Boron Toxicity and Tolerance in Crop Plants - Crop Improvement Under Adverse Conditions - page 330

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.