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Mechanisms enabling plants to tolerate aluminium toxicity

CONTENTS

S.No		Pg. No
1	Introduction	3
2	Aluminium Toxicity in Plants	4
3	Resistance of Wheat to Aluminium Toxicity	4
4	Aluminium Exclusion Mechanism in Wheat	5
5	Role of Calcium Ions in Aluminium Toxicity Resistance	5
6	Effect of Aluminium toxicity on Sorghum	6
7	Resistance of Sorghum to Aluminium Toxicity	7
8	References	9

 

Introduction                   

Aluminium is the third most found metallic element in soil. Mostly plants are not harmed by Aluminium, it is only when the soil becomes acidic and pH decreased to 5.5 or less that many plants suffer from toxicity (Kochian, Piñeros and Hoekenga, 2005). The main result of Aluminium toxicity is a drop in plant production rate and cessation of growth of root hair cells. It is also stipulated that inhibition of root growth might be the cause of decrease in production rate (Silva, 2012). This paper discusses the mechanisms which cause some plants to tolerate high levels of Aluminium concentrations and how these mechanisms developed in various strains of wheat and sorghum.

The main compounds of aluminium which make the soil acidic and causes damage to plants are aluminium hydroxide and maluinium hydrate. Aluminium toxicity is one of the most major restrictions for growth of crops due to acidic soil which covers l aarge part of land area in the topical and boreal regions. When the pH becomes less than 5, aluminium ions are released into the soil and are absorbed by the tips of roots. This results in cessation of plant growth.

Aluminium causes cessation of cell elongation and division, causing stunted growth of the root and makes it less able to take up water and nutrients (Bose, Babourina and Rengel, 2011). There are many genes which are identified which are affected by aluminium toxicity. At the tissue level, the distal part of the transitional zone is affected the most.

At the molecular level, the DNA, mitochondria, and many other proteins are implicated in aluminium toxicity. Currently it is difficult to pin point the exact mechanism through which the metal ions act on the metabolic pathways occurring in plant cells however it is known that some plants are more resistant to aluminium toxicity than others (Panda, Baluška and Matsumoto, 2009).

Aluminium Toxicity in Plants

The most sensitive part of the plant to aluminium is the root. As root elongation is a process which requires frequent cell division, aluminium toxicity inhibits this division and leads to the apices of root to become swollen and impaired. This leads to defect in the function of the roots and the plant becomes devoid of ions as well as water. The primary action of aluminium ions is at the meristematic cells which transit from the dividing phase to the F-actin dependent elongation phase (Verbelen et al., 2006).

Aluminium with iron can also disrupt the plasma membrane by reacting with the lipids through the process of lipid peroxidation. It principally binds to the phospholipid component which is present in the membrane. On the other hand, according to a recent study it has been found that the exact mechanisms underlying Aluminium toxicity are not well understood.

This is because the ions can interact with various extracellular and intracellular chemicals, structures and enzymes. Few of the various mechanisms include cell membrane disruption, inhibiting ion transport across the cells, interruption of the signalling pathways as well as binding to the DNA of the cell (Kochian, Piñeros and Hoekenga, 2005).

Aluminium induced production of reactive oxidative species also leads to damange of mitochondria and activation of oxidising enzymes which further contribute to the deteriorating condition of the plant cells. However, the role of Magnesium is not studied nor received any attention in ROS production due to Aluminium toxicity so far (Bose, Babourina and Rengel, 2011).

Raised Aluminium concentrations lead to oxidative stress in the cell which causes damage to the cellular components, especially the membrane. Oxidative stress is a condition which occurs due to disruption in the oxidative homeostasis mechanism of the cell. Aluminium leads to production of reactive oxygen species which then react with different proteins, lipids and other substances causing cell death.

Cell wall pectin and components of the outer surface of cell membrane seem to be the target of Aluminium induced oxidative stress (Zheng and Yang, 2005). Aluminium induced lipid peroxidation is seen in sorghum (Peixoto et al., 1999).

Nutritional imbalance inducted by high levels of Aluminium was also reported in some plants including wheat. Aluminium causes a decrease in the uptake of macro-nutrients like calcium and magnesium as well as micro-nutrients such as manganese and zinc. It is known that the micro nutrients are more severely affected (Mariano and Keltjens, 2005).

In wheat, the sensitive as well as tolerant strains showed a decrease in potassium and magnesium in the root cells. It was also noticed that the sensitive wheat strains showed a higher level of nutritional imbalance in shoots and roots as compared to the resistant strains (Silva, 2012).

Wheat strains which were exposed to Aluminium also showed the formation of callose in their root tips. It is now a good indicator of Aluminium toxicity. It not only accumulates in the meristematic regions but deposits in the mature root zones as well. Sensitive wheat strains showed progressive accumulation of callose in the inner cell layers as opposed to the tolerant strains (Bhuja et al., 2004).

Resistance of Wheat to Aluminium Toxicity

Plants which grow in acidic soil contaminated with high concentrations of Aluminium have adapted to their conditions and developed mechanisms which can be classified as those which cause exclusion of Aluminium ions from the root hair cells and those which cause the plant to tolerate high concentrations of Aluminium ions which it has entered the root and shoot cells (Bose, Babourina and Rengel, 2011). Wheat is a highly studied crop in terms of aluminium toxicity resistivity.

There is evidence which suggests that exudation of Malate occurs from wheat in response to Aluminium toxicity via the activation of an anion channel which is present in the plasma membrane of wheat cells (Kochian, Piñeros and Hoekenga, 2005). It is possible the Aluminium binds to the channel directly and activates the channel or indirectly through components in the cytoplasm (Zhang, 2001).

With respect to genetics, the tolerance gene identified in wheat is ALMT1 which codes for a malate transporter in wheat species of Triticum aestivum. In species of wheat, the most resistant strains are those which originated from Brazilian ancestor…