No description available.

Understanding and addressing conflict between farmers and wildlife due to crop raiding is of increasing conservation concern. Raiding impacts farmers’ livelihoods, reduces tolerance to wildlife and often results in lethal methods of retaliation. Although crop raiding occurs on commercial as well as subsistence farms, there are very few quantitative accounts of on-farm primate behaviour or techniques to deter primates from raiding commercial farms. Working in partnership with commercial crop farmers, this study was conducted in Blouberg Municipality, South Africa. Using systematic behavioural observations, camera trapping techniques, vegetation transects, interviews and a workshop, this research adopts an interdisciplinary approach to examine farmers’ perceptions of nature, behaviour of primates, and crop damage by other wildlife to understand the nature and extent of crop raiding. This information was used to develop and evaluate effective and locally appropriate deterrents to wildlife crop raiding. The farmer-baboon relationship is complicated and filled with ambiguity. Farmers are happy to see baboons in the wild, but on the farm baboons are not welcome. High population numbers and the inability to control baboons are particular concerns for commercial farmers. Baboons were the dominant raiders, whose rates of raiding were influenced most by natural food availability. Vervet monkey raiding was also frequent and was influenced by the presence of baboons on the farm. In addition to primates, 18 other wildlife species were observed within crop fields. Farmers’ perceptions were influenced by duration of raiding, average group size and overlap between farmer activity and crop raiding. Farmers underestimated crop loss to wildlife, but were able to accurately estimate where most damage occurs. The use of bells as an alarm system was not effective at alerting field guards to the presence of vervet raiders. Motion-activated sounds were effective at reducing baboon raiding for a short time, but baboons soon habituated. Electric fencing was effective at keeping most wildlife out of crop fields. The information obtained throughout the thesis was used to provide recommendations to commercial crop farmers to reduce crop raiding by wildlife.

In this project, plant-pathogen-, plant-salt-, pathogen-salt-, plant-pathogen-salt interactions were studied. The interactions between tomato and lucerne and isolates of the vascular wilt fungus <i>Verticillium albo-atrum</i> were investigated under non-saline and saline conditions. Pathogenicity trials indicated that isolates of V1 and V2 were pathogenic to tomato, however, the degree of pathogenicity of the isolates were affected by the particular cultivar, plant age, method of inoculation and temperature. The pathogenic effects of <i>Verticillium </i> on tomato did not increase with increasing spore concentration or when isolates were co-inoculated. NaC1 both delayed and reduced the germination rate of seeds and caused severe symptoms on plants. The effect of NaC1 was also detrimental on the germination and the growth rate of the fungus. Progress of the disease was markedly accelerated by salt stress. Various growth parameters were lower in the group of plants both inoculated with <i>V. albo-atrum </i>& treated with NaC1 than they were for plants inoculated only with <i>V. albo-atrum</i> or only treated with NaC1. As a defence response, an increase in PAL activity occurred when lucerne cells were treated with an elicitor from <i>V. albo-atrum; </i>a further increase was evident when cells were exposed to both elicitor and NaC1. However, the increase in PAL activity was minimised both at high concentrations of elicitor and NaC1. Disease-resistant and salt-tolerant plants showed resistance to V1 under non-saline and saline conditions; however, disease-susceptible and salt-tolerant plants did not show resistance to V1 under those conditions. Development of salt tolerant crops should therefore, involve selection of strains that maintain or improve its resistance to pathogens.

No description available.

No description available.

