Filner, Philip (1965) Studies on exponential cultures of plant cells. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-04082003-112136
The properties of tobacco cells growing exponentially in a chemically defined liquid medium were investigated for the purpose of characterizing the exponential plant cell culture and to explore its potential as a developmental model system.
The cells multiply exponentially with a generation time of 2 days, and exponential multiplication persists for 4.5 generations. The replication of DNA was studied by means of N[superscript 15] density labeling. All the DNA in the cells replicates in about one generation, and replication proceeds by a semiconservative mechanism. The average cell properties of the culture are not constant during the exponential phase, indicating that the population is not in a steady state condition. The number of cells per group, the water content per cell, the soluble protein content, and the rate of RNA synthesis vary. Enzyme levels of the soluble protein fraction do not vary in parallel with the protein content, but rather they vary in their own characteristic ways.
The changes in cell properties are related to changes in the chemical environment which the cells themselves bring about. The cells deplete the medium of phosphorus after three generations, and they deplete it of potassium and nitrogen by the end of the exponential phase. Nitrogen is the growth limiting nutrient, and nitrogen may be supplied either as nitrate or as a complete amino acid mixture. The nitrate reductase (NR) level in the culture falls as the nitrate supply is depleted, so that the enzyme must be induced before the cells can grow when subcultured into a medium in which nitrate is the sole nitrogen source. There is a lag in the onset of mitosis which correlates with the time necessary for NR induction. The initial and terminal events of the exponential phase have been tentatively identified as the induction of NR and the depletion of nitrate, respectively.
The regulation of NR involves both substrate induction and end-product repression. NR is induced by nitrate and repressed by a complete amino acid mixture in proportion to the ability of the mixture to meet the nitrogen requirement of the cells. If amino acids are available at a sub-optimal level, and nitrate is also present, sufficient NR is induced and nitrate reduced to bring the nitrogen supply up to the optimal level.
The amino acids may be classed as repressors or derepressors, according to their action in the regulation of NR. The derepressors are arginine, lysine, cysteine and isoleucine. A single repressor inhibits the growth of the cells on a nitrate medium as a consequence of the condition of nitrogen starvation which results from NR repression. In the presence of a repressor and an appropriate derepressor, NR is induced and the cells grow. Since the growth of the cells can be controlled through the NR regulatory mechanism, it has been suggested that this mechanism may be used by the whole organism to regulate the growth of its parts.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Degree Grantor:||California Institute of Technology|
|Thesis Availability:||Public (worldwide access)|
|Defense Date:||1 January 1965|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Imported from ETD-db|
|Deposited On:||08 Apr 2003|
|Last Modified:||26 Dec 2012 02:37|
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