Submitted to: Plant Cell and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/1/2010
Publication Date: 4/1/2011
Publication URL: http://naldc.nal.usda.gov/catalog/49429
Citation: Sun, J., Zhang, J., Larue, C.T., Huber, S.C. 2011. Decrease in leaf sucrose synthesis leads to increased leaf starch turnover and decreased RuBP-limited photosynthesis but not Rubisco-limited photosynthesis in Arabidopsis null mutants of SPSA1. Plant Cell and Environment. 34(4)592-604. Interpretive Summary: Sucrose is an important carbohydrate in plants. In most species, it is a major end product of leaf photosynthesis that is translocated to non-photosynthetic plant parts to support their growth and development. Plants contain one enzyme that functions in sucrose biosynthesis, known as sucrose-phosphate synthase (SPS), and in all plants studied to date this enzyme is encoded by a small gene family. For example, in the model plant, Arabidopsis, SPS is encoded by four genes that are all known to be expressed at the transcript level and thus are active genes. However, it is not clear which of the SPS isoforms encoded by the four different genes plays a major role in leaf sucrose biosynthesis. As an initial approach to address this question, we obtained null mutants (referred to as knockouts) for each of the four SPS genes and determined the impact on photosynthesis and plant growth. Interestingly, only two of the four SPS genes appear to be expressed to a significant extent at the protein level, and one of these (designated as SPSA1) accounts for the majority (85%) of the SPS activity in a leaf extract. It is only when this gene is inactivated (referred to as the spsa1 mutant) that there is an impact on photosynthesis. Under normal conditions, photosynthetic rate is unaltered but plants accumulate substantially more starch during the day and then mobilize it at night. Overall growth of the spsa1 mutant is similar to wild type plants, indicating that plant growth is quite plastic and can accommodate substantial changes in the temporal distribution of photosynthate throughout the plant. The spsa1 mutant provides a novel experimental platform for future experiments to determine domains of the protein that impact activity, including those controlling regulatory parameters. A fundamental understanding of key genes regulating important plant processes may yield new approaches to improve crop productivity.
Technical Abstract: SPS (Sucrose phosphate synthase) isoforms from dicots cluster into families A, B and C. In this study, we investigated the individual effect of null mutations of each of the four SPS genes in Arabidopsis (spsa1, spsa2, spsb and spsc) on photosynthesis and carbon partitioning. Null mutants spsa1 and spsc led to decreases in maximum SPS activity in leaves by 80% and 13%, respectively, whereas null mutants spsa2 and spsb had no significant effect. Consistently, isoform-specific antibodies detected SPSA1 and SPSC protein in leaf extracts, but not SPSA2 or SPSB. Leaf photosynthesis measured under ambient [CO2] was not different among the genotypes but when measured under saturating [CO2] levels, photosynthesis was 20% lower in spsa1 mutants compared with wild type plants and the other null mutants. However, leaf carbon partitioning was altered in the spsa1 null mutant as evidenced by 50 to 70% increase in leaf starch content and nocturnal starch mobilization under ambient [CO2]. Rosette growth was indistinguishable among the mutants and wild type plants when grown in soil or on agar plates with or without sucrose, although root growth of the spsa1 mutant was slightly slower on agar plates without sucrose. Cold treatment of plants (4⁰ C for 96 h) increased leaf soluble sugars and starch and increased the leaf content of SPSA1 and SPSC proteins 2 to 3-fold. SPSA1 and SPSB proteins were not below detection, and of the four null mutants, only spsa1 reduced leaf nonstructural carbohydrate accumulation in response to cold treatment. SPS genes had distinct expression patterns and generally correlated with their contribution to total leaf SPS activity: expression of SPSA1 was higher than SPSC, which in turn was higher than SPSA2 and SPSB. Expression of SPSA1 exhibited strong diurnal patterns in the leaves with maximum expression during the light period whereas expression of the other SPS genes did not vary diurnally. Expression of SPSA1 increased and decreased during the night period. We concluded that SPSA1 plays the major role in photosynthetic sucrose synthesis in Arabidopsis leaves, and decreases in leaf SPS activity leads to increased starch synthesis and starch turnover, decreased RuBP-limited photosynthesis but not Rubisco-limited photosynthesis. At atmospheric [CO2], adjustments in carbon allocation and utilization result in no impact of the spsa1 null mutation on growth.