Existing evidence suggested that nursery inoculation with Trichoderma harzianum and arbuscular mycorrhizal fungi (AMF) could reduce deleterious effects of biotic and abiotic stresses and improve seedling quality, fruit yield and quality of tomato (Solanum lycopersicum L.). However, studies of their combined inoculation on seedling growth, fruit yield and quality of tomato plants are not well-documented. Experiments were carried out to investigate the combined effect of T. harzianum and AMF on tomato crop performance under various conditions. When combined with a T. harzianum and AMF mixture, seaweed extract from Ecklonia maxiama inhibited AMF root colonisation of tomato seedlings. Treating seedlings with a mixture of T. harzianum and AMF reduced the incidence of Verticillium wilt in tomato grown in a nethouse at early season, with negligible effect on fruit yield. Further investigations were initiated to find out whether T. harzianum and AMF were efficient when applied as a mixture or alone, at different inoculation times. Co-inoculation with T. harzianum and AMF (Glomus mosseae) improved seedling growth and development, except when both fungi were simultaneously applied two weeks after sowing. When the seedlings were allowed to grow up until full harvest in a greenhouse, both fungal inoculants increased total yield and marketable yield, but these increases were not significant. Furthermore, inoculation with AMF increased the percentage of extra-large fruit. Field experiments conducted under commercial tomato production confirmed greenhouse studies. Inoculation of tomato with T. harzianum and AMF, either alone or in combination increased early fruit yield (four first harvesting weeks). Throughout the studies, percentage AMF root colonisation in seedlings and plants remained low, despite nursery inoculation. Field experiments investigated the effects of AMF-inoculated transplants combined with biochar-amended soils on AMF root colonisation and their resultant effects on overall crop performance and microbial community structure. Biochar had no effect on AMF root colonisation, and also when combined with AMF, it had no influence on tomato productivity. Interestingly, biochar altered the fungal community while AMF might have influenced the bacterial community such as plant-growth promoting rhizobacteria, which are associated with improved plant growth, nutrient uptake and disease control in the rhizosphere. These benefits could contribute to improved yield and fruit quality. In conclusion, although the results were variable, there was a clear indication that T. harzianum and AMF can play an important role in tomato production. / Thesis (PhD)--University of Pretoria, 2012. / Plant Production and Soil Science / unrestricted

Arbuscular mycorrhizal (AM) fungi are obligate biotrophs that formsymbiosis with plants by colonizing their roots. The fungus supplies thehost plant with various nutrients, including phosphorus. Phosphorus iscrucial for the development of plants and is hard to acquire in soilsince it can be scarce and has a slow motility. The fungus utilizes itslong hyphal threads to contact more soil to obtain phosphorus andtransport it back to the plant. It does so with its use of differentphosphorus transporters (PTs) located in its membranes. Here we havedone a phylogenetic analysis of different PTs from a variety of fungifrom different phyla together with plants and new sequence data from AMfungi. In total, 955 genomes were screened, 26 of which belong to AMfungi. This work resulted in a database of 1351 PT sequences, 907 fromfungi (243 from AM) and 444 from plants, and two phylogenetic trees tovisualize the data. One phylogeny was made of the branch of the PT Pho87which was selected for building a Hidden Markov model, which canfacilitate future searches of PTs.

Arbuscular mycorrhizal fungi (AMF) are a group of root obligate symbionts that are part of the fungal sub-phylum Glomeromycotina, which provide water, nutrients, and pathogen protection to about 80% of land plants in exchange for their photosynthetic products. AMF thus act as “biofertilizers”, have a profound effect and influence on the biodiversity of plants, and play a major role in life on land. From an evolutionary point of view, AMF are a puzzling group of organisms, thought to have propagated for over 400 million years without sexual reproduction, a rarity among eukaryotes. However, this assumption is largely based on the absence of definitive observations of sexual reproduction through microscopic tools. One clue into the sexual activity of AMF is evidence of a dikaryotic-like genome organization in their multi-nucleated mycelium. The recent identification of multi-allelic mating-type loci (MAT locus) potentially places AMF among other heterothallic or bipolar species, who’s mating compatibility is determined by their MAT locus. The presence of a hidden sexual cycle in AMF is still a possibility, and recent findings on the meiotic gene content of AMF suggests an alternative narrative to how these fungi have escaped extinction for so long. Seven meiosis-specific genes (MSG) were found to exist in AMF, indicating that these fungi are likely undergoing a cryptic sexual cycle. The main goal of this research is to determine if/when MSG are expressed in an in-vitro model of AMF. To build onto this research, we established crossings between isolates with hypothetically compatible mating types, in order to determine if fusion of their hyphae can trigger the expression of MSG. Together, these experiments will assess expression at varying stages of the putative cycle of sexual reproduction and give further insight into the elusive sexual life of AMF.

