Aberrations in plastid transcripts and deficiency of plastid DNA in striped and albino mutants in maize

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, ii1 (iojap 1), two albino mutants, w1 (white 1) and w2 (white 2), and their double mutants with l (luteus). Homozygous ij1 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 (+/+). In contrast to albinos that are blocked in the biosynthetic pathway of carotenoids, w1 and w2 seedlings fail to accumulate chlorophyll and carotenoids up to the normal level even under dim light conditions. In ij1-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 ij1 and w1 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 ij1- and w1-affected plastids may have altered synthesis, processing or stability. Therefore, the block in expression of the plastid genome by the nuclear mutants ij1 and w1 may be due to alterations in the transcriptional or post-transcriptional processes. The fact that ij1 and maternal-exception progeny show almost identical patterns of transcripts indicates that the effects of ij1 on plastid gene expression persist in the subsequent generation even after the nuclear gene, Ij1, restores the normal function. In contrast to ij1 and w1, the levels of all plastid transcripts in w2 seedlings, whether l or +, 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.