Molecular and functional analysis of trehalose metabolism during pea seed development
Final Report Abstract
Tre-6-P represents an important signal to integrate and coordinate plant metabolism with development. Levels of Tre- 6-P are particularly elevated at the transition phase of embryo development and are indispensable for storage phase initiation. Pea lines expressing a bacterial TPS (OtsA) or TPP (OtsB) were generated under control of seed-specific promoters to modulate Tre-6-P levels. Repression of Tre-6-P levels causes a wrinkled seed phenotype, impairs starch synthesis and increases sucrose levels. In contrast, seeds with elevated levels of Tre6-P have viviparous-like seed and decreased sucrose levels. Measurements of metabolite levels and enzyme assays related to starch biosynthesis revealed that AGPase activity is stimulated by Tre-6-P while sucrose synthase activity is inhibited. Transcript profiling provides evidence for regulation of AGPase at the transcript level since post translational alteration via redox activation could not be detected. Transcript levels of sucrose synthase 1 are unchanged, hence activation of enzyme activity under Tre-6-P deficiency is likely based on post translational mechanisms (protein phosphorylation). It was also found that kinase activity of SnRK1 is inhibited in a post-translational manner by Tre-6-P. A model is proposed how Tre-6-P regulates starch biosynthesis in pea embryos. The synthesis of Tre-6-P is directly linked to availability of UDPGlc and Glc-6-P. In heterotrophic tissue Glc-6-P is produced from sucrose breakdown via sucrose synthase. Tre-6-P levels reflect the cytosolic UDPG and Glc-6-P status and subsequently hexose pools are balanced by either inhibition of sucrose breakdown or stimulation Glc-6-P-consumption via Tre-6-P. If supply of sucrose is sufficient, the elevation of Tre-6-P stimulates starch biosynthesis by activation of AGPase on the transcript level. As a consequence, UDPG and Glc-6-P become metabolised. Conversely, if levels of UDPG and Glc-6-P are relatively low, Tre-6-P levels tend to decline. This happens when sucrose breakdown limits the consumption of Glc-6-P. In order to re-balance the UDPG and Glc-6-P pool, the decrease of Tre-6-P causes a stimulation of sucrose breakdown by posttranslational activation of sucrose synthase on the one hand and a reduction of starch biosynthesis on the other hand. It is unknown how the production of the signal compound Tre-6-P itself is regulated. In Arabidopsis a number of 21 genes coding for isoforms of TPSs and TPPs were identified. Intersecting expression profiles and lacking catalytic activity of particularly class II TPS suggest a regulatory function of many of these genes. In a large-scale attempt for protein-protein interaction were analysed among members of Arabidopsis TPS and TPP and a plethora of interactions were found. It is proposed that single isoforms represent putative subunits of a hetero-oligomeric protein complex, which comprises all classes of TPS and TPP enzymes. Thus, Tre-6-P synthesis seems to be dynamically regulated by variation of the TPS/TPP complex composition.
Publications
- (2011) Hybrid embryos of Vicia faba develop enhanced sink strength, which is established during early development. Plant J. 65, 517-531
Tobias Meitzel, Ruslana Radchuk, Adriano Nunes-Nesi, Alisdair R. Fernie, Wolfgang Link, Winfriede Weschke and Hans Weber