No description available.

No description available.

Soils contain the largest terrestrial pool of carbon (C), but the magnitude and distribution of the soil C sink may be sensitive to climate change. My dissertation aims to identify key processes that mediate patterns of belowground carbon storage across the globe and quantify the effect of environmental perturbations associated with global change on existing soil carbon stocks in peatland ecosystems. Using meta-analysis, I show that the relationship between plant growth, C allocation, and soil nutrient availability varies on a global scale and high-latitude ecosystems allocate >60% of fixed C to belowground structures. As high latitude ecosystems are warming faster than the global mean, the future of this belowground C store is potentially sensitive to climate change. In high latitude ecosystems in particular, I further show that belowground warming increases the rate of peatland carbon dioxide (CO2) and methane (CH4) losses, although CH4 emissions are more sensitive to warming than CO2 emissions, which is likely to shift the nature of greenhouse gas emissions and increase the importance of CH4 as a radiative forcing agent in the near-term. I also use a natural peatland water table gradient to identify the effect of water table reduction on peatland C and N cycling and find that microbial community shifts in C and N demand may attenuate production of C-degrading enzymes and C mineralization in the presence of plant roots and in areas with low water tables. Together, my dissertation work highlights the important role of belowground plant and microbial processes in high latitude ecosystems, and identifies the potential influence of factors associated with global change on belowground C and nutrient cycling.

Arbuscular mycorrhizal fungi (AMF) are an ancient group of obligate symbionts, colonizing the roots of over 72% of land plants, increasing the uptake of nutrients from the soil, and providing many fitness benefits to their host plants. The multinucleate and coenocytic nature of AMF have interested researchers for decades, leading to many theories of the evolution, and genetic organization of these organisms. Recent findings propose that AMF carry two types of strains, identified based on putative MAT-loci, as either homokaryotic, carrying multiple and genetically similar nuclei, or dikaryotic with co-existing nuclei deriving from two parental strains. In other fungi, hyphal fusions, or anastomosis, between compatible strains results in nuclear transfer, creating heterokaryotic spore progeny. It has been hypothesized that dikaryotic AMF strains arose from the anastomosis between compatible homokaryons harbouring different nucleotypes. The goal of this research is to determine whether anastomosis events, known to occur in other fungi, like homokaryon-homokaryon, homokaryon-dikaryon (Buller phenomenon), and/or dikaryon-dikaryon nuclear exchanges also occur in AMF. To achieve this, the anastomosis frequencies between 15 crosses of homokaryotic and dikaryotic strains of the model AMF species Rhizophagus irregularis were examined using microscopy and droplet digital PCR (ddPCR) to determine if nuclear transfer between strains is possible. Overall, these experiments build on the existing evidence of compatible interactions between strains of R. irregularis.

The mutualistic associations between Arbuscular mycorrhizal (AM) fungi and plant roots are ancient and ubiquitous across the plant kingdom, where AM fungi provide Phosphorus, Nitrogen, and water to the plant, and receive photosynthetically fixed Carbon in the form of fatty acids and sugars in return. Moreover, AM fungi are associated with increased plant resistance to both abiotic and biotic stressors such as drought and viral pathogens. Frequently used in agriculture, AM fungi are observed to increase crop yields and decrease chemical fertilizer needs for many economically important plant species. The potential to increase AM fungal effectiveness remains a driving force for current research. To determine their role in establishing and/or supporting AM symbiosis, we propose a reverse genetic study of two genes in the model legume Medicago truncatula. Based on RNA sequencing data indicating increased expression during AM symbiosis, we selected one gene that encodes for NAC TF-like protein, which belongs to a large family of plant transcription factors primarily involved in regulating the secretion of defence hormones. The second gene selected, PALM1, was recently discovered to play a role in the regulation of the trifoliate leaf structure of M. truncatula. We hypothesize that the genes under study play mechanistic roles in regulating AM fungal symbiosis and that we will observe a difference between the colonization rates of corresponding gene mutants and control groups. Firstly, we explored the involvement of the PALM1 and NAC TF genes by examining the root developmental phenotype of Medicago truncatula mutants. Secondly, we employed symbiosis assays to investigate the colonization rates of the genes in question. Results indicated that the NAC TF gene had no consistent role in the AM symbiosis, while the PALM1 gene revealed promising results, where significant increases in colonization rates were observed in PALM1 mutants throughout repeated experiments. Future research involves using this study to help pursue more effective ways to use AM fungi symbiosis in sustainable agro ecosystems.

