Bundle sheath leakiness and light limitation during C4 leaf and canopy CO2 uptake

Perennial species with the C(4) pathway hold promise for biomass-based energy sources. We have explored the extent that CO(2) uptake of such species may be limited by light in a temperate climate. One energetic cost of the C(4) pathway is the leakiness () of bundle sheath tissues, whereby a variable proportion of the CO(2), concentrated in bundle sheath cells, retrodiffuses back to the mesophyll. In this study, we scale from leaf to canopy level of a Miscanthus crop (Miscanthus x giganteus hybrid) under field conditions and model the likely limitations to CO(2) fixation. At the leaf level, measurements of photosynthesis coupled to online carbon isotope discrimination showed that leaves within a 3.3-m canopy (leaf area index = 8.3) show a progressive increase in both carbon isotope discrimination and as light decreases. A similar increase was observed at the ecosystem scale when we used eddy covariance net ecosystem CO(2) fluxes, together with isotopic profiles, to partition photosynthetic and respiratory isotopic flux densities (isofluxes) and derive canopy carbon isotope discrimination as an integrated proxy for at the canopy level. Modeled values of canopy CO(2) fixation using leaf-level measurements of suggest that around 32% of potential photosynthetic carbon gain is lost due to light limitation, whereas using determined independently from isofluxes at the canopy level the reduction in canopy CO(2) uptake is estimated at 14%. Based on these results, we identify as an important limitation to CO(2) uptake of crops with the C(4) pathway.     

Here comes the sun: How optimization of photosynthetic light reactions can boost crop yields

Photosynthesis started to evolve some 3.5 billion years ago CO₂ is the substrate for photosynthesis and in the past 200–250 years, atmospheric levels have approximately doubled due to human industrial activities. However, this time span is not sufficient for adaptation mechanisms of photosynthesis to be evolutionarily manifested. Steep increases in human population, shortage of arable land and food, and climate change call for actions, now. Thanks to substantial research efforts and advances in the last century, basic knowledge of photosynthetic and primary metabolic processes can now be translated into strategies to optimize photosynthesis to its full potential in order to improve crop yields and food supply for the future. Many different approaches have been proposed in recent years, some of which have already proven successful in different crop species. Here, we summarize recent advances on modifications of the complex network of photosynthetic light reactions. These are the starting point of all biomass production and supply the energy equivalents necessary for downstream processes as well as the oxygen we breathe.     

Vitiligo puzzle: the pieces fall in place

Over the years, the role of biochemical, immunological, genetic, and other biological aspects in the pathogenesis of vitiligo has been studied. So far, no convincing model describing the interplay of these contributing factors has been formulated. Based on existing research, we propose that vitiligo has a multi-factorial etiology, characterized by multiple steps, but always involving an increase of external or internal phenol/catechol concentration, serving as a preferred surrogate substrate of tyrosinase, competing with its physiological substrate tyrosine. The conversion of these substrates into reactive quinones is reinforced by a disturbed redox balance (increasing hydrogen peroxide). Such reactive quinones can be covalently bound to the catalytic centre of tyrosinase (haptenation). This could give rise to a new antigen, carried by Langerhans cells to the regional lymph node, stimulating the proliferation of cytotoxic T cells. However, the activation of such cytotoxic cells is only a first step in skin melanocyte killing, which also depends on a shift in the balance between immune defence and tolerance, e.g. resulting from a decrease in properly functioning T-regulatory cells. With this new model, based on a synthesis of several of the existing theories, in mind, the external and internal factors involved in the etiopathogenesis of vitiligo are reviewed, against the background of reported clinical data, experimental studies and existing and potential new therapies. A similar complex mechanism may also lead to some other autoimmune diseases.     

Predicting light-induced stomatal movements based on the redox state of plastoquinone: theory and validation

Photosynthesis Research - Prediction of stomatal conductance is a key element to relate and scale up leaf-level gas exchange processes to canopy, ecosystem and land surface models. The empirical...     

Generation of a human melanocyte cell line by introduction of HPV16 E6 and E7 genes

Summary Availability of a standard human melanocyte cell line with unlimited growth potential and otherwise normal melanocytic properties will greatly facilitate research in melanocyte biology and in vitro studies on the etiology of pigmentary disorders and melanoma. Using a retroviral vector, E6 and E7 open reading frames of human papilloma virus type 16 (HPV 16) have been introduced into cultured normal human melanocytes. Cells selected by increased resistance to geneticin conveyed by the vector and expressing E6E7 mRNA have been cloned to ensure genetic homogeneity. Since their establishment as primary cells, cloned PIG1 cells have undergone more than twice the amount of population doublings of senescent parental cells. Moreover, in passage numbers when parental cells had become senescent, proliferation of clonal cells was retained at levels exceeding those of normal human melanocytes in third passage by 100%. Further characterization has revealed that the cells remain dependent on tetradecanoyl phorbol 13-acetate (TPA) for growth and do not proliferate in soft agar nor form tumors in nude mice. The antigenic profile of the cells was slightly altered as compared to parental cells, but was incomparable to that of M14 melanoma cells. Importantly, PIG1 cells contain more melanin pigment than parental cells.     

