Starch synthesis and degradation in storage organs:
We aim to understand how starch granules are synthesised and degraded in the major starch-storing organs of crop plants. Historically, we studied peas carrying mutations that affect starch synthesis in the seed, and transgenic potatoes with reduced activities of specific proteins involved in starch synthesis. Our recent work is on starch synthesis and degradation in cereal seeds, and we are also interested in the synthesis of starch in cassava.
The Smart Carbohydrate Centre
The aim of this project (completed 2010) was to provide new resources for end-users and breeders of wheat and barley, paving the way for the development of new cultivars with improved or novel food and industrial value, and of new, renewable raw materials for industry.
With Kay Trafford (now at NIAB) and Andy Greenland at NIAB http://www.niab.com/pages/id/70/Professor_Andy_Greenland we exploited a collection of mutant lines of barley with unusual starches to generate novel variation for starch properties in an elite UK cultivar of barley. Four mutants lacking individual enzymes important for normal starch synthesis were crossed with elite cultivar NFC Tipple, and the progeny were used at NIAB to generate Tipple-like lines with radically altered starches. These lines have provided new information about the influence of the mutations and genetic background on starch and grain properties. They also enable the importance of starch properties to be tested for commercial applications. Prior to this project, very little variation for starch properties was available in UK-adapted cereals so the importance of this parameter could not be assessed. Grain from these lines has been supplied for testing to maltsters, food manufacturers, distillers, food processers and nutritionists interested in the links between diet and diabetes.
For several lines in the collection of mutant barley, the nature of the mutation affecting starch properties is not known. Work to map and thus discover these mutations is continuing, in collaboration with Cristobal Uauy (Department of Crop Genetics, JIC https://www.jic.ac.uk/profile/Cristobal-Uauy.asp).
Starch granules in normal barley (left) and a mutant with altered starch properties (right)
Howard TP, Fahy B, Craggs A, Mumford R, Leigh F, Howell P, Greenland A, Smith AM (2012) Barley mutants with low rates of endosperm starch synthesis have low grain dormancy and high susceptibility to pre-harvest sprouting. New Phytol 194, 158-167
Barley starch degradation
With Kay Trafford (now at NIAB) and Rob Field (Biological Chemistry Department, JIC https://www.jic.ac.uk/profile/Rob-Field.asp ) we are studying the control of starch degradation in germinating barley seeds. This process is central to the production of malt for beer and whisky manufacture. It is also relevant to understanding pre-harvest sprouting and related problems in cereal crops, and to the production of bioethanol from cereals. A great deal is known about the synthesis and structure of the individual enzymes involved, and many genetic factors that influence the malting process have been identified. Much less is known about how the rate of starch degradation is controlled in vivo during germination. We have taken two approaches:
- Chemical genetics. With Rob Field and colleagues, we are discovering new specific and class inhibitors for the major enzymes believed to be involved in starch degradation, and applying libraries of compounds to germinating seeds to identify novel inhibitors. This research is in collaboration with Birte Svensson, Department of Systems Biology, Danish Technical University.
- Reverse genetics. Using RNAi technology, Wendy Harwood and the BRACT transformation service http://www.bract.org/ have produced lines of barley with altered amounts or properties of the major enzymes of starch degradation.
In an initial project we used iminosugar inhibitors and transgenic plants to evaluate the precise role and importance of the α-glucosidase (maltase) thought to convert maltose to glucose during starch mobilisation in the endosperm of germinating seedlings. We confirmed the importance of the enzyme for maltose metabolism, and demonstrated that it is not necessary for starch degradation.
We also discovered that some inhibitors of α-glucosidases (naturally-occurring compounds called imnosugars) also inhibit starch degradation and rootlet growth in the seedling. This shows that they have other targets, in addition to the maltase. This result is important because loss of starch and the growth of rootlets during the commercial malting process both reduce the efficiency of malting, adversely affecting alcohol yields during brewing and distilling.
To investigate the potential value of iminosugars for preventing malting losses we are collaboration with David Cook (Department of Brewing, Nottingham University https://www.nottingham.ac.uk/biosciences/people/david.cook) and Robbie Waugh (James Hutton Institute, Dundee http://www.hutton.ac.uk/staff/robbie-waugh) in a BBSRC-funded project to discover the targets of iminosugars in the germinating grain, to test whether the targets are associated with malting quality (using association genetics in elite barley germplasm) and to investigate whether iminosugars can increase malting efficiency in micromalting trials.
Barley seeds germinated in the absence (left) and presence (right) of the iminosugar deoxynojirimycin (DNJ). 2012 field trial of barley lines, JHI, Dundee.
Rejzek M, Stevenson CE, Southard AM, Stanley D, Denyer K, Smith AM, Naldrett MJ, Lawson DM, Field RA (2010) Chemical genetics and cereal starch metabolism: structural basis of the non-covalent and covalent inhibition of barley ß-amylase. Mol BioSystems 7, 718-730
Stanley D, Rejzek M, Naested H, Smedley M, Otero S, Fahy B, Thorpe F, Nash RJ, Harwood W, Svensson B, Denyer K, Field RA, Smith AM (2011) The role of α-glucosidase in germinating barley grains. Plant Physiol 155, 932-943
Cassava starch synthesis
With Kay Trafford, we participated in a screen for altered starch structure and composition on mutated, inbred cassava lines developed by Hernan Ceballos and colleagues at CIAT, Colombia http://www.ciat.cgiar.org/AboutUs/People/Pages/profile_hernan_ceballos.aspx.
Using a simple, iodine based screen, a waxy (amylose-free) mutant and a mutant with abnormal, small starch granules were discovered. The waxy mutant is in trials in Thailand and shows much commercial promise.
Iodine-stained section of root of normal (left) and waxy (right) cassava. The black staining of normal cassava is due to the amylose component of starch, which is missing in the waxy mutant.
Ceballos H, Sáchez T, Morante N, Fregene M, Dufour D, Smith AM, Denyer K, Pérez JC, Calle F, Mestres C (2007) Discovery of a waxy starch mutant in cassava. J Agric Food Chem 55, 7469-7476.
Ceballos H, Sánchez T, Tofiño AP, Rosero EA, Denyer K, Smith A, Dufour D, Morante N, Pérez JC, Fahy B, Fregene M (2008) Induction and identification of a small-granule, high-amylose mutant in cassava (Manihot esculenta Crantz).J Agric Food Chem 56, 7215–7222