Gautheret ( 2 5 9 ) observed that strains of normal carrot tissue capable of culture in the absence of added IAA gradually altered in their response to this auxin. The proliferation of freshly isolated explants was stimulated by additions of IAA at appropriate concentrations. However, after a period of culture in the presence of IAA, the tissue no longer responded in this way. Later, Gautheret ( 2 6 1 ) found that tissues of Scorzonera when first isolated had an absolute requirement for auxin if they were to grow in culture. However, after numerous transfers in a culture medium containing auxin, there arose sectors in the cultures which possessed the ability to grow continuously and rapidly in an auxin-free medium. Their growth was not further enhanced by adding auxin to the culture medium. These modified tissues differed from the normal tissues, not only in their insensitivity to IAA but in being translucent, friable, and incapable of initiating roots. Various other investigators
( 3 1 9 , 3 6 5 , 4 9 8 ) have reported, in work with various other callus tissues, a similar change in response to auxin. Gautheret referred to this phe-nomenon first as "accoutumance a l'auxine" ( 2 5 8 , 2 6 1 ) and later as
"anergie a Tauxine" ( 2 6 6 ) . American and British authors have generally used the term habituation.
Kulescha ( 3 9 8 ) has shown that such habituated tissues contain a higher content of free auxin than normal tissues although the auxin composition of such tissues has not been satisfactorily described in
chemical terms nor has it been established whether the higher auxin content is a consequence of an enhanced rate of auxin biosynthesis.
Furthermore, there is evidence that stable changes affecting other aspects of metabolism can occur in cultured tissues and that the phenomenon of habituation should not be conceived of as restricted to auxin metabolism.
Thus, Gautheret (264, 266) cites the case of a willow (Salix caprea) tissue which was at first cultured successfully only by using a complex medium but which became able to grow actively without several of the growth factors initially important for its proliferation; in particular, the tissue lost its need for an external supply of pantothenic acid. Similarly, the virus tumor tissue of Rumex acetosa has recently been reported
(535) no longer to need thiamine. Normal tissue of Catharanthus roseus (Vinca rosea) has a requirement for some seven growth factors. How
ever, Braun (101) has now reported that from a large number of frag
ments of such tissue transferred to White's basal medium in which the content of each inorganic salt was raised sixfold, a tissue fragment has been obtained capable of continued growth in such a medium. This tissue was incapable of culture on unmodified White's basal medium.
It could, however, be grown on this medium modified by raising the concentration of only a single salt, potassium iodide. Similarly, it could be cultured in the presence of the normal inorganic salt mixture pro
vided the basic medium was supplemented by either 1 : 3 diiodotyrosine, kinetin, or cytidylic acid. The modified tissue no longer required for growth an external supply of auxin, rat/o-inositol, or a source of reduced nitrogen, such as glutamine or asparagine. This modified tissue during serial propagation in White's medium containing the enhanced content of inorganic salts has now yielded a further sector which no longer re
quires either additional iodide or any growth factor. It grows well on unmodified White's basal medium like fully transformed crown gall tumor tissue of Catharanthus roseus.
Habituated tissues show more or less developed tumor-forming prop
erties as evidenced by grafting into plants of the species from which they are derived (103, 140, 319). In tobacco the tumor-forming property of habituated tissue has been particularly well demonstrated; it was more malignant than the crown gall tissue (426). The rather weak tumor-forming properties of certain other habituated tissues may reflect the degree of habituation involved, for both Morel (498) and Gautheret (264) have emphasized that there are degrees of habituation, and Braun (101) clearly obtained both partially and fully habituated tissues of Catharanthus roseus.
Gautheret (266) has postulated that habituation is a form of enzymatic adaptation since its reproducibility, particularly in certain tissues, seems
to preclude somatic mutation or selection of cells with a special geno
type. Evidence that enzymatic adaptation occurs in cultured organs has already been discussed (page 2 3 ) . Gautheret agrees with Braun in ex
cluding mutation as the cause of the altered state and suggests that habituation may be reversible under appropriate conditions yet to be discovered. Since fully habituated tissue can be regarded as a form of tumor tissue, it follows that transformation to the tumorous state does not necessarily involve the intervention of an external agent (bacterial product or virus). From this it can be suggested that such tumor-inducing agents act indirectly to create a physiological state which can also be induced by the accumulation or depletion of natural cell con
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