Graduate Thesis Or Dissertation
 

Developmental mutants of Phaseolus vulgaris L.

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  • Hybrids with abnormal development were obtained when particular genotypes of Phaseolus vulgaris L. were crossed. The phenotypic aberrations of the F₁s included retarded growth, chlorosis of the trifoliate leaves and the formation of adventitious roots on the hypocotyls. The expression of the abnormalities was dependent on the temperature. At high temperature regime (30°C/25°C, day/night), the onset of the abnormal phenotypes occurred at an early stage of development (two weeks after planting) and essentially no new growth was observed between the second and the sixth week. However, at lower temperature regimes (25°C/20°C and 20°C/15°C, day/night), the F₁s exhibited normal and vigorous growth. The abnormalities observed on F₁s at high temperature is referred to as the "crippled" phenotypes. Genetic analysis indicated that the abnormal development is conditioned by two loci, DL₁ and DL₂ (for dosage-dependent lethal). The severity and the expression of the mutant phenotypes are dependent on the allelic dosage as well as the temperature. F₁s are heterozygous for DL₁ and DL₂. Plants homozygous dominant for both loci (four dominant alleles) are classified as lethals and perish soon after germination. Plants homozygous for one of the locus but heterozygous for the other locus (three dominant alleles) are classified as sub-lethals which also perish but at a slower rate than the lethals. The expression of the lethal and the sub-lethal phenotypes is not influenced by the temperature. The developmental controls of DL₁ and DL₂ were examined through grafting experiments and hydroponic studies. Phenotypes of mutant classes were duplicated by unions of scions and stocks derived from different genotypes. These results indicate that DL₁ and DL₂ regulate a root and shoot factor respectively which contribute to the mutant phenotypes. The allelic dosage of DL₁ in the root and DL₂ in the shoot rather than the genotype of the whole plant determine the severity of the abnormal development. Plants heterozygous for both loci with a temperature-dependent expression of the mutant phenotype were used to determine physiological components involved. One of the primary morphogenetic events associated with mutant expression is the restricted root growth at high temperature which can be alleviated by the addition of cytokinin in hydroponic solution. The differential expression of DL₁ and DL₂ in the root and the shoot systems and the effect of cytokinin in restoring normal growth of heterozygotes suggest that the two loci may be related to the regulation of hormonal biosynthesis or metabolism.
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