Agriculture Reference
In-Depth Information
Nonetheless, genotypic differences in low P tolerance or high yield at low P
availability were often associated with root growth properties or P uptake capability
in crops (Hammond et al.
2009
; Pariasca-Tanaka et al.
2009
; Yao et al.
2011
;Li
et al.
2008a
,
b
; Zhu and Lynch
2004
; Zhu et al.
2005a
,
b
; Gahoonia et al.
1996
;
1997
) showing that there is a large exploitable genetic variation in the root
acquisition trait. There exists considerable genotypic variation in root hairs in
barley and wheat cultivars showing that root hair length was strongly correlated
to the rhizosphere P depletion ability (Gahoonia et al.
1997
). A root hairless mutant
in maize conferred significant grain yield loss (Hochholdinger et al.
2008
). Similar
observations were made when comparing different maize lines and their root hair
length, plasticity and subsequent performance under low P (Zhu et al.
2010
). A
positive and negative regulatory role for transcription factors like PHR1 WRKY75
and BHLH32 has been suggested in
Arabidopsis
(Chen et al.
2007
; Devaiah
et al.
2007a
,
b
; Bustos et al.
2010
). Unfortunately, the underlying genetic mecha-
nisms of germplasm variation for the root hair trait are not yet determined. MAS
may facilitate root trait selection for breeding more P efficient cultivars, exempli-
fied by studies showing that root morphology QTLs are likened to P acquisition
efficiency in maize (Zhu et al.
2005a
,
b
), wheat (Ren et al.
2012a
,
b
) or soybean
(Liang et al.
2010
). Gene modification is a means of enhancing low P tolerance
(Wang et al.
2013
). For example, a ß-expansin gene in soybean, Gm-EXPB2,
enhanced P uptake when it was over-expressed (Guo et al.
2011
). Expansins are
involved in cell wall extension (Zhao et al.
2012
), including root hair formation
(Yu et al.
2011
) and are among up-regulated genes during P starvation (Calder
´
n-
V
´
zquez et al.
2008
), suggesting a remodelling of the cell wall structure and
integrity. Root morphology related genes,
Rtcs (rootless concerning crown and
lateral seminal roots;
Hochholdinger and Zimmermann
2008
),
Bk2 (brittle stalk-2
;
Brady et al.
2007
)
and
Rth3 (root hairless 3
; Hochholdinger et al.
2008
), were
determined as being related to differential P responses and PAE capability in the
seedling stage in two contrasting maize lines (de Sousa et al.
2012
). However, even
if observable only under low P conditions, root traits exhibited high heritability and
a low coefficient of variation making them exploitable for breeding (de Sousa
et al.
2012
). Genes which were investigated belonged to a family of glycosylpho-
phatidylinositol (GPI)-anchored proteins involved in root cell expansion, cell wall
biosynthesis and root hair formation (de Sousa et al.
2012
). Comparing the prote-
ome of a low P tolerant and a sensitive oilseed rape genotype revealed that proteins
related to lateral root formation such as auxin-responsive family proteins and
sucrose-phosphate synthase-like proteins were up-regulated in roots and leaves
(Yao et al.
2011
) suggesting sources of tolerance lying in the P starvation signalling
mechanism. Furthermore, Li et al. (
2008a
,
b
) revealed an increase of low-P
tolerance when the phosphoprotein, phosphatase 2A isoform 4, was increased in
abundance in the more tolerant genotype. This protein family is involved in auxin
transport and reduced activity altered lateral root growth in
Arabidopsis
(Rashotte
et al.
2001
). Significant increases of CDC48 and other regulators of cell division
and cell cycle, including Ran GTPase, MCM6 and importins, were assumed to be
important factors mediating a better root development and accelerated cell prolif-
eration in the meristem under P starvation (Li et al.
2008a
,
b
).
