Agriculture Reference
In-Depth Information
Fig. 15.6
Relative expres-
sion of
HvBor2
as a function
of root boron concentration
in barley cultivars grown in a
nutrient solution containing
5 mM boron. (Reprinted
from Reid (
2007
) with
permission)
4
3
2
R
2
= 0.99
1
0
0
1
2
3
4
5
Root B concentration (mM)
ally led to the identification of a NAC-like transcription factor with a single nucleo-
tide polymorphism between the sensitive and tolerant varieties (Ochiai et al.
2011
).
The deletion of the single nucleotide appeared to confer tolerance by disruption of
the gene in the tolerant cultivars. Hence, the functional gene, which was named
BET1
(
B
oron
E
xcess
T
olerant 1), should best be described as a boron-sensitive gene
rather than a tolerance gene. Suppression of
BET1
expression by RNAi increased
tolerance to boron (Ochiai et al.
2011
). Since these changes in tolerance occurred in
the absence of differences in root or shoot boron concentrations (Ochiai et al.
2008
),
this mechanism must be independent of boron efflux.
5.3 Boron Efflux Transporters
The identification of boron transporter genes involved in boron tolerance was
greatly assisted by the discovery of a boron efflux transporter in
Arabidopsis
that
pumped boron into the root xylem under deficiency conditions (At
Bor1
) (Takano
et al.
2002
). However, under high boron conditions, the transporter was found to
be degraded (Takano et al.
2005
), which eliminated any role in boron tolerance.
Four homologues of At
Bor1
were found in rice. Reid (
2007
) used primers prepared
from sequences of these rice genes to probe expression of related genes in sensitive
and tolerant wheat. From these experiments, a gene with high similarity to Os
Bor2
was sequenced and named Ta
Bor2
. A gene with 90 % similarity to Ta
Bor2
at
the
amino acid level was subsequently identified in barley (Hv
Bor2
). Expression of
both of these genes was shown to be high in tolerant cultivars and low in sensi-
tive cultivars, with expression negatively correlated with root boron concentra-
tions (Fig.
15.6
) (Reid
2007
). Sutton et al. (
2007
) using positional cloning methods
also reported a gene from barley with the same sequence as Hv
Bor2
which they
named Bot1. In the same year, Miwa et al. (
2007
) showed that overexpression of
At
Bor4
in the distal regions of
Arabidopsis
roots resulted in tolerance to high boron
concentrations.