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can be increased by overexpressing an Arabidopsis
acetyl-CoA carboxylase gene (Klaus et al ., 2004).
In this study, overexpression led to a c .40% in-
crease in total fatty acid content and changes in
the ratios of linoleic and a-linolenic acids. Again,
the effects on the organoleptic quality of cooked
tubers from these lines have not been reported.
One approach to identifying candidate genes
that underpin flavor differences between potato
germplasm is to use transcriptomics. Transcrip-
tomic studies in potato have benefitted from
the development of a near whole-transcriptome
microarray produced by the Potato Oligo Chip
Initiative (POCI) consortium that contains
gene probes based on 42,034 potato unigene
sequences, using the custom Agilent platform.
A  detailed description of the array has recently
been published (Kloosterman et al ., 2008). Fol-
lowing the elucidation of the potato genome,
a more comprehensive version of the POCI array
has been developed (P.E. Hedley, UK, 2011, per-
sonal communication), and in the near future
RNAseq methodologies will ensure transcrip-
tomic studies have comprehensive coverage of
the potato transcriptome. Using the POCI micro-
array, we were able to identify consistent differ-
ences in gene expression profiles between Phure-
ja and Tuberosum cultivars, including genes
likely to impact on flavor and other quality traits.
As an example of the utility of the approach,
functional analysis of a sesquiterpene synthase
gene that was expressed consistently at higher
levels in Phureja tubers demonstrated that this
gene encoded an a-copaene synthase.
As a-copaene was one of the major volatile
differences between Phureja- and Tuberosum-
type potatoes, and it was established that
a-copaene had high odor impact in lettuce and
carrots (Nielsen and Poll, 2007), the effects of
overexpressing a-copaene synthase on potato
flavor were investigated. In fact, the a-copaene-
producing transgenic lines could not be distin-
guished from parental control by taste panelists
(Morris et al ., 2011), indicating that a-copaene
was probably not a major factor in potato flavor,
a conclusion that concurred with the correl-
ation matrix developed by Morris et al . (Morris
et al ., 2010 ; Fig. 19.3 ) .
The results of the transcriptomic comparison
of Phureja and Tuberosum germplasm identified
other potential flavor genes. Esterified branched-
chain carboxylic acids such as 2- methylpropionic
acid methyl ester and 2- methylbutanoic acid
methyl ester are other volatiles produced in
much higher levels from cooked Phureja tubers
than from Tuberosum (Shepherd et al ., 2007).
These esters are derived from branched-chain
amino acids such as valine, leucine, and isoleu-
cine, in a reaction pathway that is thought to
involve branched-chain amino acid aminotrans-
ferase (BCAAT; Beck, 2005). A BCAAT gene was
expressed as much higher levels in Phureja than
in Tuberosum tubers, and again suggests that a
transgenic route in which the gene is overex-
pressed or silenced may resolve the issue of its
contribution to overall volatile level and flavor.
Glutamate is likely to be a determinant of
flavor in potato tubers, due to its contribution to
the umami taste (Morris et al ., 2007). Genes en-
coding glutamate ammonia ligase and glutam-
ine synthetase I (enzymes involved in glutamate
biosynthesis) were expressed at higher levels in
Phureja cultivars. Conversely, another gene in-
volved in glutamate biosynthesis, g-aminobutyric
acid transaminase, was expressed at a higher
level in Tuberosum cultivars. Further investiga-
tion of gene expression profiles of these and
other genes involved in tuber glutamate biosyn-
thesis may reveal those that account for higher
levels of glutamate in Phureja tubers.
19.7 Future Directions
Compared with other fruit and vegetables such
as tomatoes, there are still significant gaps in our
knowledge linking metabolites to flavor metabol-
ites. The analytical tools are now available to ad-
dress this issue; however, there are still relatively
few studies linking metabolites and quantitative
descriptive analysis. The use of germplasm that
shows wide variation in flavor characteristics
such as Phureja compared with Tuberosum has
proved useful, but only begins to investigate the
full diversity available within potato germplasm
collections. Producers, processors, and con-
sumers alike wish to have potato tubers that show
consistent and high-quality flavor properties,
and to achieve this goal we need to link the me-
tabolites with quality traits. The interaction be-
tween genotype, environment, and agronomy in
developing potato flavor is an area that requires
urgent attention. This will result in a metabolite
 
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