What is the basis for beneficial induced mutations?
There were a number of futile attempts to link the successful performance of crop plant mutants to the mutagen and the applied dose by which they were originally induced, but there are hardly any investigations about the molecular changes of genes in the genomes of improved mutant cultivars, and only few concerning specific crop plant mutants.
Mutation experiments on the seed quality of cereals induced by various mutagens may shed some light on the issue. AMANO and co-workers studied carefully the changes induced in the waxy loci of rice and maize using EMS, UV, thermal neutrons, and gamma-rays. They looked at the expression of the waxy gene in pollen and in the endosperm (AMANO 1981, 1985; YATOU and AMANO 1991). They have confirmed a destructive effect of all the mutagens used, but the kind and degree of destruction differed between the mutagens applied, from impairment of proper code transcription (mainly by EMS) to total inactivation or even complete deletion of the locus. Another interesting insight can be derived from studies on the effect of chemical mutagens on a particular locus in barley (Mla12), responsible for a â€žspecific" resistance to Erysiphe graminis (JĂ-RGENSEN 1996). Mla12 is a dominant gene whose function is to recognize attack by the mentioned fungus. The recognition is restricted to a particular product of the corresponding fungal â€žavirulence" gene. Mutagenesis was tried to alter the dominant, resistance activating allele into a recessive or the null-allele and to see what happens. The (somewhat unexpected) results were: (a) mutants with mutations in the Mla12 locus causing reduced, but not completely lost ability to recognize the corresponding specific pathogen type; (b) mutations outside this locus, which reduced gradually the defence effectiveness. These mutations apparently occurred in genes (activated by the intact dominant Mla12 gene) whose proper functioning is required for performing an effective defence. (For incomprehensible reasons the mutated genes were named suppressor (sup), although they were originally active defence genes, now to some extent damaged by a mutation).
Accepting it as a fact, that conventional mutagens have destructive effects on genetic material (MALUSZYNSKI et al. 1995), one could postulate, that those successful mutants mentioned before (e.g. listed in the FAO/IAEA Data Base) carry alterations only in genes, where even a â€žnull-allele" causes tolerable changes in the plant's ontogenesis and physiology. But this does not make it plausible, why they became useful from the point of view of a crop breeder or farmer.
If one looks into the literature or at one's own experiments, one would be able to list numerous examples of induced or spontaneous mutants, which could be explained by an alteration or complete loss of a regulation (e.g. MICKE et al. 1980; YAMAGATA 1981; GOTTSCHALK and WOLFF 1983; DONINI et al. 1984; KONZAK 1984; MICKE 1988). Some specific examples could be listed as follows:
- loss of daylength sensitivity in rice, cotton, castor bean (GUSTAFSSON and LUNDQUIST 1976; MICKE 1979; DONINI et al. 1984)
- loss of apical dominance leading to profuse branching in sweet clover (MICKE 1958; SCHEIBE and MICKE 1967)
- altered control over ontogenetic development, leading to short culm and increased tillering in cereals (MALUSZYNSKI et al. 1986)
- loss of control of root nodulation towards non-, inefficiently or abundantly nodulating pea, bean, chickpea, faba bean, soybean etc. (JACOBSEN and FEENSTRA 1984; CARROLL et al. 1985; PARK and BUTTERY 1989; etc.)
- altered control over endosperm composition in cereals (GOTTSCHALK and MĂśLLER 1983; IIDA et al. 1993; VON WETTSTEIN 1995; HABBEN and LARKINS 1995)
- altered control over compounds deposited as reserves in cotyledons of oil seeds (GREEN and MARSHALL 1984; RĂ-BBELEN 1982) and of grain legumes (TAKAGI et al. 1989; BHATTACHARYYA et al. 1993)
- loss of control over regular leaf and floral organ development in sweet clover (SCHEIBE and MICKE 1967), in faba bean and in pea (GOTTSCHALK 1971)
- improved resistance of barley to powdery mildew by loss of Ml-o regulated inhibition of callose apposition (JĂ-RGENSEN 1992; MICKE 1992)
- increased resistance to rusts by deletion of inhibitor genes in wheat (KERBER and GREEN 1980; DYK 1982; WORLAND and LAW 1991; WILLIAMS et al. 1992)
- altered control over deposition or recycling of secondary metabolic products in lupin (MICKE and SWIECICKI 1988), in sweet clover (MICKE 1962 a and b; SCHĂ-N 1966), in rape seed (RĂ-BBELEN 1990).
Pleiotropy (often seen as a typical attribute of induced mutations) and its modification through crosses (GAUL et al. 1968) could reasonably well be explained by mutational changes in genes functioning as regulators for several other genes, but should then be distinguished from â€ždownstream" effects of mutant genes, acting â€žearly" in metabolic pathways.
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