Agriculture Reference
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et al. 2009 ; Li et al. 2010 ; Yao et al. 2011 ) exposed to a short-term P starvation
period; particularly transcriptome, proteome or metabolite profiling studies (Wang
et al. 2002 ; Wasaki et al. 2003 ; Hammond et al. 2004 ; Calder ´ n-V ´ zquez
et al. 2008 ; Huang et al. 2008 ; Oono et al. 2011 ; Liang et al. 2010 ; Oono
et al. 2013 ). Even if these profiling studies provided a useful tool to study the
response mechanisms with regards to adaptation to nutrient stresses (Hammond
et al. 2004 ; Nilsson et al. 2010 ), the majority used hydroponically grown plant
material exposed to short-term P starvation. However, Hammond et al. ( 2011 ) used
the transcriptional profiling technology to identify a predictive diagnostic gene set
for detecting the physiological P i status of potato under field conditions and at a
range of P fertiliser application rates. This group of genes were determined by
investigating the transcriptional P starvation responses in potato leaves grown
hydroponically and were validated for exposure to various nutritional and abiotic
stresses using the Arabidopsis orthologs (Hammond et al. 2011 ). This aspect is
more focused on increasing the precision of fertiliser application but may also be a
potential tool for genotypic screening PUE and PUE under agronomic conditions in
the future.
The second approach deals with the over-expression of target genes resulting in
partly contradictory observations due to the expression level of their role in the P
sensing and regulating network (Rae et al. 2004 ; Zhou et al. 2008 ; Ren et al. 2012a ,
b ; Tian et al. 2012 ; Guo et al. 2013 ; Wang et al. 2013 ).
The third approach uses quantitative trail loci (QTL) analysis to dissect the
genetic basis of P efficiency and identify superior alleles or loci in different
germplasm (Wissuwa et al. 2005 ; Zhu et al. 2005a , b ; Su et al. 2006 ; Liang
et al. 2010 ; Yang et al. 2011 ; Gamuyao et al. 2012 ) which should lead, if successful,
to marker-assisted selection (MAS) in breeding for improved nutritional traits. All
three approaches will be described and discussed in more detail within the sections
below.
Root Morphology and Organic Acid Secretion
Due to the low mobility of P in the soil, the root architecture of crops in agricultural
systems is strongly related to P distribution in the soil profile, determined by tillage,
fertiliser and cultivation practices, which influence in turn the chemical dynamics
of soil P and the rhizosphere (Niu et al. 2012 ). Phenotyping of root (architectural)
parameters are difficult to evaluate as selection criteria as they are both time
consuming and destructive (de Sousa et al. 2012 ), which makes the in vitro QTL
analysis approach described below very attractive for breeders using MAS (Liang
et al. 2010 ). Root morphology or primary root growth in maize seems not to be
affected by P availability (Mollier and Pellerin 1999 ) and the extensive shoot-born
root system and different root types of cereals (Hochholdinger and Zimmermann
2008 ) emphasises regulatory differences in crops compared to model plants such as
Arabidopsis .
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