A functional water-conducting system, the tracheary elements of the xylem, is required to sustain plant growth and development. Tracheary element formation is dependent on many biological processes terminated by programmed cell death and cellular autolysis. The final two processes are probably dependent on the activity of hydrolytic enzymes such as XCP1 and XCP2 known to be expressed in tracheary elements during these final two processes. Thus, the transcriptional regulation of XCP1 and the function of XCP2 were investigated. Qualitative and quantitative assessments of GUS activity as directed by various fragments of the XCP1 promoter showed that a 237-bp internal region was able to drive GUS expression in a tracheary element-specific manner in Arabidopsis. A 25-bp deletion at the 3' end of this region abolished GUS expression. The 237-bp region served as bait in a yeast one-hybrid analysis. Screening of yeast colonies retrieved 109 putative positive interactions, which included a potential transcriptional regulator, indole acetic acid-induced protein 8 (IAA8). An auxin responsive element that potentially binds auxin responsive transcription factors was found within the 25-bp deletion. Cis-elements were predicted by Genomatix and Athamap computer programs. The cis-elements form pyrimidine and gibberellic acid responsive elements that can potentially bind Dof and Myb transcription factors, respectively. In an independent effort, attempts to develop a mapping population to isolate upstream regulators of XCP1 expression did not succeed. Functionally, tracheary element-specific expression of XCP2 in Arabidopsis suggested a specialized role for XCP2 in final phases of tracheary element differentiation. The function of XCP2 was assessed using T-DNA insertional mutants, post-transcriptional gene silencing, and through tracheary element-specific expression of the cysteine protease inhibitor, soyacystatin N in Arabidopsis. Our findings revealed that the absence of XCP2 expression due to T-DNA insertional mutagenesis did not affect plant growth and development in the laboratory. Soyacystatin N was an effective in vitro inhibitor of cysteine proteases. Plants expressing 35S-driven cytosolic form of soyacystatin exhibited stunting and reduced apical dominance. Plants expressing pXCP1-driven cytosolic soyacystatin did not differ from wild type plants. Additionally, transgenic plants expressing pXCP1- and 35S-directed XCP2-double-stranded RNA for the silencing of XCP2 showed no unusual phenotypes compared to their wild type counterparts / Ph. D.

The process of xylem tracheary element differentiation involves the coordination of vascular cambium activity, cell fate determination, cell expansion/elongation, secondary wall synthesis, programmed cell death, and cellular autolysis. The end result of tracheary element differentiation is a cellular corpse lacking a protoplast and consisting of a thickened cell wall composed mostly of lignin and cellulose. Little is known about the genetic mechanisms regulating the process of tracheary element differentiation. XCP2 expression localizes to tracheary elements according to two independent methods of analysis: promoter reporter experiments and immunogold localization by electron microscopy. XCP2 may be involved in catalyzing the degeneration of the protoplast during the final autolytic stages of tracheary element differentiation. To this date XCP2 function has not been directly demonstrated. In principle, any tracheary element-specific markers can be linked to upstream regulatory genes with roles in tracheary element differentiation. To develop the XCP2 promoter as a tool for identification of transacting factors, a promoter deletion analysis was carried out. Utilizing information from 5â and 3â deletion constructs, a 70-bp region upstream of the XCP2 translational start site is both necessary and sufficient for TE-specific expression of the UidA reporter gene. Mutational analysis of the ACTTTA element at position -113-bp strongly suggests it is a cis element required for XCP2 expression. In silico analysis of an 18-bp promoter region located within 200-bp of the translation start site and including the ACTTTA element revealed high indentity shared between xylem-specific XCP2 homologs from Zinnia elegans, Populus trichocarpa, and XCP1 from Arabidopsis thaliana. / Master of Science

A biochemical characterization of ubiquitin (Ub)-proteasome pathway activity was conducted in <I>Zinnia</I> mesophyll cell cultures to examine potential differences between differentiating cells of tracheary element (TE) cultures and non-differentiating cells of control cultures. The pathway is highly active throughout development of differentiating TEs, a programmed cell death (PCD) process during which the majority of cellular proteins and biochemical processes are expected to be down-regulated in activity and/or expression. Addition of the proteasome inhibitors <I>clasto</I>-lactacystin Beta-lactone (LAC) and carbobenzoxy-leucinyl-leucinyl-leucinal (LLL) at culture initiation prevented TE differentiation in this system. Proteasome inhibition at 48h did not alter the final percentage of TEs compared to controls. However, proteasome inhibition at 48 h delayed the differentiation program by approximately 24 h, as indicated by examination of morphological markers and the expression of putative autolytic cysteine proteases.These results suggest that proteasome activity is required both for induction of TE differentiation and for progression of the TE program in committed cells. Treatment at 48 h with LLL resulted in partial uncoupling of autolysis from differentiation. Results of protease activity gel analysis suggest that incomplete autolysis was due to the ability of LLL to inhibit TE cysteine proteases. A characterization of phytohormone-stimulated growth of non-differentiating cultured <I>Zinnia</I> cells is also presented. Differential effects on radial cell expansion versus cell elongation were observed for the four plant growth regulators examined. Auxin (naphthaleneacetic acid, NAA) and a brassinosteroid (2,4-epibrassinolide, BI) stimulate only cell elongation. Cytokinin (N-6-benzyladenine, BA) has a greater effect on growth in cell girth rather than length. Gibberellic acid (GA₃) has equivalent effects on expansion in both dimensions. These results demonstrate that radial cell expansion and cell elongation can be uncoupled, and therefore, may be controlled by different mechanisms. Additionally, this study establishes the utility of <I>Zinnia</I> suspension cultures as a valuable model for studies of cell expansion. Finally, we modified <I>Arabidopsis</I> plant growth conditions to promote proliferation of secondary tissues, permitting the separation of secondary xylem from bark (phloem plus nonvascular) tissues using hypocotyl-root segments. Dissected tissues were used for semi-quantitative and quantitative RT-PCR and for the construction of bark and xylem cDNA libraries for PCR-based screening of several Ub pathway components, including Ub-conjugating enzymes (<I>UBCs</I>), deubiquitinating enzymes (DUBs), and an Alpha (<I>PAF1</I>) and Beta (<I>PAF1</I>) subunit of the proteasome. All targeted <I>UBC</I> families, candidate <I>UBCs</I> and DUBs, and proteasome subunits are expressed in secondary xylem and bark in this system. / Ph. D.

