Control of the Metabolic Flux through the MEP Pathway

Dr. Louwrance Wright

Plants produce an enormous variety of metabolites which can be categorised into those responsible for primary metabolism and those synthesizing secondary metabolites. Although secondary metabolites are not directly involved in the plant’s growth and development, they are essential for the plants survival through ecological interactions with the environment. A very large amount of secondary compounds are synthesized by the modification of common backbone structures. This structural variation of secondary metabolites is important for the ability of plants to adapt to environmental pressures such as pests and diseases. The isoprenoids (also called terpenoids) are the most diverse group of secondary metabolites with tens of thousands of compounds identified. Certain isoprenoid compounds are also involved in primary metabolism with functions in respiration, photosynthesis, and regulation of growth and development. Many isoprenoids are also involved in the defence response of plants, including indirect defence through the emission of volatile signals that can attract herbivore predators. Despite their diversities, all isoprenoids are biosynthesized from the same five-carbon (C5) isoprene building blocks, isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP). It is to be expected that a metabolic pathway responsible for the biosynthesis of such a wide variety of compounds should be finely regulated to meet the demands of different tissues, organs and growth stages.



Interplay between the plant defence response, photosynthesis and the MEP pathway

Dr. Sirsha Mitra

Recent studies showed that 2-C-methylerythritol-2,4-cyclodiphosphate (MEcPP) is involved in the plastid-to-nucleus retrograde signaling in response to stress. Upon abiotic stresses, namely wounding and high light, an increase in MEcPP levels coincided with increased expression of genes in the hydroperoxide lyase (HPL) branch of the oxylipin metabolism pathway as well as increased levels of salicylic acid, a phytohormone. This increase in MEcPP levels and proposed export out of the chloroplast to fulfill its function as signal molecule, predict significant effects on the MEP pathway. We are interested in elucidating the effect of abiotic as well as biotic stresses on the in vivo kinetics of the MEP pathway and its effects on MEP pathway end products.

The major end products of the MEP pathway in Arabidopsis thaliana are the carotenoids and phytol chain of chlorophyll, integral components of the photosynthesis apparatus. Furthermore, it is well known that the MEP pathway is directly dependent from the Calvin-Benson-Bassham cycle for its substrates, glyceraldehyde 3-phosphate (GAP) and pyruvate. The balance between the tetrapyrrole pathway and the MEP pathway in chlorophyll biosynthesis was also recently shown to be crucial to prevent the production of deadly photo-oxidative stress. It is therefore of interest to understand the relationship between the flux in the MEP pathway and photosynthesis.



Compartmentalization of isoprenoid biosynthesis

Chenyong Lang

Plants contain two separate pathways for producing the universal isoprenoid precursors, IPP and DMAPP. The mevalonate pathway occurs in the cytoplasm and uses acetyl-CoA as substrate, whereas the MEP pathway occurs in the plastids and uses GAP and pyruvate as substrates. Numerous studies, using pathway specific inhibitors and or labelled precursors, showed that a certain amount of cross talk occurs between the MEP and mevalonate pathway. More recent research revealed the export of intermediates out of the MEP pathway under certain conditions, suggesting that the MEP pathway is not necessarily linear and not totally isolated from the cytosol. Using fractionation techniques together with 13C labelling, we are interested to investigate the fate of the exported MEP pathway intermediates and quantify the cross-talk between the mevalonate and MEP pathways.



Catabolism of exported MEP intermediates

Diego Gonzalez Cabanelas

It was shown recently that 2-C-methylerythritol-2,4-cyclodiphosphate (MEcPP) acts as a retrograde signal in plastid-to-nucleus communication. This MEcPP mediated signaling activity is transient and occur under specific abiotic stresses. It stands to reason that MEcPP should be converted to another form to fulfil its signaling function. We are therefore investigating the catabolism of MEcPP after its export out of the MEP pathway.

Regulation of the MEP pathway in isoprene-emitting plants

Diego Gonzalez Cabanelas

It is known that some plants emit isoprene in big amounts, while isoprene emission is totally absent in others. Isoprene is biosynthesized through the methylerythritol phosphate (MEP) pathway, which is also responsible for the synthesis of many important molecules involved in primary and secondary metabolism in plants. The MEP pathway is also a component of the defence signal cascade in plants and responsible for the emission of other ecologically important terpenoid volatiles. It is therefore important to understand the regulation of the MEP pathway and its influence on the emission of volatiles. Since isoprene emission is responsible for the majority of the flux through the MEP pathway in isoprene emitting plants, it stands to reason that the regulation of the MEP pathway in these plants will differ from non-emitting plants. This project aims to elucidate the regulatory steps involved in isoprene biosynthesis and the interaction between the MEP pathway, isoprene emmission and the environment. Furthermore, the regulation of the MEP pathway in Populus species, as isoprene emitter, will be compared with the regulation of the MEP pathway in the isoprene non-emitting plant Arabidopsis thaliana.