The classic model for explaining the biosynthesis of starch in plant leaves has been seriously called in to question. While to date the accepted belief has been that starch biosynthesis is produced solely in the chloroplast, biologist Nora Alonso Casajús’ PhD (photo) provides evidence to show that the greatest part of the precursor molecule in starch biosynthesis – known as ADPG – accumulates in the cytosol of the plants. This finding has meant a great advance in the race to obtain vegetables that can produce large quantities of starch, a substance the annual production of which is about a thousand million tonnes and which has become an essential raw material in multiple sectors of modern industry such as biofuels or biodegradable plastics.

The thesis, entitled, Factors involved in the regulation of starch and glycogen production in plants and bacteria was recently defended the Public University of Navarra’s Institute of Agrobiotechnology.

Biosynthesis of starch

According to the classical model for explaining the biosynthesis of starch, sucrose and starch are final products of two unidirectional routes that take place in the cytosol and the chloroplast respectively. Moreover, this model takes it that the ADPglucose pyrophosphorylase (AGP) is the only enzyme responsible for the biosynthesis of the starch precursor, ADPglucose (ADPG). Over the last few years there have been numerous indications suggesting the involvement of another enzyme, sucrose syntase (SuSy), in the production of the cytosolic ADPG needed for the synthesis of starch.

To analyse which of the models was the correct one this biologist determined the subcellular location of the ADPG linked to the starch biosynthesis, the tool used being plants that superexpress bacterial ADPG hydrolase, both in the cytosol and in the chloroplast.

With the research results, it has been possible to conclude that, contrary to that proposed in the classical model, most ADPG linked to starch biosynthesis is concentrated in the cytosol. This cytosolic location of the ADPG suggests, moreover, that the enzyme responsible for the biosynthesis of the ADPG is not the plastidial AGP, but the sucrose syntase. This is why the researcher went on to produce and characterise plants that superexpressed SuSy.

Her research concluded, primarily, that the ADPG produced by SuSy is linked to the biosynthesis of starch; secondly, that SuSy has significant control over this biosynthesis process and, thirdly, that it is SuSy and not the AGP that catalyses the production of ADPG that accumulates in the leaves.

Glycogen in bacteria

If starch is the main form of energy reserve for the plants, glycogen is the essential way in which bacteria accumulate energy. Nevertheless, according to Nora Alonso, “information about the possible involvement of glycogen in multiple metabolic processes is scarce and still fragmented”, the reason why part of the PhD was given over to study how the breakdown of the bacterial glycogen comes about and how the biology of this polyglucane functions. Thus, it was shown that glycogen acts as a “a carbon capacitor that helps to preserve osmotic homeostasis in the bacteria".

The possible involvement of glycogenphosphorylase (GlgP) in the breakdown of glycogen has, until now, been based on indirect evidence of a biochemical nature, as bacteria with altered levels of GlgP have never been produced or characterised. Nevertheless, in this PhD work the production and characterisation of bacteria with altered levels of GlgP have enabled an elucidation of the fundamental role of this enzyme, both in the breakdown of the glycogen and in the production of precursors for the synthesis of maltodextrines.

Moreover, Nora Alonso concluded that the control that GlgP has on the breakdown of glycogen and on the biosynthesis of maltodextrines is notably different for different strains of E. coli. Finally, Ms Alonso has shown that the GlgP acted during the process of accumulation of glycogen.