Pectin engineering to modify product quality in potato Heather A. Ross 1 , Wayne L. Morris 1 , Laurence J.M. Ducreux 2 , Robert D. Hancock 1 , Susan R. Verrall 1 , Jenny A Morris 2 , Gregory A. Tucker 3 , Derek Stewart 1 , Pete E. Hedley 2 , Gordon J. McDougall 1 and Mark A. Taylor 1, * 1 Plant Products and Food Quality Programme, Scottish Crop Research Institute, Dundee, UK 2 Genetics Programme, Scottish Crop Research Institute, Invergowrie, Dundee, UK 3 School of Biosciences, University of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire, UK Received 1 October 2010; accepted 19 December 2010. *Correspondence (fax +44 (0) 1382 562426; email Mark.Taylor@scri.ac.uk) Keywords: cell wall, pectin, pectin methyl esterase, potato, texture, tuber. Summary Although processed potato tuber texture is an important trait that influences consumer pref- erence, a detailed understanding of tuber textural properties at the molecular level is lacking. Previous work has identified tuber pectin methyl esterase (PME) activity as a potential factor impacting on textural properties, and the expression of a gene encoding an isoform of PME (PEST1) was associated with cooked tuber textural properties. In this study, a transgenic approach was undertaken to investigate further the impact of the PEST1 gene. Antisense and over-expressing potato lines were generated. In over-expressing lines, tuber PME activity was enhanced by up to 2.3-fold; whereas in antisense lines, PME activity was decreased by up to 62%. PME isoform analysis indicated that the PEST1 gene encoded one isoform of PME. Analysis of cell walls from tubers from the over-expressing lines indicated that the changes in PME activity resulted in a decrease in pectin methylation. Analysis of processed tuber texture demonstrated that the reduced level of pectin methylation in the over-expressing transgenic lines was associated with a firmer processed texture. Thus, there is a clear link between PME activity, pectin methylation and processed tuber textural properties. Introduction The texture of plant products is an important quality attribute that underpins aspects of consumer acceptance and preference (Szczesniak, 1971). Despite this, the factors that drive food textural properties are not well understood. In potato, consis- tency of tuber texture is important for manufacturers of pro- cessed potato products and textural properties are known to be affected by pre-processing procedures such as blanching, peeling and by storage (Shomer and Kaaber, 2006; Thybo et al., 2006). Several factors may influence processed potato tuber texture (reviewed in Taylor et al., 2007) including starch content and distribution within the tuber, starch swelling pressure, cell size, cell wall structure and composition, and the breakdown of the cell wall middle lamella during cooking. Thus, it is clear that tex- ture is likely to be a very complex trait involving many genes. Recently, it was demonstrated that tubers from some accessions of Solanum tuberosum group Phureja (Phureja) have a greatly reduced cooking (by steaming or boiling) time than typically observed for those from Solanum tuberosum group Tuberosum (Tuberosum) and hence represent an extreme variant in textural properties (Ducreux et al., 2008). The development and applica- tion of a wedge fracture test to cooked potato tubers enabled the quantification of textural properties (Ross et al., 2010a). Interestingly cooked tuber textural properties were shown to be largely independent of dry matter content as tissue from the tuber pith with a dry matter content of ca. 25% lower than in other tuber tissues had essentially the same cooked textural properties (measured with the wedge fracture test). As starch contributes approximately 80% of dry matter content (Storey, 2007), the implication is that starch content is not a major factor in determining cooked tuber textural properties, at least as assessed by this method. Further work considered differences in cell wall components as potentially underpinning the Phureja ⁄ Tuberosum differences in textural properties (Ross et al., 2010b). In potato, pectin con- tributes between 52-55% of cell wall polysaccharides (Vincken et al., 2000) and is largely comprised of homogalacturonan (HGA) and rhamnogalacturonan (RG) I polymers (Carpita and Gibeaut, 1993). HGA is an unbranched polymer of (1-4) linked a-D-galacturonic acid residues. Non-methylated D-galacturonic acid sequences are sites for cross-linking polymeric chains through the site-specific interaction with Ca 2+ (Pilnick and Vor- agen, 1991). Rhamnogalacturonan I (RGI) is a heteropolymer of alternating (1-2)-a-L-rhamnose and (1-4)-a-D-GalA residues that can be substituted with side chains of arabino-galactan or arabinan polymers, with a low degree of branching. The potato tuber RGI backbone (Schols and Voragen, 1994) has a high degree of acetylation (90%) but a relatively low methyl-ester content (13%). Chemical analysis of tuber pectin clearly demonstrated the strong link between pectin methyl esterase (PME) activity, the degree of methylation of cell wall pectin and cooked tuber tex- tural properties, with the Phureja tubers exhibiting ca. 10-fold lower total PME activity and significantly elevated levels of methylated pectin (Ross et al., 2010b). Furthermore, there was a greatly elevated expression level of a gene encoding PME in the tubers of Tuberosum compared with those from Phureja. The involvement of PME in potato tuber texture has long been mooted (van Buren, 1970; Bartolome and Hoff, 1972). The removal of methyl groups from pectins may promote cell–cell adhesion, particularly in the pectin-rich middle lamella between cells (Jarvis, 1999), by increasing the likelihood of Ca 2+ -bridges between the free acid groups of adjacent chains (Ng and Waldron, 1997). Also, demethylated pectins are less susceptible to b-elimination cleavage during cooking (Sajjaanantakul et al., 1989). ª 2011 The Authors 848 Plant Biotechnology Journal ª 2011 Society for Experimental Biology, Association of Applied Biologists and Blackwell Publishing Ltd Plant Biotechnology Journal (2011) 9, pp. 848–856 doi: 10.1111/j.1467-7652.2011.00591.x