Difference in pathogenesis between vitiligo vulgaris and halo nevi associated with vitiligo is supported by an HLA association study

Human leukocyte antigen (HLA) class II associations with two subtypes of vitiligo: vitiligo vulgaris and halo nevi associated with vitiligo were investigated. In previous studies associations between vitiligo and HLA antigens have been reported but these two subtypes have never been taken into account. However from a clinical and histological point of view, a difference in (auto)-immune pathogenesis can be expected. This difference might be reflected in an association with different HLA alleles. Seventy-six unrelated Dutch Caucasians, 40 with vitiligo vulgaris and 36 with halo nevi associated with vitiligo were included. A panel of randomly chosen HLA typed healthy Dutch blood donors (n = 2400) served as control population. HLA-DR and -DQ typing was carried out on blood samples by amplifying genomic DNA using polymerase chain reaction followed by dot blot hybridization with sequence specific oligonucleotides. The main outcome measures were odds ratio (OR), uncorrected P-value (P(u)) and corrected P-value. There were distinct differences in the clinical manifestations between vitiligo vulgaris and halo nevi associated with vitiligo with respect to precipitating factors, extent and progress of the disease and the association with other auto-immune diseases in the two subtypes and their respective first degree family members. Our stratification reveals differences in HLA class II between both subtypes and between subtypes and controls. A case-control association study showed a significant positive association of HLA-DR4 (OR = 2.787, P(u) = 0.0022) and DR53 (OR = 2.249, P(u) = 0.0153) and a negative association of HLA-DR3 (OR = 0.195, P(u) = 0.0024) with vitiligo vulgaris. The group with halo nevi associated with vitiligo did not show these associations, but had a significant negative association with HLA-DR11 (OR = 0.083, P(u) = 0.0067). In conclusion, the differences in HLA association within clinical subtypes of vitiligo support our suggestion that vitiligo vulgaris and halo nevi associated with vitiligo have distinct pathogenic mechanisms.     

PIG3V, an immortalized human vitiligo melanocyte cell line, expresses dilated endoplasmic reticulum

Summary Vitiligo is an enigmatic pigmentary disorder of the skin. Factors potentially involved in the progressive loss of melanocytes from the basal layer of the epidermis include genetically determined aberrancies of the vitiligo melanocyte. It follows that analysis of melanocytes cultured from vitiligo donors can contribute to a further understanding of the etiopathomechanism. A setback for vitiligo research has been the limited availability of vitiligo-derived melanocytes. To overcome this limitation, we have generated a vitiligo melanocyte cell line according to a protocol established previously for the immortalization of normal human melanocytes. Vitiligo melanocytes Ma9308P4 were transfected with HPV16 E6 and E7 genes using the retroviral construct LXSN16E6E7. Successful transformants were selected using geneticin and subsequently cloned to ensure genetic homogeneity. The resulting cell line PIG3V has undergone more than 100 cell population doublings ince its establishment as a confluent primary culture, whereas untransfected melanocytes derived from adult skin senesce after a maximum of 50 population doublings. Cells immortalized by this transfection procedure retain lineage-specific characteristics and proliferate significantly faster than parental cells. In this study, the phenotype of PIG3V resembled melanocytes rather than melanoma cells in culture. Tyrosinase was processed properly and melanosomes remained pigmented. Importantly, ultrastructural characterization of PIG3V cells revealed dilated endoplasmic reticulum profiles characteristic of vitiligo melanocytes. An explanation for this dilation may be found in the retention of proteins with molecular weight of 37.5, 47.5, and 56.5 kDa, as determined by gel electrophoresis of microsomal proteins isolated from radiolabeled cells. Presented in part at the Annual Meeting of the Panamerican Society for Pigment Cell Research, Aspen, Colorado, 1998.     

One crop breeding cycle from starvation? How engineering crop photosynthesis for rising CO2 and temperature could be one important route to alleviation

Global climate change is likely to severely impact human food production. This comes at a time when predicted demand for primary foodstuffs by a growing human population and changing global diets is already outpacing a stagnating annual rate of increase in crop productivity. Additionally, the time required by crop breeding and bioengineering to release improved varieties to farmers is substantial, meaning that any crop improvements needed to mitigate food shortages in the 2040s would need to start now. In this perspective, the rationale for improvements in photosynthetic efficiency as a breeding objective for higher yields is outlined. Subsequently, using simple simulation models it is shown how predicted changes in temperature and atmospheric [CO₂] affect leaf photosynthetic rates. The chloroplast accounts for the majority of leaf nitrogen in crops. Within the chloroplast about 25% of nitrogen is invested in the carboxylase, Rubisco, which catalyses the first step of CO₂ assimilation. Most of the remaining nitrogen is invested in the apparatus to drive carbohydrate synthesis and regenerate ribulose-1:5-bisphosphate (RuBP), the CO₂-acceptor molecule at Rubisco. At preindustrial [CO₂], investment in these two aspects may have been balanced resulting in co-limitation. At today''s [CO₂], there appears to be over-investment in Rubisco, and despite the counter-active effects of rising temperature and [CO₂], this imbalance is predicted to worsen with global climate change. By breeding or engineering restored optimality under future conditions increased productivity could be achieved in both tropical and temperate environments without additional nitrogen fertilizer. Given the magnitude of the potential shortfall, better storage conditions, improved crop management and better crop varieties will all be needed. With the short time-scale at which food demand is expected to outpace supplies, all available technologies to improve crop varieties, from classical crop breeding to crop genetic engineering should be employed. This will require vastly increased public and private investment to support translation of first discovery in laboratories to replicated field trials, and an urgent re-evaluation of regulation of crop genetic engineering.