Developmental Loss of Photosystem II Activity and Structure in a Chloroplast-Encoded Tobacco Mutant, Lutescens-1

Spontaneous Chloroplast Mutants Mostly Occur by Replication Slippage and Show a Biased Pattern in the Plastome of Oenothera

Spontaneous plastome mutants have been used as a research tool since the beginning of genetics. However, technical restrictions have severely limited their contributions to research in physiology and molecular biology. Here, we used full plastome sequencing to systematically characterize a collection of 51 spontaneous chloroplast mutants in Oenothera (evening primrose). Most mutants carry only a single mutation. Unexpectedly, the vast majority of mutations do not represent single nucleotide polymorphisms but are insertions/deletions originating from DNA replication slippage events. Only very few mutations appear to be caused by imprecise double-strand break repair, nucleotide misincorporation during replication, or incorrect nucleotide excision repair following oxidative damage. U-turn inversions were not detected. Replication slippage is induced at repetitive sequences that can be very small and tend to have high A/T content. Interestingly, the mutations are not distributed randomly in the genome. The underrepresentation of mutations caused by faulty double-strand break repair might explain the high structural conservation of seed plant plastomes throughout evolution. In addition to providing a fully characterized mutant collection for future research on plastid genetics, gene expression, and photosynthesis, our work identified the spectrum of spontaneous mutations in plastids and reveals that this spectrum is very different from that in the nucleus.

Nitrogen-dependent regulation of photosynthetic gene expression

Nitrogen-limited Chlamydomonas reinhardtii is chlorotic and very deficient in chlorophyll a/b light-harvesting complexes (LHC). Rates of synthesis of photosynthetic proteins, but especially the LHC apoproteins, are reduced 10- to 40-fold. Moderately high levels of chloroplast transcripts accumulate in nitrogen-limited cells, and there is a correlation between chloroplast DNA levels and chloroplast mRNA abundance. In contrast, nuclear transcripts encoding LHCII and ribulose 1,5-bisphosphate carboxylase small subunits are markedly reduced. Thus, nitrogen availability affects chloroplast protein synthesis by inhibition of translation and, to a lesser extent, chloroplast DNA amplification. Regulation of nuclear-encoded photosynthetic proteins by nitrogen is achieved through mechanisms affecting transcription and/or mRNA stability.

Alterations in chlorophyll a/b binding proteins in Solanaceae cybrids

In this study we have constructed a number of plants (cybrids), in which the nuclear genome of Nicotiana plumbaginifolia is combined with the plastome of Atropa belladonna, or the nuclear genome of N. tabacum with plastomes of Lycium barbarum, Scopolia carniolica, Physochlaine officinalis or Nolana paradoxa. Our biochemical and immunological analyses prove that in these cybrids the biogenesis of the chlorophyll a/b binding proteins (CAB) of the light harvesting complex II (LHCII) is altered. Besides normal sized CAB polypeptides of 27, 25.5 and 25 kDa, which become less abundant, the cybrids analyzed have additional polypeptides of 26, 24.5 and 24 kDa. Direct protein micro-sequencing showed that at least two truncated 26 kDa CAB polypeptides in plant cells containing a nucleus of N. plumbaginifolia and plastids of A. belladonna are encoded by the type 1 Lhcb genes. These polypeptides are 11-12 amino acids shorter at the N-terminus than the expected size. Based on the available data we conclude that the biogenesis of the LHCII in vivo may depend on plastome-encoded factor(s). These results suggest that plastome-encoded factors that cause specific protein degradation and/or abnormal processing might determine compartmental genetic incompatibility in plants.

