The CO2 transfer conductance in leaves quantifies the ease with which CO2 can diffuse from sub-stomatal cavities to sites of carboxylation within the chloroplast. The aim of this work was to test the hypothesis that the CO2 transfer conductance is proportional to the surface area of chloroplasts exposed to intercellular airspaces. We compared two genotypes, wild-type and transgenic tobacco, that had been transformed with an antisense gene directed at the mRNA of the Rubisco small subunit. Transgenic tobacco had lower rates of CO2 assimilation than wild-type but similar chlorophyll contents. Leaf anatomy was altered by growing plants in two different environments: a high daily irradiance in a growth cabinet (12 h photoperiod of 1 mmol quanta m-2 s-1) and a sunlit glasshouse. The growth cabinet gave at least twice the daily irradiance compared to the glasshouse. The CO2 transfer conductance was calculated from combined measurements of gas exchange and carbon isotope discrimination measured in 2% oxygen. Following gas exchange measurement, leaves were sampled for biochemical and anatomical measure- ment. In transgenic tobacco plants, Rubisco content was 35% of that found in the wild-type tobacco, the CO2 assimilation rate was 50% of the wild-type rate and the chlorophyll content was unaltered. While leaf mass per unit leaf area of transgenic tobacco was 82% of that of the wild-type, differences in leaf thickness and surface area of mesophyll cells exposed to intercellular airspace per unit leaf area (Smes) were small (92 and 87% of wild-type, respectively). Leaves grown in the growth cabinet under high daily irradiance were thicker (63%), had a greater Smes (41%) due to the development of thicker palisade tissue, had higher photosynthetic capacity (27%) and contained more chlorophyll (58%) and Rubisco (77%), than leaves from plants grown in the glasshouse. Irrespective of genotype or growth environment, CO2 transfer conductance varied in proportion to surface area of chloroplasts exposed to intercellular airspaces. While the method for calculating CO2 transfer conductance could not distinguish between limitations due to the gas or liquid phases, there was no reduction in CO2 transfer conductance associated with more closely packed cells, thicker leaves, nor with increasing chloroplast thickness in tobacco.

No abstract

In situ hybridisation techniques have been used to determine the distribution of mRNAs for ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco: EC 4.1.1.39) and Rubisco activase in leaves of Atriplex patula L. (C3) and A. rosea L. (C4). In A. patula, mRNA for Rubisco small subunit (encoded by the rbcS gene family) was found to accumulate in the mesophyll and bundle sheath, while in A. rosea it accumulated in the bundle sheath only, as shown previously for the C4 monocot Zea mays L. The spatial distribution of rca transcripts for Rubisco activase paralleled that for the rbcS transcripts in both C3 and C4 Atriplex species, providing evidence that Rubisco activase is required in cells only where Rubisco is present.

mRNA coding for the abundant broad-range plasma proteinase inhibitor alpha 1-inhibitor III (alpha 1I3) was detected only in rat liver, while mRNA for the related proteins alpha 1-macroglobulin and alpha 2-macroglobulin was also found in a variety of nonhepatic tissues. cis-Acting control elements necessary for the hepatic transcription of alpha 1I3 were mapped by transfection and expression studies of control-region constructs in cultured hepatic and nonhepatic cells. The promoter-proximal 5'-flanking region contained four control elements, I to IV, located between -109 and -196 base pairs upstream of the transcriptional start site relevant for the hepatic transcription of this gene. Elements II and III were essential, and I and IV exerted strong modulatory effects. Elements I to III acted as positive regulators, and IV acted as a negative element. Element II contained the sequence TGGCA and is probably a binding site for a nuclear factor related to the known transcription factor NF1. The other three elements did not resemble consensus binding sites for known transcription factors that are involved in the hepatocyte-specific transcription of other well-characterized plasma protein genes, such as the prototype factor HNF-1. Thus, the alpha 1I3 gene achieves its highly hepatocyte-specific transcription through a novel combination of cis-acting control elements and trans-acting factors.