Wound Healing in Plant Cells Term Paper

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Wound Healing in Plant Cells

The Current study will attempt to further clarify and utilize Arabidopsis thaliana in studying wound healing in plants as well as the most effective means in studying the process. Root hairs are not essential for plant growth and development and are convenient to study since they are on the exterior of the root. The simplicity of the patterning and the range of mutants with defects in hair pattern and morphology make the Arabidopsis root hair a useful model for the study of plant cell growth and for tip growth in particular. There are likely to be many parallels between the growths of the different types of tip-growing cells, e.g. plant pollen tube growth, fungal hyphal growth, and algal rhizoids. Previous studies indicate that, mutations have been identified that affect Arabidopsis root epidermis development. These mutations define genes that influence early stages of epidermal cell type specification, genes that affect the initiation of root hairs, and genes that affect root hair enlargement.In root hairs, calcium must flow into the tip from outside the hair in order for the hair to grow. If the external calcium is removed or if the calcium ion channels are blocked, a young root hair stops growing. Old hairs stop growing because of modifications to the number or activity of their calcium ion channels - calcium simply stops flowing into the hairs, and growth stops. However, calcium ions are not needed for the outgrowth of a new root hair, just for tip growth. Since rhd7 root hairs usually rupture during outgrowth, we can manipulate calcium ions to determine if calcium is needed for wound plugging. This will be tested in the current study.

INTRODUCTION:

Arabidopsis thaliana has been utilized in several previous studies due to its small size and the ability to conduct molecular genetic analysis easily. (Galway, Heckman, & Schiefelbein, 1996, p.209) Arabidopsis thaliana is interesting to use due to the previous use as a model plant in the study of cell differentiation. Researchers have reported that Arabidopsis root consists of root hair-producing cells they are derived from atrichoblasts. (Ryan, Steer, & Dolan, 2000, p.140)

Root hairs are not essential for plant growth and development and are convenient to study since they are on the exterior of the root. The simplicity of the patterning and the range of mutants with defects in hair pattern and morphology make the Arabidopsis root hair a useful model for the study of plant cell growth and for tip growth in particular. There are likely to be many parallels between the growths of the different types of tip-growing cells, e.g. plant pollen tube growth, fungal hyphal growth, and algal rhizoids. (Carol, 2002, p.815)

In most cells of the root, hypocotyls and leaf epidermal trichomes, growth is spread over a large proportion of the cell surface, producing diffusely growing cells. Development of some cells, such as leaf epidermis pavement cells, might involve a combination of localized tip growth and diffuse growth to produce strangely shaped interlocking cells. In higher plants, amongst all other cellular components, the two major cytoskeleton elements actin microfilaments and microtubules appear to play a decisive role in the shape-determining process. Treatment of plant cells with cytoskeleton-interacting drugs has been particularly helpful in developing this view and has allowed particular changes in cell morphology to be linked to altered activity of specific cytoskeletal proteins; microtubules are implicated in the establishment and maintenance of polar growth directionality. (Mathur, 2004, p.584)

Many of the important unanswered questions in root development involve events that occur at the root apex. Little about the nature of the stem cells, how cell files are established, how cell numbers and vascular patterning are determined, what controls the organization and size of the meristem, how root hair initials are formed, how cell expansion is regulated, and, perhaps most important, what controls the cell cycle and the planes of cell division. Many of these are general questions that apply equally well to morphogenesis in other parts of the plant. However, several aspects of root morphogenesis serve to simplify the study of these basic questions. (Schiefelbein & Benfey, 1991, p.1147)

Insights into cell growth and patterning obtained from the root hair are derived from a multifaceted approach to understanding the biology of this cell type. The characterization of proteins required for various cellular activities that localize to the growing tip is providing instructive information regarding tip growth. Researchers have shown that Rop GTPases localize to the tip of the growing hair, and disruption of their function results in the development of abnormal hairs. Likewise, the genetic dissection of the process of tip growth is providing a genetic framework on which to build a molecular analysis of tip growth. (Dolan, 2001, p.553)