Biochemical and physiological responses of Gracilaria tenuistipitata uuder two different nitrogen treatments

The influence of nitrogen on ribulose-1.5-bisphosphate carboxylase/oxygenase (Rubisco. EC 4.1.1.39) content is poorly understood in macroalgae. N-deficient Gracilaria tenuistipitata Zhang et Xia var. liui was cultivated in the laboratory under constant light intensity and temperature. Biochemical and physiological variables were monitored after a high (1 mM) or low (o. 1 mM) nitrate pulse. Rubisco content in crude extracts was estimated by SDS-PAGE with the Coomassie Blue Staining procedure. Nitrate was consumed immediately in the low-N treatment, but there was always an external nitrate source in the high-N treatment. Total soluble proteins and phycobiliproteins decreased as internal nitrogen diminished in the low-N treatment, but kept fairly constant in N-sufficient conditions. However, Rubisco content increased until the 7th day and then started to decrease in both cases. Fresh weight increment showed a better correlation with Rubisco than with pigment content.

Molecular analysis of an aurea photosynthetic mutant (Su/Su) in tobacco: LHCP depletion leads to pleiotropic mutant phenotypes

Su is a nuclear encoded, semi-dominant aurea mutation in Nicotiana tabacum L. The homozygous plants (Su/Su) are pale yellow and non-photosynthetic while the heterozygous (Su/+) are photosynthetically competent and have a yellow-green phenotype which is distinct from that of green wild-type plants (+/+). We have examined the RNA and protein levels for a number of nuclear and plastid encoded chloroplast proteins under high and low light plant growth conditions. Under high light conditions, the light-harvesting chlorophyll a/b binding proteins (LHCP) were undetectable in the homozygous Su/Su plants, and the large subunit (LSu) and the small subunit (SSu) of ribulose bisphosphate carboxylase (Rubisco) and cytochrome b559 were severely deficient. However, only the nuclear encoded cab and plastid encoded psbE mRNA (encoding LHCP and cytochrome b559 respectively) were reduced significantly. In heterozygous Su/+ plants, the level of LHCP was reduced to 25% of that in wild-type plants while cab and psbE mRNA, LSu, SSu and cytochrome b559 remained at normal levels, suggesting that LCHP is more immediately affected by the Su mutant gene product than the rest of the photosynthetic proteins and mRNA examined. Under low light conditions, the levels of cab and psbE mRNA, LSu, SSu and cytochrome b559 in homozygous Su/Su plants were equivalent to those in wild-type plants except LHCP which remained undetectable. Similarly, the LHCP level in low light grown Su/+ plants still remained at 25% of wild-type level. These results indicate that the decrease in LHCP is independent of light conditions and has not resulted from photooxidation, whereas the depletion of other proteins and mRNA examined under high light growth conditions is a consequence of photooxidative damage to Su/Su plastids. Furthermore, transgenic Su/Su and Su/+ plants with a cauliflower mosaic virus 35S (CaMV 35S)-cab construct constitutively maintained high levels of cab mRNA but displayed the same pattern of diminished LHCP accumulation as their non-transformed counterparts when grown under both high and low light conditions. These results indicate that the Su mutation primarily causes depletion of LHCP. The depletion of LHCP leads to photooxidative damage which results in decreased cab mRNA levels and other pleiotropic lesions in Su/Su plants.

Light-dependent accumulation and localization of photosystem II proteins in maize

We have raised antibodies against several major components of photosystem II. These antisera, which are directed against the apoproteins of two chlorophyll-binding proteins (CPa-1 and CPa-2), the apoprotein of light-harvesting complex II and the 33-kDa extrinsic protein of the oxygen-evolving complex, were used to examine the light regulation of photosystem II assembly in maize. The principal findings of this study are as follows. The 33-kDa protein is present in dark-grown maize and the content increases 5-10-fold upon illumination. The level of the protein is mediated at least in part by phytochrome and is independent of the accumulation of chlorophyll. In contrast, none of the three chlorophyll-binding proteins examined was detectable in leaves of maize grown in darkness or under other light regimes where chlorophyll does not accumulate. Even in the absence of photosystem II assembly, the 33-kDa protein is properly transported across the thylakoid into the lumen. However, the protein does not attach in the normal way to the inner surface of the membrane under these conditions.