THE GENETICS AND PLANT BREEDING OF ALLIUM CROPS - Onions and other Vegetable Alliums - page 58

The arginine substitution is close to an active binding site in the enzyme and this
probably causes its inactivation, hence blocking anthocyanidin synthesis (Kim et
al. , 2005a). Crosses between US and Brazilian yellow onions result in some red
onions, since some of the offspring inherit the alleles producing normal DFR from
the Brazilian parent along with alleles for functioning ANS from the US parent,
thereby allowing the biosynthetic pathway to cyanidin to function (see Fig 3.3).
This explains the long-recognized 'complementary gene effect', where the
interaction of two gene loci could result in red-skinned offspring from crosses
between two yellow-skinned parents (El Shafie and Davis, 1967).
Another gene variant found in some unusual gold-coloured onions involved
a small DNA change that resulted in the production of inactive chalcone
isomerase (CHI). The consequent block in flavonoid biosynthesis resulted in the
accumulation of the bright yellow pigment chalcone (see Fig 3.3; Kim et al .,
2004b). Another allele produces low levels of ANS (see Fig. 3.3), resulting in less
cyanidin production and leading to pink rather than red bulbs. This is thought to
be a result of changes in a regulatory sequence of DNA adjacent to the sequence
actually coding for the ANS protein (Kim et al ., 2005a).
The influence of qualitative genes with discrete major effects is often
enhanced or reduced by a number of minor modifier genes with a quantitative
influence. This is so for the major gene for seed stalk length described above, and
also for the gene that confers resistance to pink root disease in onion. Twenty
qualitative genes identified in edible alliums were listed by Rabinowitch (1988).
In more recent years, with application of molecular genetics to the alliums, gene
loci are being identified more rapidly. These include genetic loci for the enzymes
affecting bulb colour discussed above, for several sulfur-metabolism enzymes
involved in the synthesis of flavour compounds and for a key enzyme in fructan
biosynthesis (McCallum et al ., 2005a, b, 2006; Chapter 8, this volume). The
DNA sequences of the genes that code several of these enzymes have been
determined, and this information is helping to clarify the structure and function
of the enzymes (Shaw et al ., 2005). A genetic locus for mildew resistance has
been identified from wild alliums and is being introduced into onion (see
Breeding for Disease and Pest Resistance, below). Shigyo and his colleagues have
reported genetic loci for ten enzymes of carbohydrate and amino acid
metabolism, using different variants of the enzymes (isozymes) for the genetic
analyses and to locate on which chromosome the loci occur (Shigyo et al ., 1996;
van Heusden et al ., 2000a).
In addition to the identification of genes for individual discrete traits or
enzymes, groupings of genes or polygenic regions controlling various aspects of
biochemistry have been located to particular chromosomes or genetic linkage
regions. These include the regions coding for fructans (Havey et al., 2004),
flavour compounds (Galmarini et al ., 2001), sugar content in leaves (Tran Thi
Minh Hang et al ., 2004) and flavonoids and anthocyanins in leaf sheaths and
some of the enzymes involved in their synthesis (Shigyo et al ., 1997; Masuzaki et
al ., 2006). In addition there are 2608 published sequences of DNA from onion