Tobacco resistant cell lines

Success in regenerating salt-resistant plants from salt-resistant cells has been limited (Table 4). The most extensively quoted example is the work of Nabors et al. (1980) who found that tobacco plants regenerated from NaCl-resistant cell lines survived salinity better than did plants regenerated from unselected lines. The authors themselves were justifiably cautious. 

A frequently observed response of plant cells exposed to saline stress is the accumulation of proline. Two cell lines of tobacco, one resistant and the other sensitive to growth inhibition by NaCl, accumulated proline when exposed to 1.5% w/v NaCl in the growth media (Dix Pearce, 1981). The NaCl sensitive line accumulated proline more rapidly than did the resistant line, though the levels accumulated were not adequate to provide osmotic protection against salt stress. The authors suggested that proline accumulation may have a protective role other than osmoregulation and may be symptomatic of stress injury, the nature of which was not discussed. 

Berlin and Widholm (31) selected carrot and tobacco cells resistant to PFP. In contrast with the Ccirrot line, which showed the expected increases in free Phe, the PFP-resistant cells (TX4) had almost normal levels of Phe, but did show dramatic increases in phenolics apparently due to a 10- to 20-fold rise in phenylalanine ammonia lyase (PAL) activity as compared to control cells (TXl). It was suggested that this cell line was resistant to PFP due to the higher PAL activity, which could detoxify PFP by conversion to p-fluorocinnamic acid. If this method could be generally applied, it would allow for the positive selection of cell strains overproducing a specific class of secondary compounds. 

Borkird and Sung [1] isolated seven ABA-insensitive cell lines in a somatic embryo culture of carrot in which ABA failed to arrest the development of torpedo-stage embryos into plantlets, as occurs in wild type when ABA is added. All lines also showed reduced auxin sensitivity. Characterization of three lines showed lower levels of ABA uptake as a possible cause of ABA insensitivity. However, the uptake of 2,4-D was higher than in wild-type embryos. In tobacco, cell-lines resistant to inhibition of growth by ABA were also isolated [25], and cell proliferation continued in the presence of what would be growth-inhibiting ABA concentrations for normal cell lines. 

The inhibitor DFMO has also been used for screening plant cell lines resistant to this drug in order to isolate enzymatic overproducer variants. Thus, it has been shown that a tobacco cell line resistant to DFMO has a very low level of ODC activity (Malmbeig et al., 1985), but high levels of free putrescine due to a 20-fold increase in ADC activity (Hiatt and Malmberg, 1988). As discussed above, compensatory mechanisms regulating putrescine biosynthesis in inhibitor-treated systems may also occur in this totocco cell line. 

Two mutants with altered ADC activities have been identified. A Dfr-l tobacco cell line resistant to DFMO with low levels of ODC, but high levels of both ADC and putrescine (Hiatt and Malmberg, 1988), and a petunia mutant (alf) with developmentally regulated high levels of ADC (Gerats et al, 1988). 

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