The Role of the Ubiquitin-Proteasome Pathway During Xylem Differentiation in Zinnia elegans Mesophyll Cells and Arabidopsis thaliana
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A characterization of phytohormone-stimulated growth of non-differentiating cultured Zinnia 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 Zinnia suspension cultures as a valuable model for studies of cell expansion.
Finally, we modified Arabidopsis 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 (UBCs), deubiquitinating enzymes (DUBs), and an Alpha (PAF1) and Beta (PAF1) subunit of the proteasome. All targeted UBC families, candidate UBCs and DUBs, and proteasome subunits are expressed in secondary xylem and bark in this system.
- Doctoral Dissertations