Arbuscular mycorrhizal (AM) fungi are root and soil inhabiting symbionts with higher plants. The fungi are especially nutrient-function efficient in nutrient deficient soils. There have been innumerable studies of AM fungal facilitation of plant nutrient uptake in controlled environments. Comparatively little similar investigation has been undertaken in natural soils, including investigation of taxon specific nutrient-function efficiency in the phylum. Plant diversity and frequency, soil chemistry statuses, and AM spore diversity and abundance were sampled in an interrupted-belt transect in an aggrading dune sytem on west-coast India, followed by foredunes and transect nutrient amendment experiments in selected plant species. The transect extends 175 m inland from mean high-water mark (MH-WM). Examination showed nutrients were consistently deficient. A plant zonation pattern and increasing frequency over the transect were indicated, as well as decreasing pH and increasing organic matter (OM)-amendment AM species diversity gradients. Plant zonation does not correlate with soil chemistry. There was a distinct soil transition at the 175 m point and evidence of further system partition between foredune and behind-foredune regions. Plant and AM demographies bore no resemblance suggesting neither is driven by the other. Four AM genera were recovered, Acaulospora, Gigaspora and Scutellospora in high abundance, Glomus in comparatively low abundance. The two co-dominant species, A. spinosa and Gi. margarita, displayed divergent strategies in OM amendment. Certain AM taxa may be functionally associated with particular soil nutrients. There was no evidence of taxon-specific nutrient-function efficiency.

