Expression Profiling of Potato Germplasm Differentiated in Quality Traits Leads to the Identification of Candidate Flavour and Texture Genes
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Journal of Experimental Botany, Vol. 59, No. 15, pp. 4219–4231, 2008 doi:10.1093/jxb/ern264 Advance Access publication 5 November, 2008 This paper is available online free of all access charges (see http://jxb.oxfordjournals.org/open_access.html for further details) RESEARCH PAPER Expression profiling of potato germplasm differentiated in quality traits leads to the identification of candidate flavour and texture genes Laurence J. M. Ducreux1, Wayne L. Morris1, Ian M. Prosser2, Jenny A. Morris3, Michael H. Beale2, Frank Wright4, Tom Shepherd1, Glenn J. Bryan3, Pete E. Hedley3 and Mark A. Taylor1,* 1 Plant Products and Food Quality, SCRI, Invergowrie, Dundee DD2 5DA, UK 2 PSC Department, Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, UK 3 Genetics, SCRI, Invergowrie, Dundee DD2 5DA, UK 4 Biomathematics and Statistics Scotland (BioSS), Invergowrie, Dundee DD2 5DA, UK Received 20 June 2008; Revised 3 October 2008; Accepted 6 October 2008 Abstract Major differences in the expression levels of genes involved in cell wall biosynthesis (and potentially Quality traits such as flavour and texture are assuming texture) were also identified, including genes encoding a greater importance in crop breeding programmes. pectin acetylesterase, xyloglucan endotransglycosy- This study takes advantage of potato germplasm lase and pectin methylesterase. Other gene expression differentiated in tuber flavour and texture traits. A differences that may impact tuber carotenoid content recently developed 44 000-element potato microarray and tuber life-cycle phenotypes are discussed. was used to identify tuber gene expression profiles that correspond to differences in tuber flavour and Key words: a-Copaene, flavour, microarray, Solanum texture as well as carotenoid content and dormancy tuberosum group Phureja, Solanum tuberosum group characteristics. Gene expression was compared in two Tuberosum, texture, volatile. Solanum tuberosum group Phureja cultivars and two S. tuberosum group Tuberosum cultivars; 309 genes were significantly and consistently up-regulated in Phureja, whereas 555 genes were down-regulated. Introduction Approximately 46% of the genes in these lists can be identified from their annotation and amongst these are Recent phylogenetic analysis of a wide range of wild candidates that may underpin the Phureja/Tuberosum potato species, landraces, and cultivars, indicates that trait differences. For example, a clear difference in cultivated potatoes have a monophyletic origin, arising the cooked tuber volatile profile is the higher level of from a group of extremely similar wild species classified the sesquiterpene a-copaene in Phureja compared in the Solanum brevicaule complex (Spooner et al., 2005). with Tuberosum. A sesquiterpene synthase gene was Thus the gene pool used to develop cultivated potatoes is identified as being more highly expressed in Phureja considered to be fairly narrow and has not widely utilized tubers and its corresponding full-length cDNA the full diversity (and hence characteristics) of potato was demonstrated to encode a-copaene synthase. germplasm available. Efforts to broaden the genetic base Other potential ‘flavour genes’, identified from their of cultivated potatoes have been made, with some success differential expression profiles, include those encod- (Bradshaw and Ramsay, 2005). Despite the purported ing branched-chain amino acid aminotransferase genetic bottleneck associated with the domestication pro- and a ribonuclease suggesting a mechanism for 5#- cess, the eight cultivar groups recognized by Huaman and ribonucleotide formation in potato tubers on cooking. Spooner (2002) are highly variable phenotypically, with many potential uses in potato breeding programmes. * To whom correspondence should be addressed: E-mail: [email protected] ª 2008 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 4220 Ducreux et al. A valuable resource in this regard is S. tuberosum group sands) sesquiterpene volatiles produced by plants (Glasby, Phureja, differentiated from S. tuberosum group Tuberosum 1982). Recently a-copaene has been shown to be an on the basis of a number of important tuber quality traits important aroma compound in several food plants in- such as flavour, texture, colour, and reduced tuber cluding lettuce and carrots (Nielsen and Poll, 2007). A dormancy (De Maine et al., 1993, 1998; Dobson et al., specific a-copaene synthase has not yet been identified 2004; Morris et al., 2004; Ghislain et al.,2006).Despite from any plant species, although a-copaene is a minor being able