BB30129: Mineral Toxicity and Photoremediation
Aluminium toxicity is a major limitation to crop
production on acid soils
Acid soils
Affects 40% of world land area including parts of USA & other
developed countries
Exacerbated by intensive farming e.g. ammonia fertilisers
Acid soils make up 12% of world agricultural land
And 60% of tropical soils
Lime can alleviate acid soil but is expensive & can take years – does
not act quickly enough to meet the demands of agricultural
production
Crops affected
Acid soils affect 20% of world maize production and 13% of world
rice production
Aluminium
Most abundant metal on earth, & third most abundant metal in
earth’s crust
Phytotoxic forms are insoluble at most soil pHs – they therefore
cannot interact with the plant
At pH 5 or below, phytotoxic forms of Al become soluble &
accumulate, can harm plants
Plant responses to Aluminium toxicity
Symptoms of Al-toxicity
Rapid inhibition of root growth – occurs within minutes of the root
apex (most sensitive region to Al toxicity) being exposed to high Al
concentrations
Even low (uM) concentrations of Al are sufficient to cause this
Study: Al-resistant Atlas & Al-sensitive Scout wheat varieties grown
for 4 days on aluminium chloride at pH 4.5. As concentration of Al
increases from 0-50uM, Al-sensitive plants show stunted root
growth. At higher Al concentrations (50uM), even Al-resistant plants
showed stunted root growth (but still grew more successfully than
Al-sensitive)
Numerous ways in which Al inhibits root growth:
Blockage of K+ & Ca+ channels – this disrupts second messenger
signalling, leading to oxidative stress
Enhanced susceptibility to drought & nutrient deficiency stresses
, – inhibited root growth means fewer root cells available to take up
water and obtain other nutrients from the soil (macronutrient
deficiencies)
Distribution of Al ion species is dependent on pH
Al3+ is the most abundant ion below pH 5 & is rhizotoxic (inhibitory
to root elongation)
As pH decreases from 6 -4, concentration of this harmful Al ion
species rapidly increases
Other Al ion species decrease at low pH (e.g. Al(OH)2+, Al(OH)4-)
Aluminium resistance mechanisms
Two strategies of Al resistance, both of which are employed by
different species and in different ways within the same species.
1)Al-induced secretion of organic acids that chelate
Al3+
Al-tolerant wheat roots release malate (organic acid, binds Al) within
minutes of Al exposure, whereas Al-sensitive wheat variety does not
Magnitude of this response is related to Al dose – malate exported
by Al-tolerant plant increases linearly with Al concentration
Not all plants employ malate as a Al chelator: maize & soybean use
Al-dependent citrate release, Al-tolerant buckwheat secretes
buckwheat.
Aluminium toxicity is a major limitation to crop
production on acid soils
Acid soils
Affects 40% of world land area including parts of USA & other
developed countries
Exacerbated by intensive farming e.g. ammonia fertilisers
Acid soils make up 12% of world agricultural land
And 60% of tropical soils
Lime can alleviate acid soil but is expensive & can take years – does
not act quickly enough to meet the demands of agricultural
production
Crops affected
Acid soils affect 20% of world maize production and 13% of world
rice production
Aluminium
Most abundant metal on earth, & third most abundant metal in
earth’s crust
Phytotoxic forms are insoluble at most soil pHs – they therefore
cannot interact with the plant
At pH 5 or below, phytotoxic forms of Al become soluble &
accumulate, can harm plants
Plant responses to Aluminium toxicity
Symptoms of Al-toxicity
Rapid inhibition of root growth – occurs within minutes of the root
apex (most sensitive region to Al toxicity) being exposed to high Al
concentrations
Even low (uM) concentrations of Al are sufficient to cause this
Study: Al-resistant Atlas & Al-sensitive Scout wheat varieties grown
for 4 days on aluminium chloride at pH 4.5. As concentration of Al
increases from 0-50uM, Al-sensitive plants show stunted root
growth. At higher Al concentrations (50uM), even Al-resistant plants
showed stunted root growth (but still grew more successfully than
Al-sensitive)
Numerous ways in which Al inhibits root growth:
Blockage of K+ & Ca+ channels – this disrupts second messenger
signalling, leading to oxidative stress
Enhanced susceptibility to drought & nutrient deficiency stresses
, – inhibited root growth means fewer root cells available to take up
water and obtain other nutrients from the soil (macronutrient
deficiencies)
Distribution of Al ion species is dependent on pH
Al3+ is the most abundant ion below pH 5 & is rhizotoxic (inhibitory
to root elongation)
As pH decreases from 6 -4, concentration of this harmful Al ion
species rapidly increases
Other Al ion species decrease at low pH (e.g. Al(OH)2+, Al(OH)4-)
Aluminium resistance mechanisms
Two strategies of Al resistance, both of which are employed by
different species and in different ways within the same species.
1)Al-induced secretion of organic acids that chelate
Al3+
Al-tolerant wheat roots release malate (organic acid, binds Al) within
minutes of Al exposure, whereas Al-sensitive wheat variety does not
Magnitude of this response is related to Al dose – malate exported
by Al-tolerant plant increases linearly with Al concentration
Not all plants employ malate as a Al chelator: maize & soybean use
Al-dependent citrate release, Al-tolerant buckwheat secretes
buckwheat.
