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et al. 2010a) and a further restriction of leaf
conductance under high vapor-pressure deficit
conditions (Kholova et al. 2010b). The expres-
sion of these traits, at the vegetative stage in the
absence of water stress, leaves water available in
the soil profile eventually leading to stay-green
expression differences (Vadez et al. in prepara-
tion). Similar findings have been reported in stay-
green
Miscanthus
genotypes, where the stay-
green genotype Sin-H6 appeared to have a lower
leaf conductance (Clifton-Brown et al. 2002). In
the case of pearl millet, several QTLs have been
identified for these water-saving traits (Kholova
et al. 2012). Interestingly, water-saving traits,
measured in pots, and stay-green expression and
yield measured under field conditions co-map
to the same genomic regions (Sehgal et al. in
preparation).
Other possibilities for saving water during
the vegetative growth stage, before any stress
occurs, involve the development of smaller
leaf area. One recent report shows that hav-
ing a faster leaf-appearance rate reduced tiller-
ing and then decreased the overall plant leaf
area at anthesis. The effect was to decrease
water use prior to anthesis, leading to higher
grain yield under terminal drought conditions
(van Oosterom et al. 2011). In our current work
at ICRISAT-Patancheru, we have also demon-
strated the capacity of certain stay-green QTLs
from donor parent B35 to reduce the leaf size in
S 35 background (Kholova et al. unpublished).
However, it was also shown that faster leaf-
appearance rate was sensitive to temperature and
that the beneficial effects were reduced in higher
temperature environments (van Oosterom et al.
2011). Similarly, leaf expansion is highly depen-
dent on both the evaporative demand and soil
moisture in maize (Reymond et al. 2003). There-
fore, future challenges with the use of stay-green
expression will also be to better understand how
some of the explanatory mechanisms of stay-
green, like the leaf-area development addressed
here, respond to the environment. Unless this is
precisely known, the prediction of the effect of
stay-green mechanisms will be inaccurate and
the use of stay-green in breeding will be a blind
exercise at best. Therefore, the use of stay-green
in the future will very likely evolve to intro-
gressing genomic elements involved in its key
mechanisms rather than introgressing QTLs for
stay-green per se. This implies that a more thor-
ough undrestanding is needed that can decipher
the mechanisms underlying stay-green expres-
sion in sorghum (and other crops in which this
trait might be found useful), and the interaction
of these mechanisms with the environment. This
work is on-going at ICRISAT in India and in
Niger.
Advances inSorghum Genomics
and Applications forStay-Green
Research
Among the available marker systems, simple
sequence repeat (SSR) markers gained breeders'
interest for mapping and introgression of differ-
ent traits in crop species because these mark-
ers are amenable to simple assays, multiplexing,
and reproducibility, and more importantly are co-
dominantly inherited. SSR markers have been
greatly exploited for the mapping of different
traits in sorghum, and the stay-green trait is no
exception (Haussmann et al. 2002, Harris et al.
2007, Habyarimana et al. 2010, and Sabadin et al.
2012). The major limiting factor for utilization
of SSR markers is their resolution power. Recent
advances in sorghum genomics, including avail-
ability of an aligned sorghum genome sequence
(Paterson et al. 2009), access to larger numbers
of markers including both SSRs (e.g., Ramu et al.
2010) and DArTs (Mace et al. 2010), with very
large numbers of GbS-SNPs on the way (Elshire
et al. 2011, Nelson et al. 2011). Alignment of
major trait genes and QTLs to integrated link-
age and physical maps (Mace et al. 2011) has
strengthened the foundation for better integra-
tion of molecular marker technologies in applied
sorghum breeding programs.
With the invention of next generation
sequencing (NGS) technologies, identification
of a large number of markers, especially single
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