Agriculture Reference
In-Depth Information
Root system modification and organic acid secretion require a carbon supply
which might be to the detriment of yield or growth but being nevertheless beneficial
under conditions of low phosphorus availability (Zhu and Lynch
2004
; Lynch and
Ho
2005
; Johnson et al.
1996
; Yao et al.
2011
). PUE in two different P efficient
maize lines growing in nutrient solution culture was related to proteins that
decreased citrate degradation, increased citrate synthesis and malate dehydrogenase
activity in the roots (Li et al.
2008a
). Furthermore, proteins related to carbon and
energy metabolism were expressed to a higher extent in a low P-tolerant
Brassica
napus
genotype compared to a low P-sensitive one (Yao et al.
2011
). It has been
hypothesised that transgenic plants that secret microbial phytases into the rhizo-
sphere have potential for improved acquisition of organic P sources, but when
grown in soil their growing performance matched the control plants (George
et al.
2005
; Richardson et al.
2000
). Studies on bacterial citrate synthase genes in
tobacco came to similar contradictory results (L´pez-Bucio et al.
2000
; Delhaize
et al.
2001
) and investigations of genotypic variation in root exuded wheat phos-
phatases activity could not relate their activity to the P content when plants were
grown in soil (George et al.
2008
). Nonetheless, Zhang et al. (
2009
) suggested that
improved acquisition and therefore higher P uptake efficiency of two
Brassica
napus
genotypes grow in soil was related to the ability to lower the pH or higher
acid phosphatase activity in the rhizosphere. However, higher APase activity was
observed in roots and particularly in shoot tissue in P scarce conditions in rice but
without exhibiting genotypic differences (Yao et al.
2011
). Over-expression of a
wheat malate transporter Ta-ALMT1 in barley enhanced P uptake on acid soils in
the short-term but not when the soil was limed (Delhaize et al.
2009
). Over-
expression of a root-associated purple phosphatase gene in rice, OsPAP10a, could
promote better growth and a higher tiller number compared to the wild type under P
sufficient conditions (Tian et al.
2012
). In conclusion, results from
in vivo
studies
appear contradictory when tested under soil conditions. However, genotypic vari-
ation of low P tolerance is related to root morphology and secretory traits, which
may be exploitable.
P
i
Acquisition via Phosphate Transporters
A strong induction of Pht1 transporters was reported in the majority of transcript
profiling studies where plants were exposed to a short-term P starvation period and
grown mainly in nutrient solution (Wang et al.
2002
; Wasaki et al.
2003
; Calder´n-
V´zquez et al.
2008
; Huang et al.
2008
;
2011
) and in the field (Teng et al.
2013
),
which makes them obvious targets for genetic improvement. However, enhanced
induction of TaPht1 transcripts might be either a short-term adaptation to local P
depletion or unevenly distributed patterns of P availability. High persistent induc-
tion during severe long-term scarcity as an adaptation mechanism seems question-
able and should be investigated on field-grown crops as performed by Teng
et al. (
2013
). Enhancing P acquisition by over-expressing phosphate transporter
