Lycopene ß-cyclase expression influences plant physiology, development, and metabolism in tobacco plants.

Carotenoids are important isoprenoids produced in the plastids of photosynthetic organisms that play key roles in photoprotection and antioxidative processes. ß-Carotene is generated from lycopene by lycopene ß-cyclase (LCYB). Previously, we demonstrated that the introduction of the Daucus carota (carrot) DcLCYB1 gene into tobacco (cv. Xanthi) resulted in increased levels of abscisic acid (ABA) and especially gibberellins (GAs), resulting in increased plant yield. In order to understand this phenomenon prior to exporting this genetic strategy to crops, we generated tobacco (Nicotiana tabacum cv. Petit Havana) mutants that exhibited a wide range of LCYB expression. Transplastomic plants expressing DcLCYB1 at high levels showed a wild-type-like growth, even though their pigment content was increased and their leaf GA1 content was reduced. RNA interference (RNAi) NtLCYB lines showed different reductions in NtLCYB transcript abundance, correlating with reduced pigment content and plant variegation. Photosynthesis (leaf absorptance, Fv/Fm, and light-saturated capacity of linear electron transport) and plant growth were impaired. Remarkably, drastic changes in phytohormone content also occurred in the RNAi lines. However, external application of phytohormones was not sufficient to rescue these phenotypes, suggesting that altered photosynthetic efficiency might be another important factor explaining their reduced biomass. These results show that LCYB expression influences plant biomass by different mechanisms and suggests thresholds for LCYB expression levels that might be beneficial or detrimental for plant growth.

Expression of a carotenogenic gene allows faster biomass production by redesigning plant architecture and improving photosynthetic efficiency in tobacco.

Because carotenoids act as accessory pigments in photosynthesis, play a key photoprotective role and are of major nutritional importance, carotenogenesis has been a target for crop improvement. Although carotenoids are important precursors of phytohormones, previous genetic manipulations reported little if any effects on biomass production and plant development, but resulted in specific modifications in carotenoid content. Unexpectedly, the expression of the carrot lycopene ß-cyclase (DcLCYB1) in Nicotiana tabacum cv. Xanthi not only resulted in increased carotenoid accumulation, but also in altered plant architecture characterized by longer internodes, faster plant growth, early flowering and increased biomass. Here, we have challenged these transformants with a range of growth conditions to determine the robustness of their phenotype and analyze the underlying mechanisms. Transgenic DcLCYB1 lines showed increased transcript levels of key genes involved in carotenoid, chlorophyll, gibberellin (GA) and abscisic acid (ABA) biosynthesis, but also in photosynthesis-related genes. Accordingly, their carotenoid, chlorophyll, ABA and GA contents were increased. Hormone application and inhibitor experiments confirmed the key role of altered GA/ABA contents in the growth phenotype. Because the longer internodes reduce shading of mature leaves, induction of leaf senescence was delayed, and mature leaves maintained a high photosynthetic capacity. This increased total plant assimilation, as reflected in higher plant yields under both fully controlled constant and fluctuating light, and in non-controlled conditions. Furthermore, our data are a warning that engineering of isoprenoid metabolism can cause complex changes in phytohormone homeostasis and therefore plant development, which have not been sufficiently considered in previous studies.

Identification of Two Arabidopsis thaliana Plasma Membrane Transporters Able to Transport Glucose 1-Phosphate.

Primary carbohydrate metabolism in plants includes several sugar and sugar-derivative transport processes. Over recent years, evidences have shown that in starch-related transport processes, in addition to glucose 6-phosphate, maltose, glucose and triose-phosphates, glucose 1-phosphate also plays a role and thereby increases the possible fluxes of sugar metabolites in planta. In this study, we report the characterization of two highly similar transporters, At1g34020 and At4g09810, in Arabidopsis thaliana, which allow the import of glucose 1-phosphate through the plasma membrane. Both transporters were expressed in yeast and were biochemically analyzed to reveal an antiport of glucose 1-phosphate/phosphate. Furthermore, we showed that the apoplast of Arabidopsis leaves contained glucose 1-phosphate and that the corresponding mutant of these transporters had higher glucose 1-phosphate amounts in the apoplast and alterations in starch and starch-related metabolism.