Genetic and biochemical characterization of the maize mutants floury-2 and modified opaque-2

Abstract

The maize mutations floury-2 (fl2) and opaque-2 (o2) reduce endosperm storage protein synthesis and lead to accumulation of protein fractions that are nutritionally more balanced. However, this advantage is offset by the soft and fragile texture of the endosperm, which gives the grain inferior quality for harvesting, storage, and processing. Further development and future usefulness of these materials may depend upon deciphering the genetic, biochemical and molecular mechanisms that affect seed physical structure and nutritional quality. The soft endosperm of the maize floury-2 (fl2) mutant is associated with a reduction in zein mRNA and protein synthesis, and enhanced levels of a 70-kD protein shown to be the maize homolog of the chaperonin BiP. An unusual alpha-zein protein of 24-kD was found to be consistently associated with the storage protein fraction from fl2 mutants. Increases in accumulation of this novel alpha-zein protein is correlated with increases in BiP accumulation and decreases in storage protein deposition. Also, an RFLP that is unique to fl2 was detected with a 22-KD alpha-zein probe. The synthesis of an abnormal alpha-zein protein in fl2 could explain many features of the mutant, such as abnormal protein body morphology, BiP induction and hypostasis to o2. These findings indicate that the effects of the fl2 mutation could be a response to accumulation of a defective protein that causes impairment of secretory protein synthesis. Breeding methods based on accumulation of o2 modifier genes led to the development of mutant germplasm with normal kernel phenotype and enhanced nutritional value. However, these modified mutants have not yet reached widespread commercial production because the components and mode of action of the genetic system involved in seed modification are largely unknown. Detailed genetic and biochemical characterization of endosperm modification in o2 indicated that two independent modifier genes act in a semidominant fashion to increase seed hardness and accumulation of the storage protein gamma-zein. Genetic mapping experiments together with molecular analysis indicated gamma-zein genes as components of the o2 modifier system. Also the use of RFLP, RAPD and bulked segregant analysis to locate a second endosperm modifier locus is described.