Ultrastructural and genetic characterization of a mutant exhibiting starch accumulation and premature leaf senescence in Medicago sativa

Abstract

A mutant was isolated from irradiated seed of alfalfa, Medicago sativa L. cv. Excalibur. The mutant plant, Ex-139, displayed symptoms of premature senescence in the leaf palisade mesophyll. The leaves emerged as a normal phenotype, but light microscopy revealed that they rapidly began to accumulate starch in plastids of some cells in the palisade mesophyll. This accumulation of starch was followed by general cellular autolysis leading to the formation of necrotic regions in the palisade mesophyll. The adjacent epidermal and spongy mesophyll cells were not structurally affected. The mutant otherwise exhibited normal growth and development and was fertile. Inheritance studies indicated that the trait was transmitted to the progeny, preferentially but not exclusively, through the female, which suggests that either there is differential selection among male and female gametes or the trait is controlled by extranuclear DNA. This mutant should be useful in the study of the link between senescence and carbohydrate metabolism and in alfalfa genetics. Key words: starch metabolism, plastid, chloroplast genome, biparental inheritance.