As global population expands, so does its demand for food. This will require vast amounts of plant-derived calories. This increased pressure means it is vital that we do more with less; less land, less water, less chemical input and less labour. One major threat to crop production is posed by plant pests and pathogens. In particular, filamentous plant pathogens – oomycetes and fungi – are among the most devastating organisms known to agriculture. With their rapidly evolving genomes which specialise in breaking plant immune systems and chemical control methods, control of these pathogens is becoming extremely problematic. Especially as they often encounter crops in monoculture. An often overlooked potential strategy for developing pathogen resistance is manipulation of the host targets of plant pathogen effector proteins. In order to manipulate such targets to condition enhanced resistance, one must first develop a sound understanding of the interaction between effector and target, the target’s function in planta and which processes are being perturbed by the interaction. A previously discovered interaction between a P. infestans RXLR effector and a host MAPK known to be a positive regulator of immunity, where the effector acts to perturb positive regulation of immunity, is used here to investigate the possibility of manipulating a host target for enhanced immunity. Effector-insensitive variants were produced and characterised, prior to expression in CRISPR-edited tomato plants. We tentatively suggest that at least one variant may condition enhanced resistance. Another interactor of the same effector protein was investigated for its role in immunity and evidence suggesting it may function as a helper of the effector is presented. Additionally, the expression, purification and crystallisation of native and heavy atom derivatives of a candidate effector protein from B. graminis f.sp. hordeii is demonstrated.

Myzus persicae is one of the most successful insects on the planet. It is the world’s most pesticide-resistant insect, feeds on hundreds of plant species and acts as a vector for over 100 viruses. Upon perception of M. persicae feeding, plants activate pattern-triggered immunity (PTI), a pivotal part of which is believed to be calcium signalling. The aim of this thesis is to uncover the role that calcium signalling might be playing in the interaction between M. persicae and one of its hosts: the model plant Arabidopsis. Using a fluorescent calcium sensor (GCAMP3), in vivo imaging of calcium dynamics was performed on leaves infested with M. persicae. There is a rapid and highly localised calcium burst around the feeding site in the epidermal and mesophyll cells, making it as one of the first plant responses to aphid attack. This calcium burst is triggered after perception of the aphid by the defence co-receptor BRASSINOSTEROID INSENSITIVE-ASSOCIATED KINASE 1 (BAK1), establishing it as part of PTI. Calcium is released from the extracellular space into the cell by GLUTAMATE-LIKE RECEPTORS 3.3 and 3.6 (GLR3.3 and GLR3.6), in combination with the release of intracellular calcium from the vacuole by TWO-PORE CHANNEL 1 (TPC1). Loss of BAK1, GLR3.3/GLR3.6 or TPC1 significantly attenuates the aphid-induced calcium burst and has an effect on the induction of anti-aphid defence responses. Downstream of the burst, CBL-INTERACTING PROTEIN KINASES 3, 9, 23 and 26 are activated by calcium and together mediate plant resistance to aphid attack. Furthermore, the M. persicae effector Mp10 partially suppresses the feeding site calcium burst, suggesting that the aphid is manipulating this pathway as part of its successful colonisation of the plant. Together, the data presented in this thesis provide evidence for the significant involvement of calcium signalling in the plant response to aphid attack.

This study elucidated the relationship between the potato cyst nematode, Globodera pallida and the peach-potato aphid, Myzus persicae, two pests that can infect the potato plant and often so simultaneously. The biological, biochemical and molecular responses of potato plants suffering nematode and aphid stress were characterised both singly and in combination. It was established that the reproductive success of aphids was significantly greater on potato plants pre-infected with nematodes compared to non-infected control plants. Endogenous levels of salicylic acid (SA) increased systemically in the leaves of potato plants following nematode and aphid infection singly with a corresponding increase in expression of the SA-mediated marker gene PR-5. Measurements of endogenous levels of jasmonic acid (JA) as well as the expression of JA-mediated defence genes increased in plants infected with aphids but were suppressed when plants were co-infected with both nematodes and aphids. This suppression of JA in co-infected plants shows a different and unique response to that found when the plant was infected with either pest in isolation. These results may indicate antagonistic crosstalk between molecular pathways in the plant following infection of the plant with G. pallida and M. persicae. M. persicae had a negative amensalism effect on G. pallida below-ground whereby pre-infestation of potato plants with M. persicae stimulated a significantly lower cumulative hatch of second-stage juveniles (J2s) when cysts of G. pallida were incubated in root diffusates from aphid-infested plants. A dose-dependent response was responsible for this with the presence of 50 aphids on the foliar parts of the plant causing a significant reduction in the emergence of J2s from cysts. Sequence of arrival experiments showed that pre-infestation of M. persicae significantly reduced the infection rate of G. pallida. This work comprises the first study into dual nematode and aphid attack in the potato crop and the study highlights how multiple stresses elicit a unique molecular response to attack compared to singly stressed plants.

