PHENOTYPES MEDIATED BY THE IOJAP GENOTYPE IN MAIZE

The phenotype of maize plants that are homozygous recessive for the nuclear gene iojap is commonly an "anemic," grainy green appearance accompanied by white leaf stripes, white leaf margins, and reduced growth. In plants in which the iojap effect is occurring (homozygous ij/ij), we find affected tissue varies in extent, from wide, discrete striping to layered striping to short striations; and in distribution, from white leaves to boldly white-patterned to white-margined. In homozygous iojap plants the patterns are predictable and are consistent within a particular genetic background. Leaf striping patterns in iojap seedlings and plants are expressed primarily in the leaf margins, in positional patterns that are inconsistent with cellular proliferation. Reversion sectors on iojap plants (green stripes resulting from nuclear reversion of the ij gene), however, follow clonal cell lineages and indicate either that the iojap defects do not arise until after a leaf differentiates, or that they remain correctable in leaf tissue. On transmission through the egg, iojap-affected plastids become altered to a stable, nongreening state and can be transmitted maternally to subsequent generations. Defects in eggs borne on iojap ears, as revealed by ear maps of seedling progeny, have positional rather than clonal distributions on the ear. The expression of striping nonetheless is clonal in the occasional sectored plants that occur among such maternal progeny. The contrast of maternal sectorials with iojap plants is profound, and shows the period of meiosis and gametogenesis to be a critical passage for plastids in the egg as well as in the pollen. In some genetic backgrounds aborted embryos occur at variable stages in embryo development in ij ij zygotes and in the maternal progeny from ij ij plants. These effects presumably reflect a participation of the organelles in embryogenesis that is subject to the influence of nuclear genes. We conclude that the functional basis of the effects of the iojap gene must be 1) position-dependent (i.e., dependent on the location of a cell in the meristem or organ at a particular time in development of an organ) and 2) differentiation-dependent (i.e., dependent on the attainment of a certain state, specific to the genetic background). The iojap phenotype is thus a reflection of developmental progression of the organ rather than cell-specific, clonal changes. The rate of cell proliferation and growth of one portion of an organ vs. another is related to position-specific and background-specific expressions, and the range of iojap expression is similarly related. Maternally inherited sectoring, on the other hand, shows phenotypes that are derived from clonal changes specific to each cell lineage, presumably attributable to sorting of mixed organelles in the apical meristem cells.