Biology Reference
In-Depth Information
that many plant species make use of a gener-
alized exclusion mechanism based on the chela-
tion of Al
3
+
by organic acid anions transported
out of root apex cells using specialized plasma
membrane-localized transporters. Pioneer work
established that Al tolerance in wheat was cor-
related with a strong Al-activated exudation of
malate (a dicarboxylic acid anion) (Delhaize
et al. 1993a; Delhaize et al. 1993b). Since then,
Al-tolerant genotypes from many plant species,
including maize, have been shown to make use of
this same mechanism of Al-exclusion, with the
identity of the organic acid released being the
main difference among plant species (see Table I
in Kochian et al. 2004).
Introduction
Importance of Acid Soils in Limiting
Worldwide Agriculture
Aluminum (Al) toxicity is the primary factor lim-
iting crop production on acidic soils. At soil pH
values below 5, the rhizotoxic Al species, Al
3
+
,
is solubilized into the soil solution, inhibiting
root growth and function and thus reducing crop
yields. Acid soils limit agricultural productiv-
ity in many regions of the world. Approximately
30% of the world's total land area consists of acid
soils, and it has been estimated that more than
50% of the world's potentially arable lands are
acidic (von Uexkull and Mutert 1995; Wood et al.
2000). Significant portions of the land acreage
used to produce important grain crops are also
acidic, and maize is one of the most important
grain crops grown on acid soils. Approximately
20% of all maize grown is found on acid soils.
A large proportion of the acid soils occur in
developing countries in the tropics and subtrop-
ics; it has been estimated that the humid tropics
account for 60% of the acid soils in the world.
Thus, acid soils limit maize yields in many devel-
oping countries where food production is critical.
Furthermore, in developed countries such as the
United States, high-input farming practices such
as the extensive use of ammonia fertilizers are
causing additional soil acidification of agricul-
tural soils. While liming of acid soils can ame-
liorate soil acidity, this is neither an economic
option for poor farmers nor an effective strategy
for alleviating subsoil acidity.
Progress on the Molecular Biology
of CropAlTolerance
Considerable progress has been made over the
past eight years on identifying and characteriz-
ing plant Al-tolerance genes. The first plant Al-
tolerance gene to be identified,
Ta A L M T 1
,was
cloned from wheat.
Ta A L M T 1
encodes a mem-
ber of a novel family of organic acid transporters,
the ALMT family, which when expressed het-
erologously confers Al-activated malate efflux
and increased Al tolerance in plants (Delhaize
et al. 2004; Sasaki et al. 2004). Electrophys-
iological studies established that TaALMT1,
as well as TaALMT1 orthologs identified in
Arabidopsis (
Arabidopsis thaliana)
(AtALMT1;
Hoekenga et al. 2006) and rape (
Brassica napus
;
Ligaba et al. 2006), mediate a selective efflux of
malate that is greatly enhanced by high affinity
direct binding of Al
3
+
to the transporter (Pi neros
et al. 2008; Zhang et al. 2008). The biochem-
istry and genetics of the ALMT family, and their
involvement in mediating Al
3
+
tolerance mech-
anisms based on organic anion efflux, have been
reviewed (Delhaize et al. 2007).
More recently a second class of Al-tolerance
genes was identified, which are transporters in
the MATE family that mediate root citrate efflux
and contribute to Al tolerance in a number of
plant species. MATEs were first identified as
Progress on PhysiologyofCrop
AlTolerance
Plants avoid the phytotoxic effects of Al
3
+
by
employing physiological mechanisms aimed at
excluding Al
3
+
from entering the root apex,
which is the primary site of Al toxicity (Al exclu-
sion), and/or by mechanisms that confer the abil-
ity to tolerate Al as it enters the plant symplasm
(Al tolerance). Compelling evidence indicates
Search WWH ::
Custom Search