No description available.

The plant root system is crucial for plant survival, growth and development, and it plays an important role in plant resistance to drought stress. Drought is one of the primary factors that restrict plant growth and yield, and its threat to crop yields will increase along with the growing food demand by the population of a world experiencing a changing climate. In response to drought in plants, various hormones are vital regulators, because they are able to manipulate plant development and in some cases minimise the adverse impact of drought. Therefore, understanding how the plant root system will adapt to a soil drying challenge is crucial. Of particular importance is the plant response to a non-lethal drought stress, which is often encountered in the field. Elucidation of the mechanisms underlying such responses, including hormonal regulations, may help crop scientists improve the plant performance under drought. A six-day progressive soil drying pot experiment was designed to examine the synchronisation of physiological responses in maize (Zea mays L.) roots and leaves during soil drying. It was found that maize roots showed earlier responses to soil drying than leaves in changing growth rates, water potentials and hormone levels. Root growth was stimulated at soil water content of 25−32% (ca. 41% in well-watered pots), while both root growth and leaf elongation were inhibited when soil water content was below 20%. Root abscisic acid (ABA) level gradually increased when soil water content was lower than 32% during soil drying. The stimulation and inhibition of root growth during soil drying may be regulated by root ABA, depending on the degree of the concentration increase. The ethylene release rates from leaves and roots were inhibited during soil drying, which occurred later than the increase in ABA levels. In a subsequent root phenotyping study on 14 maize genotypes, significant genetic variation was observed in root angle and size (root length, surface area and dry weight), and in the plasticity of these traits under mild and severe drought stress. Genotypes with a steeper root angle under well-watered conditions tended to display more promotion or less inhibition in root size under drought. Further analysis showed that combined traits of maize root angle, its plasticity and the root size plasticity under drought may be a better predictor for maize drought resistance than a single one of these traits. Moreover, root angle was found positively related to the leaf and root ABA levels and negatively related to the root tZ (a cytokinin) level under well-watered conditions. In another study on the crosstalk of drought-related hormones using the model plant Arabidopsis thaliana L., the biphasic responses of root elongation to ABA were confirmed, i.e. low external ABA concentrations stimulated root growth while high ABA concentrations inhibited it. Furthermore, ethylene and auxin were found to be involved in these responses. The inhibitory effect of high ABA levels on root growth was reduced or even eliminated when Arabidopsis was chemically treated to inhibit the ethylene biosynthesis or signalling, or to block auxin influx carriers. This was confirmed using mutants with blocked ethylene or auxin signalling, or a defect in the auxin influx carrier AUX1. On the other hand, the stimulatory effect of low ABA levels on root growth was lost when Arabidopsis seedlings were chemically treated to inhibit the auxin efflux carriers, and in mutants with blocked auxin signalling or with a defect in the PIN2/EIR1 auxin efflux carrier. These results indicate that ABA regulates root growth through two distinct pathways. The inhibitory effect that operates at high ABA concentrations is via an ethylene-dependent pathway and requires auxin signalling and auxin influx through AUX1. The stimulatory effect that operates at low ABA concentrations is via an ethylene-independent pathway and also requires auxin signalling and auxin efflux through PIN2/EIR1. This research contributes to our understanding of the responses of plant root system to different degrees of non-lethal drought stress, and it highlights the importance of root traits that may be important to plant drought resistance. The potential involvement of hormones (ABA, ethylene, auxin and cytokinin) in these processes is clarified. The knowledge gained may be integrated in novel crop management strategies to plan irrigation and help in the development of drought resistant crop varieties.