Abstract.To better understand the regulatory roles of nuclear genes in chloroplast genomic expression during leaf development in maize (Zea mays L.), we studied a striped mutant, iil (iojap 1), two albino mutants, wl (white 1) and w2 (white 2), and their double mutants with l (luteus). Homozygous ijl plants as a female parent produce albino seedlings, called maternal exceptions, among their progeny, even when the nuclear genotype of the male parent is normal (4-/4-). In contrast to albinos that are blocked in the biosynthetic pathway of carotenoids, wl and w2 seedlings fail to accumulate chlorophyll and carotenoids up to the normal level even under dim light conditions. In ijl-affected plastids, the plastid-encoded proteins and nuclear-encoded proteins that are associated with thylakoid membranes were not detecable. However, the 33-kDa protein of the oxygen-evolving complex and ferredoxin: NADP oxidoreductase, which are localized extrinsically, were accumulated even though the level of the proteins was decreased. Both ijl and wl albino seedlings contain a normal level of plastid DNA. However, both show similar aberrant patterns among the transcripts of all the plastid genes examined (psbB, psbH, petB, petD, atpA, psaB, psbA, and rbcL). Not only were additional transcripts detected but some of the normal transcripts were not detectable or were barely detectable by Northern hybridization. These facts indicate that the transcripts of ijl-and wl-affected plastids may have altered synthesis, processing or stability. Therefore, the block in expression of the plastid genome by the nuclear mutants ijI and wl may be due to alterations in the transcriptional or post-transcriptional processes. The fact that ijl and maternal-exception progeny show almost * Present address: Cold Spring Harbor Laboratory, Cold Spring Harbor, New York, USA ** Present address: Department of Biological Sciences, State University of New York, Binghamton, New York, USA Abbreviations: FNR = ferredoxin: NADP oxidoreductase; OEC-33=33-kDa protein of the oxygen-evolving complex; Rubisco = ribulose-l,5-bisphopsphate carboxylase oxygenase Correspondence to: E.H. Coe identical patterns of transcripts indicates that the effects of ijl on plastid gene expression persist in the subsequent generation even after the nuclear gene, ljl, restores the normal function. In contrast to ijl and wl, the levels of all plastid transcripts in w2 seedlings, whether l or 4-, are uniformly reduced. Compared to normal sibling seedlings, the patterns of the RNA species are relatively unaltered. Relative to the level of a nuclear rDNA, the plastid DNA content of w2 is decreased 20-fold. Therefore, the limited expression of the w2-affected plastids may be due to failure to maintain the copy number of plastid genomes. Thus, albinisms of these mutants result from limiting of expression of plastids due to alteration of transcripts on the one hand, or to lowered DNA content on the other.

Since the work done by R.A. Emerson in the 1930s, Inhibitor of striate (Isr) has been recognized as a dose-dependent genetic modifier of variegation in chlorotic leaf striping mutants of maize such as striate2 (sr2). We have shown thatIsr specifically inhibits proliferation and differentiation of plastid defective cells in sr2 mutants. Leaf narrowing is due to loss of intermediate veins and ground tissue located at leaf margins, and the few remaining plastid defective cells are of irregular size and aberrant organization. The Isr gene has been cloned by targeted transposon tagging. Isr mRNA is expressed throughout young leaves, but Isr chimeras indicate that the expression ofIsr at leaf margins is sufficient to suppress both the lateral expansion of sr2 leaves and the extent of striping. Isr protein appears to encode a chloroplast protein with sequence similarity to a family of bacterial phosphatases involved in carbon catabolite repression or in carbon metabolism. We propose that the action ofIsr in nuclear and plastid communication could be triggered by carbon stress.