A mutational approach to the study of seed development in maize.

The maize seed comprises two major compartments, the embryo and the endosperm, both originating from the double fertilization event. The embryogenetic process allows the formation of a well-differentiated embryonic axis, surrounded by a single massive cotyledon, the scutellum. The mature endosperm constitutes the bulk of the seed and comprises specific regions containing reserve proteins, complex carbohydrates, and oils. To gain more insight into molecular events that underlie seed development, three monogenic mutants were characterized, referred to as emp (empty pericarp) on the basis of their extreme endosperm reduction, first recognizable at about 12 d after pollination. Their histological analysis reveals a partial development of the endosperm domains as well as loss of adhesion between pedicel tissues and the basal transfer layer. In the endosperm, programmed cell death (PCD) is delayed. The embryo appears retarded in its growth, but not impaired in its morphogenesis. The mutants can be rescued by culturing immature embryos, even though the seedlings appear retarded in their growth. The analysis of seeds with discordant embryo-endosperm phenotype (mutant embryo, normal endosperm and vice-versa), obtained using B-A translocations, suggests that emp expression in the embryo is necessary, but not sufficient, for proper seed development. In all three mutants the picture emerging is one of a general delay in processes related to growth, as a result of a mutation affecting endosperm development as a primary event.

Genetic analysis as a tool to investigate the molecular mechanisms underlying seed development in maize.

BACKGROUND: In angiosperms the seed is the outcome of double fertilization, a process leading to the formation of the embryo and the endosperm. The development of the two seed compartments goes through three main phases: polarization, differentiation of the main tissues and organs and maturation. SCOPE: This review focuses on the maize kernel as a model system for developmental and genetic studies of seed development in angiosperms. An overview of what is known about the genetic and molecular aspects underlying embryo and endosperm formation and maturation is presented. The role played by embryonic meristems in laying down the plant architecture is discussed. The acquisition of the different endosperm domains are presented together with the use of molecular markers available for the detection of these domains. Finally the role of programmed cell death in embryo and endosperm development is considered. CONCLUSIONS: The sequence of events occurring in the developing maize seed appears to be strictly regulated. Proper seed development requires the co-ordinated expression of embryo and endosperm genes and relies on the interaction between the two seed components and between the seed and the maternal tissues. Mutant analysis is instrumental in unravelling the genetic control underlying the formation of each compartment as well as the molecular signals interplaying between the two compartments.

Programmed cell death during embryogenesis in maize.

Programmed cell death (PCD) in plants is considered an integral part of development. Evidence of DNA fragmentation, occurring at specific sites and times during embryo formation in maize (Zea mays L.), was obtained using terminal deoxyribonucleotidyl transferase-mediated dUTP-fluorescein nick end labelling (TUNEL) and by genomic DNA ladder detection. During the crucial period of elaboration of the primary shoot and root axis (14-20 d after pollination), TUNEL-positive nuclei are present in the scutellum, coleoptile, root cap and principally in the suspensor. Additional evidence of a form of programmed cell death occurring in these tissues comes from the detection of a DNA ladder. Upon completion of the differentiation process, all embryonic cells are TUNEL-negative, indicating that possible programmed cell death events during maize embryogenesis are confined to structures or organs that do not contribute to the adult plant body.

Mutations in two independent genes lead to suppression of the shoot apical meristem in maize.

The shoot apical meristem (SAM), initially formed during embryogenesis, gives rise to the aboveground portion of the maize (Zea mays) plant. The shootless phenotype (sml) described here is caused by disruption of SAM formation due to the synergistic interaction of mutations at two genetic loci. Seedlings must be homozygous for both sml (shootmeristemless), and the unlinked dgr (distorted growth) loci for a SAM-less phenotype to occur. Seedlings mutant only for sml are impaired in their morphogenesis to different extents, whereas the dgr mutation alone does not have a recognisable phenotype. Thus, dgr can be envisaged as being a dominant modifier of sml and the 12 (normal):3 (distorted growth):1 (shoot meristemless) segregation observed in the F(2) of the double heterozygote is the result of the interaction between the sml and dgr genes. Other segregation patterns were also observed in the F(2), suggesting instability of the dgr gene. Efforts to rescue mutant embryos by growth on media enriched with hormones have been unsuccessful so far. However, mutant roots grow normally on medium supplemented with kinetin at a concentration that suppresses wild-type root elongation, suggesting possible involvement of the mutant in the reception or transduction of the kinetin signal or transport of the hormone. The shootless mutant appears to be a valuable tool with which to investigate the organization of the shoot meristem in monocots as well as a means to assay the origins and relationships between organs such as the scutellum, the coleoptile, and leaves that are initiated during the embryogenic process.