast, plants inoculated with discs treated with concentrations of orthophosphate above 10 mM together with 3 and 10 mM phosphite, lesions were reduced when compared to plants inoculated with discs treated with phosphite alone.
The inhibition of phosphatase activity in P. cinnamomi is often proposed to be a primary effect of phosphite. Treatment of P. cinnamomi colonized discs with the phosphatase inhibitors okadaic acid, sodium fluoride, and a mixture of inhibitors containing sodium vanadate, sodium molybdate, sodium tartrate and imidazole, neither decreased nor increased the development of lesions, and no change in the degree of phosphorylation of cytosolic proteins could be detected by Pro-Q Diamond phosphoproteins staining. The addition of the kinase inhibitor staurosporine (0.1 - 1 mM) reduced lesion development on lupins and this effect was augmented slightly, but significantly, by the addition of phosphite (3 mM). It was not possible to draw any conclusions from the results of experiments testing the effect of addition of exogenous cAMP or the phosphatase inhibitor phenyl arsine oxide to colonised discs on the ability of P. cinnamomi to produce lesions in lupins. These results suggest that phosphorylation reactions and cascades may not be the primary control mechanism in either initiation or inhibition of phytopathogenesis. However, the addition of glucose (30 mM) increased pathogenicity and the development of lesions.
As evidence exists that abscisic acid (ABA) increases the susceptibility of plants to infection by Phytophthora spp. and that ABA signaling involves phospholipase D (PLD) the effect of inhibitors on this signaling pathway were tested on the ability of P. cinnamomi to produce lesions. Primary, 2o and 3o butyl alcohol, as well as the guanine nucleotide exchange factor (GEF) inhibitor brefeldin A, and ABA itself were added to cultures of P. cinnamomi. The application of either 1o, 2o or 3o butyl alcohol to P. cinnamomi colonised discs had no effect on lesion development, which would be expected were the generation of phosphatidic acid per se was vital to pathogenicity. However, Brefeldrin A (10 - 250 μM) had a highly significant and concentration dependent effect on the development of lesion on lupins. These results suggested that a member of the Ras-superfamily (such as an ADP ribosylation factor) is likely to be involved in the development of lesions and that the exchange of GDP for GTP on this protein is required for pathogenesis. The results from the bioassays of addition of exogenous ABA to P. cinnamomi colonised discs were ambiguous and additional experimentation is needed to elucidate the role of ABA in the phytopathogenesis of P. cinnamomi.
Calcium ion signatures and cytosolic gradients are known to be important components of many signal transduction pathways and increased soil calcium can limit the development of Phytophthora disease. The effect of external calcium ion concentration and the calcium channel blockers ruthenium red (RR), lanthanum chloride (La3+) and the calcium ion chelator EGTA on the development of lesions was investigated. The results of the bioassays indicated that external calcium ion concentration, RR and La3+ (100 μM) reduced lesion development significantly as did EGTA (1 mM) and that this reduction was further enhanced in the presence of phosphite.
The combined role of phosphite and external calcium ion concentration was further investigated in a glasshouse pathogenicity trial using P. cinnamomi and the Australian plant Banksia leptophilia in a factorial nested pot design with foliar phosphite and soil calcium sulphate concentration as independent variables. The results one year post-inoculation confirmed that when foliar phosphite (0.1% - 0.3%) was used in conjunction with soil supplementation with calcium sulphate (3 – 30 mM) disease symptoms and lesion development were significantly reduced and general plant health was improved.
The combined results of these experiments suggested not only a role for calcium ion concentration and signaling in pathogenicity but, together with the 35-fold increase in PPi concentration, imply that inhibition of the calcium dependent ATPase responsible for regulating cytosolic Ca 2+ concentration may be the cause of the antipathogenic effect of phosphite in P. cinnamomi. Calcium dependent ATPases are known to be involved in the gravitropic response of roots as well as the polar growth of pollen tubes (i.e. presence of the Ca2+ channel blocker La3+ results in inhibition of the gravitropic and polar response). Preliminary results of the effect of phosphite on the gravitropism of lupin seedling roots indicate that phosphite does inhibit the gravitropic response, suggesting that there is a causal link between the mechanism of action of phosphite and calcium-dependent ATPases
Gish Jen: Vocation of the Writer (Library Resources)
No full textA bibliography of resources available through the Holy Cross Libraries which provide additional information related to Gish Jen: Vocation of the Writer a lecture by award-winning author and speaker Gish Jen. The conference is sponsored by the Rev. Michael C. McFarland, S.J. Center for Religion, Ethics and Culture, the Creative Writing Program, and Asian Studies and was held at the College of the Holy Cross on February 27, 2018.https://crossworks.holycross.edu/bibliography_events/1012/thumbnail.jp
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