Many environmental stresses result in increased generation of active oxygen species (AOS) in plant cells, leading to the induction of protective mechanisms. In this study, signalling components linking AOS perception to downstream responses were examined, with particular emphasis on H₂O₂ signalling. All AOS investigated had an early [Ca²⁺]_cyt peak in common, but differed in other aspects of their Ca²⁺ signatures, indicating that the plant is able to discriminate between different types of AOS. An early event in AOS signal transduction may involve changes in the cellular redox balance as reduction of glutathione levels prior to stress application increased the height of the first [Ca²⁺]_cyt peak. Inhibiting or enhancing the height of the H₂O₂-triggered Ca²⁺ signature lead to inhibition or enhancement of GST1 and APX1 induction, respectively, demonstrating that the Ca²⁺ signature is required for induction of genes encoding antioxidant enzymes. OX1, encoding a putative ser/thr kinase, was shown to be involved in signal transduction in response to H₂O₂-generating stresses. Transcript levels of OX1 were increased upon treatment with H₂O₂ and a range of abiotic and biotic stresses as well as ABA, all of which have been shown to result in H₂O₂ accumulation. Inhibition of stress-induced [Ca²⁺]_cyt elevations inhibited OX1 induction, placing the OX1 kinase downstream of Ca²⁺ in the signalling chain. OX1 is required for full activation of AtMPKS and AtMPK6 in response to ozone fumigation, indicating that OX1 functions upstream of these MAP kinases. An ox1 null-mutant displayed enhanced susceptibility to infection with a virulent Peronospora parasitica isolate as well as reduced induction of several defence genes. In addition, the ox1 mutant exhibited shorter root hairs and an early flowering phenotype. AOS treatment induced several genes encoding AtERF transcription factors, but did not have an effect on other members of this family. Induction occurred in an ethylene-independent but Ca²⁺-dependent manner.

During the present studies we attempted to answer the following questions: (i) Does low temperature alter the phosphorylation level of proteins in isolated nuclei? (ii) Does the nuclear phosphoprotein population change during a prolonged exposure of seedlings to cold? (iii) Do heat-stable proteins accumulate in the nucleus during a prolonged exposure of seedlings to cold? (iv) Are the answers to the above three questions related to freezing tolerance? A possible relationship between the observed cold-induced changes in phosphoproteins and the level of freezing tolerance was explored by comparing the results of experiments conducted on two cultivars (Apica and Trek) of alfalfa (Medicago sativa L.) which markedly differ in their capacity for cold acclimation. / We show that the phosphorylation level of several nuclear proteins is subject to rapid and reversible enhancement by low temperature. Several phosphoproteins were found to be constitutively present in the nucleus of both cultivars. The cold-induced stimulation of the phosphorylation of many of these proteins was much greater in the relatively freezing tolerant cultivar Apica than in the relatively freezing sensitive cultivar Trek. Population of nuclear phosphoproteins was found to be considerably more complex in Apica than in Trek. During a prolonged exposure of the seedlings to 4$ sp circ$C, additional phosphoproteins were imported into the nucleus of Apica seedlings but not those Trek. / Some heat-stable proteins were constitutively present in the nucleus of both cultivars. However during the 4-day cold treatment, a large accumulation of several additional heat-stable proteins was observed in the tolerant, but not the sensitive, cultivar. (Abstract shortened by UMI.)

Plants possess highly sensitive perception systems by which they recognize signal compounds originating from microbes. These molecular cues play an important role in both symbiotic and pathogenic relationships. Establishment of the soybean (Glycine max)-Bradyrhizobium symbiosis is orchestrated by specific signal molecules exchanged between appropriate plant and microbe partners: flavonoids as plant-to-bacteria signals, and Nod factor as bacteria-to-plant signals. How this signaling process interacts with stress conditions (abiotic and biotic) is the subject of this thesis. The abiotic stresses were suboptimal growth temperature, low pH, and salinity. Suboptimal growth temperatures affected the ability of the microsymbiont, Bradyrhizobim japonicum, to perceive nod gene inducers (genistein) and produce Nod factor. Nod Bj-V (C18:1, MeFuc) production by B. japonicum strains 523C and USDA110 was strongly affected by suboptimal growth temperature. Nod factor production declined with temperature, from 28 to 15°C. Strain USDA110 was more affected by decreased temperature than strain 532C. Decreased Nod factor production at low temperature was due to both decreased bacterial growth and lower production efficiency (Nod factor per cell). When a 1:1 mixture of Nod factor Nod Bj-V (C18:1, MeFuc) and Nod Bj-V (Ac, C16:0, MeFuc) was applied to soybean roots, root hair deformation increased as Nod factor concentration increased under stressfully low temperature and low pH conditions. High salinity stress strongly reduced the root hair deformation caused by Nod factor, and increasing the concentrations of added Nod factor did not over come this. Exogenous application of Nod Bj-V (C18:1, MeFuc), from strain 532C, to soybean root systems under two root zone temperatures (RZTs---17 and 25°C) reduced the progression of disease (powdery mildew---Microsphaera difussa) development on soybean leaves; this effect increased with Nod factor concentration and was gr

Thesis (Ph. D.)--University of Texas at Austin, 2004. / Supervisor: Stanley J Roux. Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 2004. / Supervisor: Stanley Roux. Vita. Includes bibliographical references.

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