Secondary xylem, commonly known as wood, is a valuable commercial commodity. Among the major components of wood are the elongated, thick-walled water-conducting cells known as tracheary elements. Understanding tracheary element differentiation and maturation is of scientific and commercial importance as it may lead to broad understanding of cellular differentiation processes as well as ways to increase both the quality and quantity of wood produced by economically important tree species. One way to begin to understand the regulation of tracheary element differentiation is to identify elements that control expression of genes associated with tracheary elements. In Arabidopsis thaliana, Xylem Cysteine Protease 1 (XCP1) is specifically expressed in tracheary elements where it catalyzes microautolysis. Thus XCP1 can serve as a useful model for identifying factors that regulate tracheary element-specific gene expression. A deletion analysis of the XCP1 promoter was conducted to identify promoter elements that are necessary and sufficient for tracheary element-restricted gene expression. Two regions required for tracheary element-specific gene expression were identified. One of these was assembled as a multimeric bait construct and used in yeast one-hybrid assays to identify candidate transcription factors that bind to the XCP1 promoter region. Subsequently, a southwestern blot analysis was used to identify transcription factors displaying specific binding to a previously reported cis-element, CTTCAAAGCCA, found in the XCP1 promoter and other tracheary element-associated genes from multiple species. / Master of Science

Cysteine proteases are associated with xylem tracheary element differentiation. XCP1 was recently identified as a xylem-specific cysteine protease in Arabidopsis (Zhao, et al., 2000). For this study a recombinant polyhistidine-tagged XCP1 (XCP1H6) was expressed and purified from an E. coli expression system. A polyclonal anti-XCP1 antibody was produced using purified XCP1H6. Immunoblot analysis of a developmental time course of xylem and bark protein extracted from root-hypocotyl segments demonstrated that XCP1 was expressed in xylem only. Further analysis under optimized immunoblot conditions, however, revealed that anti-XCP1 antibody reacted with protein present in both xylem and bark. The vast majority of immunoreactivity, however, was restricted to xylem. Cell-type localization of GUS expression under the control of a putative XCP1 promoter indicated that the XCP1 promoter specifies expression of XCP1 in tracheary elements in leaves, stems, roots and flowers. XCP1 promoter-driven GUS activity was not associated with senescing tissues. / Master of Science

The evolutionary success of land plants was fostered by the acquisition of the xylem vascular tissue which conducts water and minerals upwards from the roots. The xylem tissue of flowering plants is composed of three main types of cells: the sap-conducting tracheary elements (TE), the fibres which provide mechanical support and the parenchyma cells which provide metabolic support to the tissue. Both the TEs and the fibres deposit thick polysaccharidic secondary cell walls (SCWs), reinforced by a rigid phenolic polymer called lignin. The cell walls of TEs form efficient water conducting hollow tubes after the TEs have undergone programmed cell death (PCD) and complete protoplast degradation as a part of their differentiation. The work presented in this thesis studied the regulation of TE PCD by characterizing the function of the candidate PCD regulator METACASPASE 9 (MC9) in Arabidopsis thaliana xylogenic cell suspensions. These cell suspensions can be externally induced to differentiate into a mix of TEs and parenchymatic non-TE cells, thus representing an ideal system to study the cellular processes of TE PCD. In this system, TEs with reduced expression of MC9 were shown to have increased levels of autophagy and to trigger the ectopic death of the non-TE cells. The viability of the non-TE cells could be restored by down-regulating autophagy specifically in the TEs with reduced MC9 expression. Therefore, this work showed that MC9 must tightly regulate the level of autophagy during TE PCD in order to prevent the TEs from becoming harmful to the non-TEs. Hence, this work demonstrated the existence of a cellular cooperation between the TEs and the surrounding parenchymatic cells during TE PCD. The potential cooperation between the TEs and the neighbouring parenchyma during the biosynthesis of lignin was also investigated. The cupin domain containing protein PIRIN2 was found to regulate TE lignification in a non-cell autonomous manner in Arabidopsis thaliana. More precisely, PIRIN2 was shown to function as an antagonist of positive transcriptional regulators of lignin biosynthetic genes in xylem parenchyma cells. Part of the transcriptional regulation by PIRIN2 involves chromatin modifications, which represent a new type of regulation of lignin biosynthesis. Because xylem constitutes the wood in tree species, this newly discovered regulation of non-cell autonomous lignification represents a potential target to modify lignin biosynthesis in order to overcome the recalcitrance of the woody biomass for the production of biofuels.