The work in this thesis aimed to demonstrate the contribution of arbuscular mycorrhizal (AM) fungi to the yield and fruit quality of field-grown processing tomatoes, and the potential to increase the sustainability of tomato production through greater fertiliser use efficiency by inoculating tomato seedlings with beneficial AM fungi. Previously, the conclusion that tomato growth is unresponsive to AM colonisation, particularly in high-P soils, has often been based on only a part of the tomato life-cycle. However, there is increasing evidence that that positive AM yield responses can occur in soils with relatively high plant-available P, and that AM responsiveness of tomato during vegetative growth may be a poor predictor of reproductive growth. A preceding industry study found that AM colonisation of field-grown processing tomatoes was very low, mostly less than 5%. The reason for the low colonisation was unclear since previous studies have shown that tomato can become relatively highly colonised by AM fungi. It was not known if farm practices, such as soil cultivation and chemical sterilisation, which have been shown to decrease AM colonisation of tomato and other crops, could have contributed to the low colonisation. Furthermore, it was unclear what contribution AM fungi were making to the yield and fruit quality of tomato in commercial production, and what their potential contribution might be if greater AM colonisation could be achieved through inoculating seedlings. Yield and fruit quality are important to tomato growers as both are used to calculate payment when the fruits are sold. Large amounts of soluble fertilisers, particularly P, are applied during tomato production with the aim of increasing yield and quality. However, fertiliser use efficiency, particularly P, on tomato farms has been identified as being low, and needing to be improved in order to increase the economic and environmental sustainability of tomato farming. Increasing P, and also other nutrients, such as Zn and Ca, in tomatoes could also help to improve agricultural sustainability by alleviating human malnutrition in developing countries and, in the case of Ca, have the potential to reduce blossom end rot, which can severely reduce marketable yield. There is considerable potential for AM fungi to assist in the supply of these nutrients to field-grown tomatoes. AM fungi are widely accepted to increase plant uptake of P. This has mostly been demonstrated in low-P soils, as increases in plant-available P are generally known to be detrimental to AM colonisation and any subsequent growth effects. However, there is increasing evidence of the ability of AM fungi to increase P uptake and yield even in high P soils. There is also good evidence of increased Zn uptake by mycorrhizal supply to plants. Evidence for increased Ca uptake in mycorrhizal plants is in comparison limited and conflicting, but has been demonstrated in some cases. It is possible that AM fungi could allow applications of these nutrients, particularly P, to be reduced while maintaining or increasing fruit yield and quality. However, the ability of indigenous or inoculated AM fungi to do so in the relatively high-P farm soils used in this project was unknown. In order to address these uncertainties a series of pot studies and a field experiment were conducted using field soils from tomato farms and an adjacent nature reserve for comparison. Data on soil characteristics from five farms, collected during the previous industry study, was analysed in conjunction with data from another farm located nearby with contrasting soil properties. Two farm soils and an unfarmed comparison were selected on the basis of their having contrasting levels of P, Zn and Ca, and pH, with the constraint that they were located within 50 km of each other to minimise travel time in the study area. The two farmed soils had a relatively high concentration of plant-available P (103 and 58 mg/kg Colwell), while plant-available P in the unfarmed soil was probably marginal to that required for healthy tomato growth (27 mg/kg Colwell). Samples of the soils were taken soon after commencement of the work and used in pot studies. Firstly, a bioassay was conducted to establish the ability of tomato to become colonised in the three field soils. AM colonisation of tomato and medic, which is known to be highly susceptible to AM colonisation, was compared between three harvests over an approx. 16 week period. Vegetative growth was also measured. The total colonisation of tomato mostly did not differ from that of medic at each harvest in any soil. Furthermore, despite the large differences in plant-available P between the three soils, colonisation and vegetative growth of tomato did not differ between soils at any harvest. In a subsequent pot experiment, the effect of colonisation by AM fungi in the three field soils on the vegetative and reproductive growth, and nutrient status of tomato was determined using the tomato mutant rmc (reduced mycorrhizal colonisation) and its progenitor 76R. A number of non-destructive vegetative and reproductive growth measurements were repeatedly measured over an approx. 24 week period. Destructive measurements were carried out at two harvests, 39 and 164 days after planting. Tomato 76R was again well colonised in all soils. Tomato rmc remained uncolonised, and was therefore an effective non-mycorrhizal control. AM colonisation had little effect on plant growth or nutrient status in any soil at the first harvest, but significant growth and nutrient responses were recorded at the second harvest. In particular, AM colonisation markedly increased vegetative growth in the unfarmed soil. AM colonisation did not affect vegetative growth in either of the farmed soils. However, AM colonisation increased reproductive growth, particularly yield over time, in all soils. AM colonisation increased shoot P concentration and content, but effects on Zn were mixed and largely inconclusive. Shoot Ca concentration and content were mostly reduced by AM colonisation. Similar patterns were observed in fruit nutrient status. The potential of pre-inoculation with AM fungi to increase AM colonisation and/or AM growth and nutrient effects in the field was considered. A commercial AM fungal inoculum was initially proposed for use, but was found to be unreliable and laboratory cultures of Scutellospora calospora and Glomus mosseae were used instead. Tomato seedlings were inoculated by amending a commercial seed-raising medium with an equal mixture of S. calospora and G. mosseae inocula. Seeds of tomato rmc, 76R and the commercial processing tomato cultivar U941 were sown and raised according to the practices followed by a commercial seedling nursery. After 9 weeks a sub-sample of inoculated seedlings of 76R and U941 had become colonised by both AM fungi, although the total colonisation was relatively low (approx. 10%). There was no difference in the shoot or root dry weights between inoculated and non-inoculated seedlings. The remaining seedlings were then used in the field experiment. Seedlings were transplanted amongst a commercial processing tomato crop on two farms and grown to maturity. A substitute farm with soil of moderate P (66 mg/kg Colwell) was used as tomatoes were no longer being grown on the initial farm with moderate P. Two P treatments, ‘normal’ and ‘reduced’ P fertilisation, were imposed in order to investigate the effect of P fertilisation on colonisation by indigenous and inoculated AM fungi, and growth and nutrient status of tomato in the field. Non-destructive growth measurements and soil core samples to assess mycorrhizal colonisation were taken mid-season (approx. 10 weeks after transplanting). Destructive growth measurements and core samples to assess colonisation were taken at harvest (approx. 19 weeks after transplanting). Colonisation of rmc was insubstantial and it again served as an effective non-mycorrhizal control to 76R. Colonisation was insubstantial in all treatments on the farm where soil had moderate plant-available P. On the other farm, where soil had relatively high plant-available P, colonisation of all plants was low mid-season, but was mostly substantial (>20%) in 76R and U941 at harvest. Low colonisation on both farms was probably the result of farming practices, particularly soil cultivation. However, a combination of inoculation and reduced P fertilisation increased colonisation. Colonisation by indigenous AM fungi had no effect on the growth or nutrient status of field grown tomatoes. In contrast, pre-inoculation with AM fungi increased fruit yield by a mean of approx. 40% in 76R and U941. This was the result of an 18% increase in the fresh weight of individual fruits and, when inoculation was combined with reduced P fertilisation, a 21% increase in the number of fruits on each plant. The increase in the number of fruits on each plant was associated with an increase in the number of flowers at the most advanced growth stage. Inoculation also increased vegetative growth, and fruit P, Zn and Ca contents. A small (4%) decrease in fruit brix was more than offset by increased yield. This study has shown that while AM fungi indigenous to tomato farm soils have the ability to substantially colonise tomato, they appear to have little effect on tomato growth, yield or nutrition in the field. In contrast, inoculation of tomato seedlings with mutualistic AM fungi during nursery production can substantially increase the growth, yield and fruit nutrient contents of field-grown tomatoes under commercial conditions. This increase could also be enhanced by a reduction in P fertilisation. Increased yield and fruit nutrient contents, and decreased P fertilisation neatly address the aims of increased agricultural sustainability. Incorporating pre-inoculation of tomato into existing farming practices has a potential to increase the productivity and sustainability of processing tomato production worldwide. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1292847 / Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2007