Improvement of Food Quality Traits through Enhancement of Starch Biosynthesis

Salinity induces carbohydrate accumulation and sugar-regulated starch biosynthetic genes in tomato (Solanum lycopersicum L. cv. 'Micro-Tom') fruits in an ABA- and osmotic stress-independent manner

Salinity stress enhances sugar accumulation in tomato (Solanum lycopersicum) fruits. To elucidate the mechanisms underlying this phenomenon, the transport of carbohydrates into tomato fruits and the regulation of starch synthesis during fruit development in tomato plants cv. 'Micro-Tom' exposed to high levels of salinity stress were examined. Growth with 160 mM NaCl doubled starch accumulation in tomato fruits compared to control plants during the early stages of development, and soluble sugars increased as the fruit matured. Tracer analysis with (13)C confirmed that elevated carbohydrate accumulation in fruits exposed to salinity stress was confined to the early development stages and did not occur after ripening. Salinity stress also up-regulated sucrose transporter expression in source leaves and increased activity of ADP-glucose pyrophosphorylase (AGPase) in fruits during the early development stages. The results indicate that salinity stress enhanced carbohydrate accumulation as starch during the early development stages and it is responsible for the increase in soluble sugars in ripe fruit. Quantitative RT-PCR analyses of salinity-stressed plants showed that the AGPase-encoding genes, AgpL1 and AgpS1 were up-regulated in developing fruits, and AgpL1 was obviously up-regulated by sugar at the transcriptional level but not by abscisic acid and osmotic stress. These results indicate AgpL1 and AgpS1 are involved in the promotion of starch biosynthesis under the salinity stress in ABA- and osmotic stress-independent manners. These two genes are differentially regulated at the transcriptional level, and AgpL1 is suggested to play a regulatory role in this event.

Temporally extended gene expression of the ADP-Glc pyrophosphorylase large subunit (AgpL1) leads to increased enzyme activity in developing tomato fruit

Tomato plants (Solanum lycopersicum) harboring the allele for the AGPase large subunit (AgpL1) derived from the wild species Solanum habrochaites (AgpL1 ( H )) are characterized by higher AGPase activity and increased starch content in the immature fruit, as well as higher soluble solids in the mature fruit following the breakdown of the transient starch, as compared to fruits from plants harboring the cultivated tomato allele (AgpL1 ( E )). Comparisons of AGPase subunit gene expression and protein levels during fruit development indicate that the increase in AGPase activity correlates with a prolonged expression of the AgpL1 gene in the AgpL1 ( H ) high starch line, leading to an extended presence of the L1 protein. The S1 (small subunit) protein also remained for an extended period of fruit development in the AgpL1 ( H ) fruit, linked to the presence of the L1 protein. There were no discernible differences between the kinetic characteristics of the partially purified AGPase-L1(E) and AGPase-L1(H) enzymes. The results indicate that the increased activity of AGPase in the AgpL1 ( H ) tomatoes is due to the extended expression of the regulatory L1 and to the subsequent stability of the heterotetramer in the presence of the L1 protein, implying a role for the large subunit not only in the allosteric control of AGPase activity but also in the stability of the AGPase L1-S1 heterotetramer. The introgression line of S. lycopersicum containing the wild species AgpL1 ( H ) allele is a novel example of transgressive heterosis in which the hybrid multimeric enzyme shows higher activity due to a modulated temporal expression of one of the subunits.

Agricultural biotechnology in developing countries

After a slow start many developing countries are now investing in agricultural biotechnology. Although these countries face several constraints, efforts are being made to promote biotechnology that requires high investment with long term returns. A number of donor agencies are providing incentives to stimulate biotechnology in the developing countries. There is however a major debate towards the development of biotechnology, especially genetically modified organisms, in the developing countries and there is a need for them to address biosafety issues and proper monitoring systems. The concern of intellectual property rights is a major issue in the developing countries in order to have access to the technologies that are often owned by multinational corporations in the industrialized countries.

Ectopic expression of a tobacco invertase inhibitor homolog prevents cold-induced sweetening of potato tubers

We have transformed potato with Nt-inhh cDNA, encoding a putative vacuolar homolog of a tobacco cell wall invertase inhibitor, under the control of the CaMV 35S promoter. In transgenic tubers, cold-induced hexose accumulation was reduced by up to 75%, without any effect on potato tuber yield. Processing quality of tubers was greatly improved without changing starch quantity or quality, an important prerequisite for the biotechnological use of Nt-inhh for potato transformation.