Biology Reference
In-Depth Information
to improve the quality of residues (van Oost-
erom et al. 1996), support the continuation of
carbon fixation and supply of starch (McBee
et al. 1983), prevent premature death and lodging
(Rosenow and Clark 1981), sustain grain-filling
under water stress (Rosenow et al. 1983; Raj-
can and Tollenaar 1999a, 199b), and improve
grain yield under stress (Borrell and Douglas
1996). Here we focus on using stay-green as a
breeding target under water-limited conditions
and review recent progress in different areas
of stay-green research, with a particular focus
on sorghum, where this trait has been most
studied.
Given the potential benefit of stay-green,
genotypes displaying this trait have been used
to identify the genomic regions responsible for
this phenotype. Several QTLs have been iden-
tified, using different breeding populations and
stay-green QTL donors, and different types of
drought stress. This information is reviewed and
the most important QTLs are identified. We also
review the experimental conditions in which
phenotyping for stay-green has taken place and
the different ways of assessing this phenotype,
either from leaf senescence curves or leaf green-
ness assessments. A following section then sum-
marizes current work being done at ICRISAT
(International Crops Research Institute for the
Semi-Arid Tropics, in Andhra Pradesh, India)
to introgress several known QTLs for stay-
green into various agronomically elite genetic
backgrounds.
Our understanding of the stay-green trait and
of the genetic regulation of mechanisms that
lead to the expression of a stay-green phenotype
in sorghum is still very incomplete. Early works
considered the benefit of stay-green in terms of
extending the period during which a leaf could
actively fix carbon (McBee et al. 1983). Subse-
quent work also related to the carbon economy
of the plant addressed the nitrogen status of the
plant and in particular the balance between nitro-
gen demand and nitrogen capture (Borrell et al.
2001). A large amount of work, mostly involv-
ing transgenics, has addressed the question of
maintaining the production of cytokinins to
prevent leaf senescence (Gan and Amasino
1995). These views, which try to address the
“symptoms” of stay-green, see in the degra-
dation of the photosynthetic pigments the key
entry point for manipulating the stay-green trait.
These approaches are probably the complete
opposite to more recent work that addresses the
“cause” of stay-green and looks at stay-green
from the angle of water supply, taking the view
that stay-green expression is a consequence
of having water available in the soil profile
during grain-filling, when stay-green is actually
measured (Vadez et al. under review). Therefore,
two sections of this review will deal with these
“early” and more recent considerations related
to the stay-green trait.
As we progress in our understanding of the
physiological mechanisms and genetic regula-
tion of the stay-green phenotype in sorghum,
manipulation of the trait is likely to evolve from
the current introgression of genomic regions
involved in expression of the stay-green pheno-
type, which we now know are likely explained
by mechanisms of varied nature, to the intro-
gression of these mechanistic components indi-
vidually. For instance, it was found recently that
B35 (
BTx642) donor parent alleles at stay-
green QTL
Stg1
contributed to increased water
extraction by moderately senescent caudatum
variety S 35 (Vadez et al. 2011), but did not
do this in the genetic background of highly-
senescent durra variety R 16. Therefore, work
will be needed to identify the best germplasm
options as donors for each of the components
of the stay-green phenotype, and these may
vary with the genetic backgrounds and specific
soil, water, and temperature regimes in which
improved drought tolerance is desired. Work will
also be needed to measure the “baseline” compo-
nent trait value of potential recipient genotypes.
This would involve both the development of high
through-put phenotyping methods for assessing
these traits, and the refinement of molecular tools
for deciphering the genetic basis of these key
traits. Current efforts in sorghum are exploiting
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