VI. GENETICS OF GLYCINE BETAINE ACCUMULATION
A. Genetic Control
Analysis of the progeny from high-GB low-GB crosses suggests that GB levels are controlled by a
small number of nuclear-coded genes [235]. The precise function of these genes is unknown. It is sug-
gested that these GB genes may have a role in the overall control of osmoregulation rather than the con-
trol of GB alone [236]. In maize, a single gene bet1located on chromosome 3 [54] confers the presence
or absence of GB. Glycine betaine deficiency in homozygous bet1plants is associated with an inability
to oxidize choline to betaine aldehyde [8,31,83,237]. A number of maize inbreds lack GB completely
[31,53,54], and this is controlled by recessive alleles of a single nuclear gene [31,54,238]. Maize lines that
are deficient in GB synthesis (homozygous bet1) are more susceptible to salinity stress than maize lines
that synthesize GB (homozygous Bet1) [83,236]. In sorghum, GB synthesis is also governed by a single
nuclear gene [39].
B. Genetic Variation
In cultivated barley and its wild progenitor (H. spontaneum), the natural variability for GB levels among
genotypes tested (71 cultivated and 268 wild types) ranged from 19 to 40 mol g^1 dwt under irrigated
conditions and 15 to 90 mol g^1 dwt under stress conditions [239]. Salt-tolerant rice cultivars CSC 1,
AU1, and Co 43 accumulated GB levels up to 16.4 to 21.3 mol g^1 dwt, whereas salt-sensitive cultivars
TKM 9, TKM 4, CSC 2, Co 36, IR 20, and GR 3 accumulated little GB during salinization [240]. The GB
concentration in Co 43, CSC 1, and AU1 was consistently higher than in the other cultivars during the en-
tire growing period of the salinization [240]. Glycine betaine–accumulating maize lines showed higher
stomatal conductance than GB-deficient lines under drought [223,236].
C. Genetic Engineering
Genetic engineering of GB synthesis in higher plants requires at least two genes: choline monooxygenase
(CMO) and betaine aldehyde dehydrogenase (BADH). Also, free choline pools in the metabolic pathway
are required, although their size is not known [36,241]. Manipulation of foliate-mediated methyl group
metabolism may be required to ensure appropriate reserves of these choline pools [242,243]. Also, trans-
port systems for choline and GB are necessary to increase salt resistance through in situ GB production
[230]. The BADH genes from E. coli[244], spinach [118], or barley [162] have been introduced into to-
bacco plants. But the BADH transgenic tobacco lines did not exhibit increased stress tolerance. In con-
trast, tobacco plants engineered to accumulate trehalose, mannitol, proline, fructan, sorbitol, inositol, and
ononitol are reported to show improved performance under drought or salinity conditions (Table 4). En-
hanced osmotic tolerance in transgenic tobacco plants engineered to accumulate proline [247], fructans
[248], and mannitol [245] is associated with increased root/shoot ratio. In sorghum and wheat, increased
OA leads to improved root growth, larger shoots, and improved yield under drought conditions [214,215].
Tobacco plants expressing a gene for trehalose synthesis lose water more slowly than nontransformed
plants [244]. However, the levels of trehalose expressed in these transgenic plants are not sufficiently high
to reduce the leaf s. It is hypothesized that trehalose preserves protein and membrane integrity under
drought and salinity [169,170,174].
VII. CONCLUDING REMARKS
Early concepts of GB as being a useless metabolic waste product (Winterstein, 1910; cited in Ref. 3) have
evolved into defining a biochemically significant role for GB in stress adaptation of plants [8,18]. How-
ever, much additional research is required to establish the importance of GB in conferring drought or
salinity resistance. Given the inherent complexity of drought and/or salinity stresses and the extensive
mechanisms evolved in plants for adaptation to these stresses [9,16,65,66,214,215] (also see Chapter 44),
it is unlikely that a single biochemical trait such as GB accumulation can have an overriding effect. How-
ever, GB accumulation could be part of multiple mechanisms that are needed to improve crop adaptation
to stresses such as drought and salinity, which defy easy genetic solutions. Salinity resistance is a combi-
GLYCINE BETAINE IN STRESS RESISTANCE 897