Agriculture Reference
In-Depth Information
15.6 DECIPHERING THE GENETIC BASIS OF NITROGEN USE
EFFICIENCY IN CROPS
There have been an increasing number of studies only performed on the
model species Arabidopsis, in an attempt to link plant physiology to whole
genome expression in order to obtain an integrated view on how the ex-
pression of genes can affect overall plant functioning [151]. When a struc-
tural or regulatory gene putatively involved in the control of a metabolic
pathway or a developmental process or both is identified, information can
then be obtained by producing overexpressors or selecting deficient mu-
tants of the gene in question. By studying the impact of the genetic modi-
fication or the mutation on the phenotype or the physiology of the plant, it
is often possible to determine whether the expression of this specific gene
is a limiting step in the development of a particular organ or of a metabolic
pathway. In general, this targeted approach, which allows the identifica-
tion of a single limiting reaction, or a co-limiting/non-limiting reaction
does not adequately take into account the variation in complex traits such
as those controlling NUE, which involves multiple genes and thus mul-
tiple enzyme reactions and regulatory factors.
Over the last ten years, quantitative genetics, through the detection
of quantitative trait loci (QTL), has become an important approach for
identifying key regulatory or structural genes involved in the expression
of complex physiological and agronomic traits in an integrated manner
and for the study of plant responses to environmental constraints [175].
When QTLs for agronomic and phenotypic traits are located on a genetic
map, it is possible to look for their genetic signifi cance by establishing the
co-location of QTLs for physiological or biochemical traits with genes
putatively involved in the control of the trait of interest (candidate genes).
Validation of candidate genes can then be undertaken using transgenic
technologies (forward genetics) or mutagenesis (reverse genetics) or by
studying the relationship between allelic polymorphism and the trait of
interest (association genetics; Figure 3) either at a single gene or genome-
wide level [176]. Positional cloning is another alternative strategy that can
be used to focus on the chromosomal region controlling the trait of interest
and that ultimately allows access to a single gene [177].