Several loci involved in Arabidopsis root hair tip growth have been isolated, including RHD2, RHD3, RHD4, and TIP1. These genes may encode products that affect known tip growth factors, such as the cytoskeleton associated proteins, or calcium ion transport. The rhd2 mutants possess "stubby" hairs that are apparently caused by an inability to expand past the initial swelling stage. The phenotypes rhd3 and rhd4 are "wavy hairs" and "bulging hairs" respectively, and have been interpreted as having defects in the control of cell expansion polarity at the root hair tip. By crossbreeding and producing double mutants, Researchers were able to show that the RHD2 gene product is required before the RHD3 or RHD4 products and that RHD3 and RHD4 products probably act in separate pathways. Although the RHD3 and RHD4 genes apparently affect root hairs specifically, the RHD3 gene is required for normal cell expansion in many plant tissues, and has a similar action to the COBRA gene. (Ridge, 1995, p.402)

Mutants with defects in potassium transport are providing insight into the role of potassium during root hair elongation. Tiny root hair mutants form short root hairs and frequently initiate more than one root hair per trichoblast. (Foreman & Dolan, 2001, p.3) Iron deficiency causes a number of visible symptoms in plants, such as severe chlorosis and changes in the morphology of the roots, enhanced formation of root hairs and the development of transfer cells in the root apical zones. In the same root zones, iron deficiency enhanced proton extrusion and induced the "Turbo reductase" system, a description that comes from the enhancement of the reduction of external ferric compounds by intact roots. This reducing capacity (in vivo activity) has been identified as the enhanced activity of a plasma membrane bound ferric chelate reductase. (Moog, van der Kooij, Bruggemann, Schiefelbein, & Kuiper, 1994, p.505)

Researchers state in journal article (#10), that rhd7 mutations are new alleles of CslD3 Two root hair mutants independently identified by J.S. Schiefelbein (rhd7-1) and G.O. Wasteneys (rhd7-4) both exhibited root hair rupture. rhd7-1 was one of three rhd7 alleles, and was previously designated RM57 (Moog et al., 1995). Complementation tests revealed that rhd7-1 and rhd7-4 were alleles. Subsequent crosses between rhd7 mutants and the kjk-2 mutant also failed to complement the mutant phenotype. Hence, Rhd7 is identical to Kojak, also designated CslD3. Here we will use rhd7, kjk and csld3 plus the allele number in order to refer to specific alleles of this gene, and CslD3 to refer to the non-mutated gene. Single point mutations were identified in the coding region of CslD3 in both rhd7-1 and rhd7-4. The mutation in rhd7-4 is identical to that reported for kjk-1, although the latter was isolated from a mutagenized Landsberg erecta population. C is replaced by T, converting the arginine codon in the highly conserved QXXRW motif to a stop codon. The novel point mutation in rhd7-1 similarly replaces an initial C. with a T, converting a glutamine codon Q951 to a stop codon. This mutation is located in the short sequence between the predicted third and fourth membrane spanning domains. (#10)

Previous studies indicate that, mutations have been identified that affect Arabidopsis root epidermis development. These mutations define genes that influence early stages of epidermal cell type specification, genes that affect the initiation of root hairs, and genes that affect root hair enlargement. Mutations affecting the RHD3 gene are unique because they alter the enlargement of root hairs as well as the root proper, the root hairs exhibit a short and wavy morphology, and the roots possess a reduced length. Detailed ultra structural analyses of growing rhd3 root hairs have shown that vacuole formation is reduced and secretory vesicle distribution is altered. Because the rhd3 mutations alter the size of roots and root hairs, the RHD3 product may be involved in a fundamental plant cell expansion mechanism. (Wang, Lockwood, Hoeltzel, & Schiefelbein, 1997, p.800)

Visual examination of roots from 12,000 mutagenized Arabidopsis seedlings has led to the identification of more than 40 mutants impaired in root hair morphogenesis. Mutants from four phenotypic classes have been characterized in detail, and genetic tests show that these result from single nuclear recessive mutations in four different genes designated RHD1, RHDP, RHD3, and RHD4. The phenotypic analysis of the mutants and homozygous double… [END OF PREVIEW]

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