to differentiate the Phureja group of landraces part of the spectrum of sesquiterpenes produced by some based on geographical origin, this group is very similar sesquiterpene synthases (Chen et al., 2003). Other genetically to the Tuberosum group (Spooner et al., 2007). reported differences in the volatile profiles from cooked Whatever the final taxonomic outcome, as many of the Phureja and Tuberosum tubers are the levels of branched Phureja group can be clearly differentiated based on tuber chain aldehydes and esters of branched chain carboxylic quality trait parameters, they form a useful resource for the acids produced both enzymatically and non-enzymatically identification of key genes that underpin these traits. during processing (Shepherd et al., 2007). However, for many quality traits, the genes responsible Texture, in common with flavour, is an important remain to be defined, impeding molecular breeding determinant of consumer preference (reviewed in Taylor approaches to develop improved potato germplasm. et al., 2007). Although quantitative texture measurement Potato flavour is a trait of increasing importance to comparisons between Phureja and Tuberosum tubers have consumers (McGregor, 2007). However, flavour analysis not been published, qualitative reports indicate that is complex and likely to depend on both volatile and Phureja accessions exhibit a boiled tuber texture described matrix associated compounds such as those that give rise as extremely floury or crumbly (De Maine et al., 1993, to the umami taste (one of the five basic tastes; Morris 1998). In addition, many Phureja accessions produce et al., 2007). The volatiles produced by raw and cooked yellow-fleshed tubers with much higher levels of carote- potatoes have been studied extensively (reviewed by noids compared with those typically found in Tuberosum Maga, 1994) and over 250 compounds have been cultivars (Morris et al., 2004). In general, Phureja identified in volatile fractions. Many attempts have been cultivars are also characterized by differences in the tuber made to discriminate which of these components are life-cycle, for example, a much shorter dormancy period important for potato flavour, which are specific to the and short-day adaptations (Ghislain et al., 2006), although method of cooking, cultivar differences, the effects of long-day adapted accessions have been developed for agronomic conditions, and the effects of storage (Duckham cultivation in different geographical locations (De Maine et al., 2001, 2002; Oruna-Concha et al., 2001, 2002a, b; et al., 2000). Dobson et al., 2004). To summarize the findings of The very close genetic similarity of the two cultivar several studies on boiled potato volatiles; potentially groups, yet substantial phenotypic differences that impinge significant compounds include those derived from lipid on important breeding objectives, suggest relatively simple degradation, the Maillard reaction, and the Strecker genetic architectures for the traits which differ so markedly. reaction, including methional, aliphatic alcohols and Thus, a comparison of gene expression profiles in aldehydes, thiols and sulphides, and methoxypyrazines representative Phureja and Tuberosum clones may reveal (reviewed in Taylor et al., 2007). More recently, boiled the identity of candidate genes that influence a wide range tuber volatiles produced by Phureja cultivars have been of traits of growing importance to consumers and compared with profiles from Tuberosum (Winfield et al., producers. 2005; Shepherd et al., 2007). Winfield et al. (2005) In order to identify candidate genes that underpin quality compared sensory profiles of 16 Phureja clones and six trait differences between Tuberosum and Phureja tubers, Tuberosum cultivars with volatile data. Generally, the transcript profiles were compared using a potato micro- Phureja clones sampled scored considerably higher on an array, recently developed by the Potato Oligo Chip ‘acceptability’ scale, with four of the Tuberosums occu- Initiative (POCI) consortium that contains gene probes pying the bottom five places in a ranked list of the 22 based on 42 034 potato unigene sequences, using the samples. The acceptability measure correlates highly with custom Agilent platform. A detailed description of the array other less subjective sensory traits, such as ‘creaminess’ has recently been published (Kloosterman et al., 2008). and ‘flavour intensity’. Principal component analysis This array has a much greater coverage of potato transcripts (PCA) of volatile profiles of boiled Tuberosum and than was previously available and so provides an opportu- Phureja, revealed that hexanal, pentanal, pentyl-furan, and nity for detailed trait dissection. Using this microarray