Sucrose-Phosphate Synthase and Sucrose Synthase contribute to refoliation in ryegrass, a grassland fructan-accumulating species.

The perennity of grassland species such as Lolium perenne greatly depends on their ability to regrow after cutting or grazing. Refoliation largely relies on the mobilization of fructans in the remaining tissues and on the associated sucrose synthesis and transport towards the basal leaf meristems. However, nothing is known yet about the sucrose synthesis pathway. Sucrose Phosphate Synthase (SPS) and Sucrose Synthase (SuS) activities, together with their transcripts, were monitored during the first hours after defoliation along the leaf axis of mature leaf sheaths and elongating leaf bases (ELB) where the leaf meristems are located. In leaf sheaths, which undergo a sink-source transition, fructan and sucrose contents declined while SPS and SuS activities increased, along with the expression of LpSPSA, LpSPSD.2, LpSuS1, LpSuS2, and LpSuS4. In ELB, which continue to act as a strong carbon sink, SPS and SuS activities increased to varying degrees while the expression of all the LpSPS and LpSuS genes decreased after defoliation. SPS and SuS both contribute to refoliation but are regulated differently depending on the source or sink status of the tissues. Together with fructan metabolism, they represent key determinants of ryegrass perennity and, more generally, of grassland sustainability.

Comparative phosphoproteomic analysis reveals that phosphorylation of sucrose synthase GhSUS2 by Ca2+-dependent protein kinases GhCPK84/93 affects cotton fiber development.

Cotton fiber elongation is a critical growth phase that affects final fiber length. Morphological analysis indicated an asynchronous fiber elongation pattern between two cotton varieties, J7-1 and J14-1. Through phosphoproteomic analysis, a total of 89 differentially-phosphorylated proteins (DPPs) were identified in elongating fibers between J7-1 and J14-1. Gene ontology (GO) analysis showed that these DPPs were mainly enriched in sucrose synthase activity, transferase activity, and UDP-glycosyltransferase activity. In J14-1, the phosphorylation level of GhSUS2, a key sucrose synthase in the sucrose metabolism pathway, was significantly higher than that in J7-1. We further revealed that GhSUS2 positively regulates fiber elongation, and GhSUS2-silenced transgenic cotton displayed the phenotype of 'short fibers' compared with the controls. During fiber development, the residue Ser11 in the GhSUS2 protein is phosphorylated by the Ca2+-dependent protein kinases GhCPK84 and GhCPK93. Phosphorylated GhSUS2 is localized in the cytoplasm, whereas unphosphorylated GhSUS2 is localized in the plasma membrane. Moreover, abscisic acid (ABA) could promote the transcription and translation of GhCPK84 and GhCPK93, thereby enhancing the phosphorylation of GhSUS2 to impede fiber elongation. Thus, our data demonstrates that GhSUS2 plays a positive role in fiber development, but its phosphorylation by GhCPK84 and GhCPK93 hinders fiber elongation of cotton.

Deficient sucrose synthase activity in developing wood does not specifically affect cellulose biosynthesis, but causes an overall decrease in cell wall polymers.

The biosynthesis of wood in aspen (Populus) depends on the metabolism of sucrose, which is the main transported form of carbon from source tissues. The largest fraction of the wood biomass is cellulose, which is synthesized from UDP-glucose. Sucrose synthase (SUS) has been proposed previously to interact directly with cellulose synthase complexes and specifically supply UDP-glucose for cellulose biosynthesis. To investigate the role of SUS in wood biosynthesis, we characterized transgenic lines of hybrid aspen with strongly reduced SUS activity in developing wood. No dramatic growth phenotypes in glasshouse-grown trees were observed, but chemical fingerprinting with pyrolysis-GC/MS, together with micromechanical analysis, showed notable changes in chemistry and ultrastructure of the wood in the transgenic lines. Wet chemical analysis showed that the dry weight percentage composition of wood polymers was not changed significantly. However, a decrease in wood density was observed and, consequently, the content of lignin, hemicellulose and cellulose was decreased per wood volume. The decrease in density was explained by a looser structure of fibre cell walls as shown by increased wall shrinkage on drying. The results show that SUS is not essential for cellulose biosynthesis, but plays a role in defining the total carbon incorporation to wood cell walls.