To assess possible roles of nuclear genes in chloroplast biogenesis, five photosynthetic mutants of maize with nuclear lesions were characterized with respect to their chloroplast proteins and chloroplast RNAs. Each mutant is deficient in a characteristic set of polypeptides. The high sensitivity and resolution of immunoblot analysis revealed that in no case are polypeptides completely missing. Mutants deficient in the cytochrome f/b6 or photosystem I complex show a coordinate reduction of all of the core subunits of these complexes such that the correct stoichiometries are maintained. RNA transcripts encoding most of the affected plastid‐encoded proteins were analyzed on Northern blots. In general, chloroplast RNAs in mutant seedlings were not altered in size or abundance, even when the corresponding protein was barely detectable. Therefore, in most cases the nuclear lesions act within the chloroplast at a post‐transcriptional level to prevent expression of chloroplast‐encoded polypeptides. One mutant, hcf‐38, does show striking alterations in the abundance and size of many chloroplast transcripts. The hcf‐38 locus might therefore encode a product that functions in chloroplast transcription or RNA processing.

Various maturity rating systems have been proposed to evaluate seed development in corn (Zea mays L.). The objective of this research was to determine the relationship of seed assimilate uptake and dry weigh to three indicators of physiological maturity: moisture concentration, black layer, and milk line development. Assimilate movement into the seed was determined by exposing the leaf subtending the ear to 14CO2 at regular intervals during seed development and measuring the 14C 24 h later in F2 seed of B73 ✕ Mo17 (1984, 1985) and the stay‐green hybrids FS854 and Pioneer Brand 3358 (1985). In 1986 and 1987, plants of the same F2 hybrids and the inbreds B73 and Mo17 and their F1 cross (B73 ✕ Mo17) were defoliated at ≈20 d after pollination and seed development was compared to that of undefoliated plants. Seed dry weight, moisture concentration, and milk line and black layer development (using a five‐stage rating system) were determined at 3‐ to 5‐d intervals. Uptake of 14C‐assimilate by seed and cob tissue decreased sharply near the end of the linear phase of seed dry weight accumulation when the milk line and black layer ratings were at Stage 3 (mid‐milk line) greater. Near Stage 4 of both black layer and milk line development, 14C in the seed had fallen to the lowest levels, maximum seed weight had been reached, and the seed were physiologically mature. Both black layer and milk line followed consistent rends throughout seed development for all hybrids (including the stay‐green genotypes), defoliation treatments, and years. Seed moisture during seed development was highly correlated with both black layer and milk line, but the concentration at black layer Stage 4 was more variable (293–415 g kg−1) across genotypes and years. The data suggest that Stage 4 of both black layer and milk line development represents a useful and reliable indicator of physiological maturity.

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The nonchromosomal stripe 2 (NCS2) mutant of maize (Zea mays L.) has a DNA rearrangement in the mitochondrial genome that segregates with the abnormal growth phenotype. Yet, the NCS2 characteristic phenotype includes striped sectors of palegreen tissue on the leaves. This suggests a chloroplast abnormality. To characterize the chloroplasts present in the mutant sectors, we examined the chloroplast structure by electron microscopy, chloroplast function by radiolabeled carbon dioxide fixation and fluorescence induction kinetics, and thylakoid protein composition by polyacrylamide gel electrophoresis. The data from these analyses suggest abnormal or prematurely arrested chloroplast development. Deleterious effects of the NCS2 mutant mitochondria upon the cells of the leaf include structural and functional alterations in the both the bundle sheath and mesophyll chloroplasts. and was absent in the mtDNA from those plants that displayed a normal phenotype (16; and our unpublished data). DNA probes corresponding to the rearranged region of the mitochondrial genome identified aberrant transcripts in the mutant, compared with the mtRNAs of the progenitor (cms-T) and normal derivative plants (6). Other mitochondrial genes and their transcripts were unaffected in the NCS2 plants. The synthesis of a single polypeptide was also specifically reduced in the NCS2 mitochondria (6). Preliminary data suggest that the NCS2 mutation is in the mitochondrial gene for subunit 4 of NADH dehydrogenase (complex I) (our unpublished data). Thus, the molecular data implicate the mitochondrion as the actual site of the NCS2 lesion.Although NCS2 plants have pale-green sectors on their leaves, there is no evidence for a chloroplast DNA mutation in addition to the mutation in the mitochondrial DNA. Both molecular and phenotypic considerations have contributed to this conclusion. In NCS2 plants, the kernels abort whereas plants that have mutations affecting chloroplasts generally have full-sized kernels (EH Coe, personal communication). Additionally, by restriction endonuclease fragment analysis, no chloroplast DNA alterations have been detected in NCS2 plants, compared with normal derivatives (16). Of course, point mutations in the chloroplast genome would go undetected by this method of analysis and therefore provide no molecular marker for genetic studies. However, it is highly improbable that the well-established genetic correlation of a specific mitochondrial DNA alteration with the NCS2 phewww.plantphysiol.org on May 9, 2018 -Published by Downloaded from

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