Abstract
The methylerythritol 4-phosphate (MEP) pathway of isoprenoid biosynthesis produces chlorophyll side chains and compounds that function in resistance to abiotic stresses, including carotenoids, and isoprene. Thus we investigated the effects of moderate and severe drought on MEP pathway function in the conifer Picea glauca, a boreal species at risk under global warming trends. Although moderate drought treatment reduced the photosynthetic rate by over 70%, metabolic flux through the MEP pathway was reduced by only 37%. The activity of the putative rate-limiting step, 1-deoxy-D-xylulose-5-phosphate synthase (DXS), was also reduced by about 50%, supporting the key role of this enzyme in regulating pathway metabolic flux. However, under severe drought, as flux declined below detectable levels, DXS activity showed no significant decrease, indicating a much-reduced role in controlling flux under these conditions. Both MEP pathway intermediates and the MEP pathway product isoprene incorporate administered 13CO2 to high levels (75–85%) under well-watered control conditions indicating a close connection to photosynthesis. However, this incorporation declined precipitously under drought, demonstrating exploitation of alternative carbon sources. Despite the reductions in MEP pathway flux and intermediate pools, there was no detectable decline in most major MEP pathway products under drought (except for violaxanthin under moderate and severe stress and isoprene under severe stress) suggesting that the pathway is somehow buffered against this stress. The resilience of the MEP pathway under drought may be a consequence of the importance of the metabolites formed under these conditions.
Highlights
Most members of the vast isoprenoid family of metabolites are produced from the two C5 diphosphate intermediates, dimethylallyl diphosphate (DMADP) and its isomer isopentenyl diphosphate (IDP)
We found drought to decrease metabolic flux through the methylerythritol 4-phosphate (MEP) pathway in P. glauca, this decline was considerably less than the declines in photosynthesis and transpiration observed, and there was no apparent decrease in the levels of most major MEP pathway products
In the moderate stress treatment, transpiration rate was reduced to 50% of that in well-watered trees, which occurred when the fraction of transpirable soil water (FTSW) was 12%
Summary
Most members of the vast isoprenoid family of metabolites are produced from the two C5 diphosphate intermediates, dimethylallyl diphosphate (DMADP) and its isomer isopentenyl diphosphate (IDP). These intermediates arise from two different pathways in plants, the mevalonate (MVA) pathway located in the cytosol, ER and peroxisomes, and the more recently identified. Eberl et al (2018) suggested a contribution to the total DMADP/IDP intermediate pool size from the MVA pathway, under fungus infestation in poplar leaves. Dudareva et al (2005) showed that the MEP pathway provides IDP precursors for both plastidial monoterpene and cytosolic sesquiterpenes synthesis in snapdragon flowers and pointed out the possibility of a cross talk between the two pathways. Other studies showed that cross talk between both pathways is not capable of rescuing a pharmacological block in either pathway (Laule et al, 2003; Rodríguez-Concepción et al, 2004)
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