Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

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K. Gasic and S.S. Korban
detoxify aluminum in the rhizosphere by releasing organic acids that chelate aluminum,
while others detoxify aluminum internally by forming complexes with organic acids (Ma
et al., 2001). Extracellular chelation by organic acids, such as citrate, oxalate, and malate,
is important in aluminum tolerance. Malate is released from roots of Al-tolerant cultivars
of wheat (Triticum aestivum); while, citrate is released from roots of Al-tolerant
cultivars of snapbean (Phaseolus vulgaris), maize (Zea mays), Cassia tora, and soybean
(Glycine max). Whereas, oxalate is released from roots of buckwheat (Fagopyrum
esculentum) and taro (Colocasia esculenta) (Delhaize et al., 1993; Pellet et al., 1995; Ma
et al., 1997a; Yang et al., 2001; Ma et al., 1997b; Ma and Miyasaka, 1998). Some plant
species, such as Al-tolerant triticale (x Triticosecale Wittmack), rapeseed (Brassica
napus), oat (Avena sativa), radish (Raphanus sativus), and rye (Secale cereale) release
both malate and citrate (Ma et al., 2000; Zheng et al., 1998; Li et al., 2000).
Sasaki et al. (2004) have cloned a wheat gene, ALMT1 (aluminum-activated
malate transporter), that co-segregates with Al tolerance in F 2
and F 3
populations derived
from crosses between near-isogenic wheat lines that differ in Al tolerance. The
ALMT1 gene codes for a membrane protein constitutively expressed in root apices of
an Al-tolerant line at higher levels than in a near-isogenic Al-sensitive line. Heterologous
expression of ALMT1 in Xenopus oocytes, rice, and cultured tobacco cells has
conferred an Al-activated malate efflux. Additionally, ALMT1 has increased tolerance
of tobacco cells to Al treatment. These findings demonstrate that ALMT1 codes for an
Al-activated malate transporter capable of conferring Al tolerance to plant cells.
It has been reported that Al treatment in Arabidopsis induces oxidative stress
genes (Richards et al., 1998). Ezaki et al. (2001) have characterized the mechanism of
action of four genes, including AtBCB (Arabidopsis blue copper-binding protein), parB
(Nicotiana tabacum gluthatione S-transferase), NtPox (tobacco peroxidase), and
NtGDI1 (tobacco GDP dissociation inhibitor), involved independently in Al resistance
using transgenic Arabidopsis lines. Apparently, these four genes have different biochemical
functions, thus suggesting that there are several different mechanisms for Al
tolerance in plants in addition to the release of organic acids. Influx and efflux experiments
of Al ions have suggested that the AtCBC gene likely suppresses Al absorption;
whereas, expression of the NtGDI1 gene promotes the release of Al in root-tips of
Arabidopsis plants. Overexpression of parB and NtPox diminishes oxidative damage
caused by Al stress. Analysis of F 1
hybrids among the four transgenic lines suggests
that enhanced resistance can be achieved by combining some of these four genes in
individual lines.
Exposure of plant cells of the cobalt hyperaccumulator Crotalaria cobalticola
and non-accumulators Raufolia serpentine and Silene cucubalus to cobalt ions have
resulted in increases of both citrate and cysteine suggesting that these two proteins are
involved in cobalt ion complexation (Oven et al., 2002a).
Under heavy metal stress, a high cysteine biosynthesis rate is required for the
synthesis of GSH and phytochelatins. O-acetyl-serine(thiol)lyase (OASTL) is a key
enzyme of a plant sulfur metabolism that catalyses the formation of Cys which serves as