Loss of the two major leaf isoforms of sucrose-phosphate synthase in Arabidopsis thaliana limits sucrose synthesis and nocturnal starch degradation but does not alter carbon partitioning during photosynthesis

Abstract

Sucrose (Suc)-phosphate synthase (SPS) catalyses one of the rate-limiting steps in the synthesis of Suc in plants. The Arabidopsis genome contains four annotated SPS genes which can be grouped into three different families (SPSA1, SPSA2, SPSB, and SPSC). However, the functional significance of this multiplicity of SPS genes is as yet only poorly understood. All four SPS isoforms show enzymatic activity when expressed in yeast although there is variation in sensitivity towards allosteric effectors. Promoter-reporter gene analyses and quantitative real-time reverse transcription-PCR studies indicate that no two SPS genes have the same expression pattern and that AtSPSA1 and AtSPSC represent the major isoforms expressed in leaves. An spsa1 knock-out mutant showed a 44% decrease in leaf SPS activity and a slight increase in leaf starch content at the end of the light period as well as at the end of the dark period. The spsc null mutant displayed reduced Suc contents towards the end of the photoperiod and a concomitant 25% reduction in SPS activity. In contrast, an spsa1/spsc double mutant was strongly impaired in growth and accumulated high levels of starch. This increase in starch was probably not due to an increased partitioning of carbon into starch, but was rather caused by an impaired starch mobilization during the night. Suc export from excised petioles harvested from spsa1/spsc double mutant plants was significantly reduced under illumination as well as during the dark period. It is concluded that loss of the two major SPS isoforms in leaves limits Suc synthesis without grossly changing carbon partitioning in favour of starch during the light period but limits starch degradation during the dark period.

Characterization of multiple SPS knockout mutants reveals redundant functions of the four Arabidopsis sucrose phosphate synthase isoforms in plant viability, and strongly indicates that enhanced respiration and accelerated starch turnover can alleviate the blockage of sucrose biosynthesis

“…Studies of the predicted amino acid sequences and gene structure have shown that the Arabidopsis SPS family consists of four SPS genes, referred to as AtSPSA1 (At5g20280), AtSPSA2 (At5g11110), AtSPSB (At1g04920) and AtSPSC (At4g10120) [14,15]. Genome-wide expression analyses (https:// www.geneinvestigator.ethz.ch) and comparative studies of SPS gene expression in Arabidopsis [14,15] provided evidence for distinct, but partially overlapping spatial and temporal expression patterns for the four SPS genes. Metabolic studies of an spsa1/spsc double knockout Arabidopsis mutant revealed effects on growth and leaf nonstructural carbohydrate metabolism in this mutant [15].…”

“…Genome-wide expression analyses (https:// www.geneinvestigator.ethz.ch) and comparative studies of SPS gene expression in Arabidopsis [14,15] provided evidence for distinct, but partially overlapping spatial and temporal expression patterns for the four SPS genes. Metabolic studies of an spsa1/spsc double knockout Arabidopsis mutant revealed effects on growth and leaf nonstructural carbohydrate metabolism in this mutant [15]. Thus spsa1/spsc plants cultured under 8 h light/16 h dark photoregime displayed a dwarf phenotype.…”

“…Also, these plants accumulated low levels of sucrose and moderately high levels of both starch and maltose when compared with wild type (WT) plants, strongly indicating that SPSA1 and SPSC have overlapping functions in aspects related with growth and leaf nonstructural carbohydrate metabolism. According to Volkert et al [15], the increase in starch was probably not due to an increased partitioning of carbon into starch, but was rather caused by an impaired starch mobilization during the night due to impairment in downstream metabolization of maltose.…”

STOREKEEPER RELATED1/G-Element Binding Protein (STKR1) Interacts with Protein Kinase SnRK1

“…At the end of the light period, the starch content in leaves of 35S-STKR1 plants was unaltered as compared with the wild-type control, while the levels of Glc, Fru, and Suc were reduced in the transgenics. The reduction in soluble carbohydrates upon the overexpression of STKR1 could be brought about by a restriction in Suc synthetic capacity, as, for instance, it is observed in plants with reduced activity of SPS, the enzyme catalyzing the rate-limiting step within this pathway (Chen et al, 2005;Volkert et al, 2014). However, a restriction in Suc biosynthetic capacity has been shown to result in increased leaf starch levels caused by an impaired starch mobilization during the night (Chen et al, 2005;Volkert et al, 2014).…”

“…The reduction in soluble carbohydrates upon the overexpression of STKR1 could be brought about by a restriction in Suc synthetic capacity, as, for instance, it is observed in plants with reduced activity of SPS, the enzyme catalyzing the rate-limiting step within this pathway (Chen et al, 2005;Volkert et al, 2014). However, a restriction in Suc biosynthetic capacity has been shown to result in increased leaf starch levels caused by an impaired starch mobilization during the night (Chen et al, 2005;Volkert et al, 2014). Starch levels in STKR1 overexpression lines were strongly reduced at the end of the dark period, arguing for an accelerated rate of starch breakdown in these plants despite the lower Suc levels also observed at the end of the dark period.…”