Starch granules: structure and biosynthesis.

A mechanism for gene conversion is proposed which overcomes many of the difficulties that any copy choice model encounters. It is suggested that along with general genetic pairing of homologous genomes at meiosis, effective pairing over short regions of the genetic material occurs at the molecular level by the separation of the strands of the DNA double helices, followed by the annealing of strands from two homologous chromatids. If the annealed region happens to span a heterozygous site, mispairing of bases will occur. Such a situation may be analogous to that in DNA which is damaged by mutagens; the same or similar repair mechanisms may operate, and these, by adjusting the base sequences in order to restore normal base pairing, would bring about gene conversion in the absence of any genetic replication. The model indicates how precise breakage and rejoining of chromatids could occur in the vicinity of the conversion, so that conversion would frequently be accompanied by the recombination of outside markers. The model also proposes that the distance between two mutant sites on a fine structure map depends not so much on the frequency of a recombinational event occurring between them, but rather on the degree of inhibition of the processes of genetic pairing by the mutants themselves. The model will explain almost all the data in a formal way, and it has the advantage over copy choice mechanisms for gene conversion in (1) being compatible with semi-conservative replication of DNA, (2) not invoking DNA synthesis during or after genetic pairing, (3) providing a molecular mechanism for close specific pairing, (4) making it unnecessary to postulate sister strand exchange or a process akin to this, (5) suggesting why rates of gene conversion in opposite directions are sometimes unequal and (6) providing an explanation of the clustering of mutant sites, a basis for map expansion and for the apparently capricious departure of fine structure maps from additivity. Although the model proposed is a general rather than a specific one, it suggests that the process of conversion and intragenic recombination is more complex than is usually believed, since it depends on several interacting factors. Nevertheless, it is hoped that the introduction of a model with this complexity will help to stimulate specific experiments, and that these will provide definitive information which would never be obtained if simpler models of conversion and intragenic recombination were believed to explain the genetic data sufficiently well.

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A mechanism for gene conversion in fungi.

Molecular identification and isolation of the Waxy locus in maize.

Carotenoids and tocopherols are the two most abundant groups of lipid-soluble antioxidants in chloroplasts. In addition to their many functional roles in photosynthetic organisms, these compounds are also essential components of animal diets, including humans. During the past decade, a near complete set of genes required for the synthesis of both classes of compounds in photosynthetic tissues has been identified, primarily as a result of molecular genetic and biochemical genomics-based approaches in the model organisms Arabidopsis thaliana and Synechocystis sp. PCC6803. Mutant analysis and transgenic studies in these and other systems have provided important insight into the regulation, activities, integration, and evolution of individual enzymes and are already providing a knowledge base for breeding and transgenic approaches to modify the types and levels of these important compounds in agricultural crops.

Vitamin synthesis in plants: tocopherols and carotenoids.

Publisher Summary In the early days of agriculture, man must have learned of the competition among individual plants within a crop or intraspecific competition, even though his knowledge was purely in empirical terms. He must have learned by experience that if the sowing rate were sparse, his harvest would be lean, and conversely that if the seed rate were increased beyond a certain value, the plants would be spindly and poorly grown. The fuller understanding of competition among plants requires a greater knowledge of the response of plants to their environment, especially of the response to the environmental stresses created by neighbors. Plant physiologists have studied the single plant, and agronomists have looked at the whole crop, but the plant within the community has scarcely been investigated. This is a field which promises both scientific depth and great potential reward in terms of crop production.

Competition Among Crop and Pasture Plants

Preliminary tests have shown that the endosperms of maize seeds homozygous for the opaque-2 mutant gene have a higher lysine content than normal kernels. As a critical test, a backcross progeny was divided into opaque-2 and normal kernels, the endosperms separated, and the amino acids determined. The opaque-2 endosperms had a different amino acid pattern and 69 percent more lysine than the normal seeds. The major reason for these changes is the synthesis of proteins with a greater content of basic amino acids in the acid-soluble fraction of the mutant endosperm. This is accompanied by a reduction in the ratio of zein to glutelin.

https://science.sciencemag.org/content/sci/145/3629/279.full.pdf

Mutant Gene That Changes Protein Composition and Increases Lysine Content of Maize Endosperm

The mutant floury-2 results in the production of maize endosperm proteins with an altered amino acid pattern. The lysine concentration is high, eapproximately equal to that in mutant opaque-2, and the methionine concentration is higher than in any other stock tested. Other mutants of similar phenotype!, opaque-1, floury-1, and soft-starch do not cause major changes in amino acid Pattern.

Second Mutant Gene Affecting the Amino Acid Pattern of Maize Endosperm Proteins

The enzymatic deficiency in the waxy mutant of maize

The maize mutant shrunken-2 synthesizes only 25 to 30 percent as much starch as normal maize; it completely lacks adenosine diphosphate glucose pyrophosphorylase activity in both endosperm and embryo tissue. Identification of the mutant block indicates that the greater portion of starch in the endosperm of normal maize is synthesized by way of enzyme systems that utilize adenosine diphosphate glucose as a substrate, and that the latter is formed chiefly by adenosine diphosphate glucose pyrophosphorylase.

Oliver E. Nelson

Papers

Mutant Gene That Changes Protein Composition and Increases Lysine Content of Maize Endosperm

Second Mutant Gene Affecting the Amino Acid Pattern of Maize Endosperm Proteins

The enzymatic deficiency in the waxy mutant of maize

Starch-Deficient Maize Mutant Lacking Adenosine Diphosphate Glucose Pyrophosphorylase Activity

The abnormal lignins produced by the brown-midrib mutants of maize. i. the brown-midrib-1 mutant.