Light induction of cell type differentiation and cell-type-specific gene expression in cotyledons of a C(4) plant, Flaveria trinervia.

In Flaveria trinervia (Asteraceae) seedlings, light-induced signals are required for differentiation of cotyledon bundle sheath cells and mesophyll cells and for cell-type-specific expression of Rubisco small subunit genes (bundle sheath cell specific) and the genes that encode pyruvate orthophosphate dikinase and phosphoenolpyruvate carboxylase (mesophyll cell specific). Both cell type differentiation and cell-type-specific gene expression were complete by d 7 in light-grown seedlings, but were arrested beyond d 4 in dark-grown seedlings. Our results contrast with those found for another C(4) dicot, Amaranthus hypochondriacus, in which light was not required for either process. The differences between the two C(4) dicot species in cotyledon cell differentiation may arise from differences in embryonic and post-embryonic cotyledon development. Our results illustrate that a common C(4) photosynthetic mechanism can be established through different developmental pathways in different species, and provide evidence for independent evolutionary origins of C(4) photosynthetic mechanisms within dicotyledonous plants.

Expression of a C3 plant Rubisco SSU gene in regenerated C4 Flaveria plants.

We report the successful transformation, via Agrobacterium tumefaciens infection, and regeneration of two species of the genus Flaveria: F. brownii and F. palmeri. We document the expression of a C3 plant gene, an abundantly expressed ribulose 1,5-bisphosphate carboxylase/oxygenase small subunit gene isolated from petunia, in these C4 plants. The organ-specific expression of this petunia gene in Flaveria brownii is qualitatively identical to its endogenous pattern of expression.

Carbon-isotope discrimination by leaves of Flaveria species exhibiting different amounts of C3-and C 4-cycle co-function.

Carbon-isotope ratios were examined as δ(13)C values in several C3, C4, and C3-C4 Flaveria species, and compared to predicted δ(13)C, values generated from theoretical models. The measured δ(13)C values were within 4‰ of those predicted from the models. The models were used to identify factors that contribute to C3-like δ(13)C values in C3-C4 species that exhibit considerable C4-cycle activity. Two of the factors contributing to C3-like δ(13)C values are high CO2 leakiness from the C4 pathway and pi/pa values that were higher than C4 congeners. A marked break occurred in the relationship between the percentage of atmospheric CO2 assimilated through the C4 cycle and the δ(13)C value. Below 50% C4-cycle assimialtion there was no significant relationship between the variables, but above 50% the δ(13)C values became less negative. These results demonstrate that the level of C4-cycle expression can increase from, 0 to 50% with little integration of carbon transfer from the C4 to the C3 cycle. As expression increaces above 50%, however, increased integration of C3- and C4-cycle co-function occurs.

Atrazine, bromacil, and diuron resistance in chlamydomonas: a single non-mendelian genetic locus controls the structure of the thylakoid binding site.

A series of Chlamydomonas reinhardii mutants were selected for resistance to the herbicides atrazine, bromacil, and diuron. Four of these have reduced herbicide binding to the thylakoid membranes and show the non-Mendelian inheritance pattern characteristic of chloroplast genes. These mutants show a variety of selective alterations in binding of the three herbicides. These changes account for the observed patterns of in vivo cross-resistance. Analyses of chloroplast gene recombination indicate that these four mutations are in the same gene. Overall, the results suggest that this gene codes for a protein component of the herbicide binding site. One of the mutants has slow phototrophic growth and altered electron transport as has been observed in atrazine-resistant higher plant varieties, but the others are normal in these respects. The slow growth characteristic of this mutant seems to be the consequence of the same mutation which confers herbicide resistance.The mutants isolated also include a large number which achieve resistance by some secondary mechanism. These are all nuclear gene mutations, and represent numerous loci. They also show a variety of patterns of cross-resistance, but the mechanisms behind them have not yet been investigated.

Linkage of a Known Chloroplast Gene Mutation to the Uniparental Genome of CHLAMYDOMONAS REINHARDII.

Data are presented that associate three new markers with the uniparental linkage group in Chlamydomonas reinhardii. One of these, mutant 10-6C, is a genetic marker for the structural gene of the large subunit of ribulose bisphosphate carboxylase. These results provide the first direct link between the uniparental gene map and the physical map of chloroplast DNA. The other two markers, Dr2 (DCMU resistant) and 8-36C (deficient in photosystem II activity), map to a single locus. The data suggest that mixing in zygotic chloroplasts may not be complete so that input genomes do not have equal opportunities to recombine. The data are not compatible with simple linear or circular maps but can be explained on the basis of the known physical structure of chloroplast DNA.

Mendelian and uniparental alterations in erythromycin binding by plastid ribosomes.

Erythromycin binds specifically to the 52S subunit of the chloroplast ribosome of Chlamydomonas reinhardi. A number of erythromycin-resistant mutants whose ribosomes have lost their affinity for the antibiotic have been isolated, but the sedimentation properties of their ribosomes are indistinguishable from those of the wild-type strain. These mutants represent at least three genetic loci. Two of them show Mendelian inheritance, and one of them is inherited in a uniparental manner.