Effects of temperature and pentachloronitrobenzene on vegetative growth and development of Sclerotinia sclerotiorum (Lib.) de Bary were studied using Newton's synthetic liquid medium. Microconidial growth increased until the eighth day of incubation at 24.4° C., after which maximum dry weights were obtained. . Growth and development of mycelia and sclerotia was most rapid at 24° C., although a similar, but slightly less response was obtained at 20° C. On a dry weight basis, inhibition of mycelial growth and. development of S. sclerotiorum by PCNB was greater in shake culture than stationary culture at room temperature. PCNB was fungistatic at the concentrations used. Pathogenicity of soil-borne inoculum of S. sclerotiorum on Penn Lake lettuce seeds was affected by temperature, inoculum density, PCNB and a Trichoderma sp. The number of emerged seedlings decreased with increasing concentration of soil inoculum and decreasing soil temperature. PCNB applied to U.C. Soil Mix C as a drench treatment not only failed to control disease caused by S. sclerotiorum, but also following use of the fungicide, disease increased appreciably at all concentrations used. Trichoderma sp. reduced disease only at 15° C. and was ineffective at lower soil temperatures. Decreases in height of lettuce seedlings were observed with increasing inoculum density of S. sclerotiorum. / Land and Food Systems, Faculty of / Graduate

RNA decay plays an essential role in the regulation of gene expression during plant development and response to environmental stimuli. The protein DXO is a 5' to 3' exonuclease that functions in RNA degradation and RNA quality control that has been studied in animals. It has not yet been identified in plants. The gene locus At4g17620 in Arabidopsis thaliana encodes a protein homolog of the mammalian DXO, termed AtDXO. Recombinantly expressed AtDXO possesses a 5'-3' RNA exonuclease activity in vitro. Loss-of-function of AtDXO in Arabidopsis generates multiple growth defects, including curled and yellowish leaves, growth retardation and limited fertility, whereas overexpression show no obvious growth phenotype. The development defect of atdxo might be attributed to aberrant RNAs, which are not degraded when AtDXO is dysfunctioning. From the RNA-Seq analysis, the transcriptome pattern of atdxo mutants shows significant disparity from wild-type. Among the differences, the defense response genes are elevated in atdxo while photosynthesis-related and plastid genesis-related genes are downregulated. The constitutive expression of defense response genes causes the autoimmune phenotypes of atdxo. This could be modulated by temperature and is partially dependent on the master immunity regulators EDS1 or NPR1. Reactive oxygen species (ROS) accumulation was also detected in the atdxo mutant, and atdxo showed insensitivity to oxidative stress imposed by paraquat. Moreover, the atdxo mutant is hypersensitive to salt stress but not sensitive to general osmotic stress. In Arabidopsis, the 5'-3' RNA decay pathway could act as a repressor of endogenous post-transcriptional gene silencing (PTGS), which is regulated by small RNAs (sRNA). The mutation of AtDXO caused productions of 24- and 25-nucleotide endogenous sRNAs. The growth defect phenotype of atdxo could not be repressed by dysfunction of the RDR6 (RNA-DEPENDENT RNA POLYMERASE 6)-dependent sRNA biogenesis pathway. These findings demonstrate that AtDXO functions as a 5'-3' exoribonuclease both in vitro and in vivo to regulate plant development and to mediate the response to environmental stresses.

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No description available.

In the context of nutrient cycling in soil, plant-mediated mineralisation of carbon (C) and nitrogen (N) is poorly understood. The broad focus of this thesis was to investigate the potential importance of plant-mediated mineralisation (i.e.priming) on C and N mineralisation in soils with contrasting crop productivities. The studies focus on two soils, which had similar chemical and physical properties but contrasting plant productivities relating to their N supply capacity. These soils were used to investigate the potential importance of priming processes in contributing to the contrasting capacities of these soils to supply N for plant growth. 13C and 15N stable isotopes were used to measure specific gross C and N fluxes. Sole C or N and combined C with N treatments were established to disentangle the effect of carbon and nitrogen availability on plant-mediated mineralisation. The addition of labile C increased gross carbon and nitrogen fluxes from native soil organic matter (SOM) but the effect was soil specific. The addition of nitrogen did not affect SOM mineralisation in either soil. The much lower C-to-N ratio of the 'primed' flux compared to the 'basal' flux indicated that the primed flux utilises different OM pools highlighting that primed and basal mineralisation may be distinct processes. The priming response (i.e. positive or negative and associated mechanisms) was different depending on carbon and nitrogen supply to the microbial community. Overall, the studies in this thesis places the microbial community as the focal point of soil N supply. This data strongly supports the concept that the release of labile carbon from plant roots functions as a nutrient acquisition response, increasing mineralisation of SOM. From the data a conceptual model of priming mechanisms, based on nutrient availability to the microbial community, was established. This could be used as the foundation to develop key concepts for sustainable agricultural practice.

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Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: advanced life support; ethylene; carbon dioxide; pisum sativum; root hypoxia; oxygen; bioregenerative life support. Includes bibliographical references (p. 53-57).

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No description available.