These included manipulating leaf photosynthesis, photosynthate partitioning, total
biomass production, and nitrogen use efficiency (NUE). NUE may be defined as the
yield as a function of available nitrogen, and therefore improvements in yield will
implicitly deliver enhanced NUE (Parry and Hawkesford 2012). Improved NUE in
crops should lead to reduced fertilizer application and thereby lower emissions of
greenhouse gases into the atmosphere (Varshney et al. 2011).
Significant advances have been made in the area of genomics over the past
10 years. Genome sequences are available now for many crop species. Furthermore,
the advent of so-called ‘‘next-generation sequencing’’ (NGS) technologies has made
it possible to sequence the transcriptomes or genomes of any species relatively
quickly and cheaply (Varshney et al. 2009). These genome or transcriptome
sequences coupled with genetic approaches can be used for identifying suitable
genes conferring stress tolerance that can be deployed in crop improvement.
The molecular breeding approach involves first identifying quantitative trait
loci (QTLs) for traits of interest, such as tolerance to abiotic stresses. Once the
markers associated with QTLs were identified, the candidate QTLs or genes can be
introgressed in elite lines through marker-assisted backcrossing (MABC), markerassisted recurrent selection (MARS), or genome-wide selection (GWS) (Varshney
et al. 2011).
The concern for global food security results from an impending imbalance
between the supply and demand of the major food crops (wheat, rice, and maize).
The predicted increase in temperatures, as well as decreased and more erratic
rainfall as a result of global climate change, are projected to decrease global yields
of crops. As a consequence, a need for a substantial acceleration in crop improvements is required (Parry and Hawkesford 2012). To do this, it is needed to increased
productivity through plant breeding and improvements in agronomy to increase both
the yield potential and the actual yields achieved by farmers. Genetic gains will be
required in addition to agronomic improvements (Leegood et al. 2010).
Molecular breeding should best focus on constitutive traits that give good
performance under drought stress. Traits related to root architecture and function
are often difficult to assess under field conditions but are obvious targets for
both selection and manipulation. Canopy traits that relate to sustaining high
photosynthetic rate at low stomatal conductance are also important (Parry and
Hawkesford 2012).
Knowledge gained from wide germplasm screening will inform on requirements for novel genes/alleles to be introduced through nonclassical means, such as
gene transformation. Mutagenesis has been widely used to generate new variations
in genomes. Mutations may be generated by irradiation or chemical treatment; the
changes can result in large-scale deletions of DNA, or may only involve point
mutations. While most often the performance of mutants is generally inferior to the
wild type, occasionally lines with improved performance with respect to any trait
can be selected. Moreover, mutagenesis also has the advantage over RNAi and
similar methods of being able to target individual members of gene families and
therefore potentially target gene function in specific tissues and at specific times
(Parry et al. 2009).
28
L. Garcia-Mier et al.
biomass production, and nitrogen use efficiency (NUE). NUE may be defined as the
yield as a function of available nitrogen, and therefore improvements in yield will
implicitly deliver enhanced NUE (Parry and Hawkesford 2012). Improved NUE in
crops should lead to reduced fertilizer application and thereby lower emissions of
greenhouse gases into the atmosphere (Varshney et al. 2011).
Significant advances have been made in the area of genomics over the past
10 years. Genome sequences are available now for many crop species. Furthermore,
the advent of so-called ‘‘next-generation sequencing’’ (NGS) technologies has made
it possible to sequence the transcriptomes or genomes of any species relatively
quickly and cheaply (Varshney et al. 2009). These genome or transcriptome
sequences coupled with genetic approaches can be used for identifying suitable
genes conferring stress tolerance that can be deployed in crop improvement.
The molecular breeding approach involves first identifying quantitative trait
loci (QTLs) for traits of interest, such as tolerance to abiotic stresses. Once the
markers associated with QTLs were identified, the candidate QTLs or genes can be
introgressed in elite lines through marker-assisted backcrossing (MABC), markerassisted recurrent selection (MARS), or genome-wide selection (GWS) (Varshney
et al. 2011).
The concern for global food security results from an impending imbalance
between the supply and demand of the major food crops (wheat, rice, and maize).
The predicted increase in temperatures, as well as decreased and more erratic
rainfall as a result of global climate change, are projected to decrease global yields
of crops. As a consequence, a need for a substantial acceleration in crop improvements is required (Parry and Hawkesford 2012). To do this, it is needed to increased
productivity through plant breeding and improvements in agronomy to increase both
the yield potential and the actual yields achieved by farmers. Genetic gains will be
required in addition to agronomic improvements (Leegood et al. 2010).
Molecular breeding should best focus on constitutive traits that give good
performance under drought stress. Traits related to root architecture and function
are often difficult to assess under field conditions but are obvious targets for
both selection and manipulation. Canopy traits that relate to sustaining high
photosynthetic rate at low stomatal conductance are also important (Parry and
Hawkesford 2012).
Knowledge gained from wide germplasm screening will inform on requirements for novel genes/alleles to be introduced through nonclassical means, such as
gene transformation. Mutagenesis has been widely used to generate new variations
in genomes. Mutations may be generated by irradiation or chemical treatment; the
changes can result in large-scale deletions of DNA, or may only involve point
mutations. While most often the performance of mutants is generally inferior to the
wild type, occasionally lines with improved performance with respect to any trait
can be selected. Moreover, mutagenesis also has the advantage over RNAi and
similar methods of being able to target individual members of gene families and
therefore potentially target gene function in specific tissues and at specific times
(Parry et al. 2009).
28
L. Garcia-Mier et al.