Increasing Sucrose Uptake Capacity of Wheat Grains Stimulates Storage Protein Synthesis  

Abstract

Increasing grain sink strength by improving assimilate uptake capacity could be a promising approach toward getting higher yield. The barley (Hordeum vulgare) sucrose transporter HvSUT1 (SUT) was expressed under control of the endosperm-specific Hordein B1 promoter (HO). Compared with the wild type, transgenic HOSUT grains take up more sucrose (Suc) in vitro, showing that the transgene is functional. Grain Suc levels are not altered, indicating that Suc fluxes are influenced rather than steady-state levels. HOSUT grains have increased percentages of total nitrogen and prolamins, which is reflected in increased levels of phenylalanine, tyrosine, tryptophan, isoleucine, and leucine at late grain development. Transcript profiling indicates specific stimulation of prolamin gene expression at the onset of storage phase. Changes in gene expression and metabolite levels related to carbon metabolism and amino acid biosynthesis suggest deregulated carbon-nitrogen balance, which together indicate carbon sufficiency and relative depletion of nitrogen. Genes, deregulated together with prolamin genes, might represent candidates, which respond positively to assimilate supply and are related to sugar-starch metabolism, cytokinin and brassinosteroid functions, cell proliferation, and sugar/abscisic acid signaling. Genes showing inverse expression patterns represent potential negative regulators. It is concluded that HvSUT1 overexpression increases grain protein content but also deregulates the metabolic status of wheat (Triticum aestivum) grains, accompanied by up-regulated gene expression of positive and negative regulators related to sugar signaling and assimilate supply. In HOSUT grains, alternating stimulation of positive and negative regulators causes oscillatory patterns of gene expression and highlights the capacity and great flexibility to adjust wheat grain storage metabolism in response to metabolic alterations.

Nitrogen allocation in barley: Relationships between amino acid transport and storage protein synthesis during grain filling

“…Takahashi et al (2003) found that seeds of globulin ''null mutants'' of soybean strongly accumulated amino acids, suggesting efficient uptake of amino acids independently of globulin biosynthesis. Grain storage protein (hordein) synthesis seem to be dependent on the available N in the grain as it is demonstrated in several plants by several authors (Balconi et al 1991;Golombek et al 2001;Miranda et al 2001;Rolletschek et al 2005;Weichert et al 2010). It appeared, thus, that other transport mechanisms such as transport (active transport) via specific amino acid transporters against the concentration gradient might hence play a role in the transport and acquisition of amino acids by the developing grains (Patrick 1997;Miranda et al 2001;Williams and Miller 2001;Takahashi et al 2003;Rolletschek et al 2005;Tislner et al 2005).…”

Enhanced sucrose loading improves rice yield by increasing grain size

“…For example, seed-specific overexpression of a potato (Solanum tuberosum) SUT increased Suc uptake and growth rates of developing pea (Pisum sativum) cotyledons (Rosche et al, 2002). In addition, the Suc uptake capacity of grains and storage protein biosynthesis was increased in transgenic wheat (Triticum aestivum) plants expressing the barley (Hordeum vulgare) SUT HvSUT1 under the control of an endosperm-specific promoter (Weichert et al, 2010). Moreover, it was recently found that these transgenic wheat plants had a higher thousand grain weight and grain width and length, as well as a 28% increase in grain yield (Saalbach et al, 2014).…”

Expression of Sucrose Transporter cDNAs Specifically in Companion Cells Enhances Phloem Loading and Long-Distance Transport of Sucrose but Leads to an Inhibition of Growth and the Perception of a Phosphate Limitation

“…With respect to ectopic expression of SUT genes to study the impact on carbon partitioning, constitutive expression of SoSUT1 (type I) of spinach (Spinacia oleracea) from the Cauliflower mosaic virus 35S promoter in potato (Solanum tuberosum) resulted in shifts in metabolite levels with little effect on tuber morphology (Leggewie et al, 2003), while OE of OsSUT5Z (type IIB) and OsSUT2M (type III) of rice (Oryza sativa) from a tuber-specific promoter was reported to increase tuber yield (Sun et al, 2011). In other species, ectopic expression of potato StSUT1 in the storage parenchyma of pea (Pisum sativum) cotyledons during seed development enhanced Suc influx and enhanced cotyledon growth rates (Rosche et al, 2002), and ectopic SUT expression in wheat (Triticum aestivum) grains increased levels of storage protein, indicating that enhanced Suc transport has benefits beyond carbohydrate accumulation (Weichert et al, 2010).…”