Review article Starches—from current models to genetic engineering Uwe Sonnewald 1 and Jens Kossmann 2, * 1 Department of Biology, Friedrich-Alexander-University Erlangen-Nuremberg, Erlangen, Germany 2 Institute of Biotechnology, Department of Genetics, Stellenbosch University, Stellenbosch, South Africa Received 10 August 2012; revised 29 October 2012; accepted 31 October 2012. *Correspondence (Tel +27 (0)21 8083836; fax +27 (0)21 8083835; email kossmann@sun.ac.za) Keywords: starch synthesis, starch degradation, regulation of starch metabolism. Summary As the world’s second most abundant biopolymer, starch serves as food, feed and renewable resource for bioenergy production and other industrial applications. Unlike storage lipids, starch is stored in the form of semi-crystalline granules, which are tissue- and species-specific in number, shape and size. Over the last decades, most biosynthetic and degradative enzymes of starch metabolism have been identified in the model species Arabidopsis thaliana. Based on this, biotechnological applications have arisen that led to a number of transgenic crop plants with elevated starch content or improved starch quality. Irrespective of this great success, there are still numerous open questions including the regulation of starch metabolism, the initiation of granule formation, the regulation of granule shape and size and many more, which will be tackled over the next decades. Here, we briefly summarize current knowledge concerning starch metabolism and its regulation and biotechnological use. Introduction Starch is one of the most abundant constituents of the world’s crops (Jobling, 2004). The majority of starch harvested in crops is consumed directly as food or feed, but a significant proportion is also directed into industrial applications (Marz, 2006), with increasing proportions being used as feedstock for bioethanol production. While the food, paper and textile industries consume a large amount of starch in their manufacturing processes, application in other industries has been limited by far more competitively priced petroleum-based products (Visser and Jacobsen, 1993). Likely future increases in the cost of fossil fuels will make starch an attractive alternative as a raw material in industrial applications (R€ oper, 2002) other than those it is already used for. Starch often requires chemical or physical treatment to alter its physiochemical properties (Jobling, 2004). To establish it as a more viable alternative to fossil fuels and petroleum-based polymers, it would, therefore, be advantageous to either increase the starch content of plant organs from which starch is usually extracted (mostly corn and potato tubers), or modify its structure in planta to make post-harvest treatments unnecessary. In plant metabolism, starch is important both for short- and long-term storage of carbohydrates, which can be accessed to drive glycolysis and biosynthesis when photosynthesis cannot. Starch is stored in the plastids of both photosynthetic and non-photosynthetic organs including seeds, fruits, tubers, roots and leaves. Its structure typically varies between species and even organs within the same plant (Kossmann and Lloyd, 2000). Leaf starch granules are generally smaller and easier to degrade as its primary function is to act as a carbohydrate reserve during times of darkness (Smith et al., 2005). This ‘transitory starch’ is broken down to glucose, which can be fed into glycolysis, providing energy to cells when photosynthesis does not occur. Long-term storage of starch usually occurs in amyloplasts that are found in organs such as potato tubers, cassava roots and cereal seed endosperm. While starch is almost solely composed of glucans, a large diversity of granules from different species has been observed. The reactions involved in catalysing starch synthesis are funda- mentally the same in all plants so the structural variability that exists must come from the relative proportions of the enzymes involved in starch metabolism, differences in the substrate specificities of the enzymes between species or the involvement of as-yet unidentified proteins/enzymes. These changes affect the physicochemical properties of the starches manufactured in the different species and therefore elucidating the differences in enzymes between species might help to understand how they affect starch structure. As most commercially produced starch is from maize, wheat, rice and potato, the carbohydrate metabo- lism of these species has been studied extensively. However, some of the most important discoveries in starch metabolism have come from studies performed in the model plant species Arabidopsis thaliana and the unicellular green algae Chlamydo- monas reinhardtii. The starch granule is composed of two distinct glucans, amylose and amylopectin, amylose being almost linear in nature, whereas amylopectin is branched, with the branches being arranged in a highly ordered fashion, giving amylopectin a crystalline nature. In addition, depending on the plant organ, amylopectin can contain significant proportions of covalently attached phosphate. It has been demonstrated in rice and potato that starch from leaves contains less than 15% amylose, while that found in storage organs can contain between 11 and 37% (Slattery et al., 2000). We will briefly describe pathways of starch synthesis and degradation, highlight progress in understanding the regulation of starch metabolism and discuss biotechnological possibilities to optimize plant organs with respect to the amount of starch they accumulate, as well as possibilities to modify starch structure to generate tailor-made starches. ª 2012 The Authors Plant Biotechnology Journal ª 2012 Society for Experimental Biology, Association of Applied Biologists and Blackwell Publishing Ltd 223 Plant Biotechnology Journal (2013) 11, pp. 223–232 doi: 10.1111/pbi.12029