Agriculture Reference
In-Depth Information
stage (or the fi rst-hollow-stem stage) and heading
date for a large sample of commercially released
winter wheat genotypes in multiple fi eld environ-
ments (Edwards et al., 2007). However, the devel-
opmental linkage between stem elongation and
heading date may not be absolute. No tight asso-
ciation was previously detected between these two
phenological events, as many genetic and envi-
ronmental factors are involved in wheat develop-
ment (Kirby et al., 1999), indicating that fl owering
time or heading date might not be the most appro-
priate phenotype for monitoring the developmen-
tal transition.
The life cycle of wheat from sowing to maturity
is marked by several critical physiological and
morphological stages, including seedling emer-
gence, stem elongation, jointing, heading, fl ower-
ing, and maturity (Hay and Kirby 1991; Snape
et al., 2001b; Gonzalez et al., 2002). At the joint-
ing stage, the plant starts to produce terminal
spikelets and should be at Zadoks stage 31, accord-
ing to scales developed for the Triticeae
(Haun
1973; Zadoks et al., 1974; McMaster 2005). At
this point, the plant apex has completed the tran-
sition from vegetative to reproductive develop-
ment. The life cycle of the wheat plant can be
more simply dissected into three phases: the fi rst
phase from seedling emergence to stem elonga-
tion (EM-SE), the second phase from stem elon-
gation to heading date (SE-HD), and the third
phase from heading date to physiological maturity
(HD-PM) (Whitechurch and Slafer 2001).
Each developmental phase plays a key role
in the life cycle, ultimate adaptation range, and
end use of wheat. The vernalization genes have
major effects on the rate of primodia production,
whereas the photoperiod genes affect the timing
of terminal spikelet production and stem elonga-
tion (Snape et al., 2001a). Delayed stem elonga-
tion may be selected for an extended vegetative
phase to generate more biomass as a forage
resource in dual-purpose production systems
(Redmon et al., 1996). A longer vegetative phase
is also a key characteristic of a cultivar better fi t
for biomass production as a supplemental biofuel
feedstock. In contrast, accelerated stem elonga-
tion may be selected to achieve a longer reproduc-
tive phase to increase the number of fertile fl orets
(Gonzalez et al., 2003). In addition, proper timing
of stem elongation is needed to avoid late-winter
freeze and early-spring frost injury (Fowler et al.,
2001). An improved genetic understanding of
each developmental phase will benefi t production
of wheat for many different purposes.
Studies in Arabidopsis established the complex
network of gene interactions that regulate the
transition from vegetative to reproductive devel-
opment for fl owering in plants. In a similar way,
such a gene network will be established in wheat
and extended to developmental phases before and
after fl owering. Allelic variation in the fl owering
time genes allowed the determination of critical
regulatory sites, facilitating further studies on
their upstream or downstream genes or proteins
that interact with cloned genes or their encoded
proteins using yeast-hybrid screen systems.
Many genes known to affect fl owering time in
Arabidopsis, such
FCA
,
GI
,
LD
,
SOC1
, have
been found to have orthologous expressed
sequence tags (ESTs) in wheat, and their func-
tions and respective phenotypes
await further
investigation. Forward genetics is still a most con-
vincing approach to discover novel fl owering time
genes unique in wheat by cloning genes respon-
sible for certain traits, such as replacement of
vernalization by short days and delay of growth
rate by low temperature and short days in winter
wheat.
It is expected that molecular markers will be
used to create specifi c gene combinations which
elicit a difference in fl owering time as small as a
few days. Such differences could be used oppor-
tunistically to avoid critical heat and drought
periods or to produce never-fl owering types as a
unique and high-quality forage resource. As
global climate is never static, this research is par-
ticularly important for winter wheat to have the
necessary plasticity to grow and thrive in chang-
ing environments of the world wheat area.
REFERENCES
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