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monosaccharides and dissolved organic carbon in comparison with those of non-mycorrhizal plants.
These exudates can chelate Cd
2+
and Pb
2+
and therefore, contribute to detoxification of the heavy
metals and improve the performance of the seedlings.
If this
first line of defense fails, most of the heavy metals that do enter the plant are then kept
in root cells, where they are detoxified by complexation with amino acids, organic acids or metal-
binding peptides and/or sequestered into vacuoles. This greatly restricts translocation to the above-
ground organs thus protecting the leaf tissues, and particularly the metabolically active photosyn-
thetic cells from heavy metal damage.
A further defense mechanism generally adopted by heavy metal-exposed plants is enhance-
ment of cell antioxidant systems, which counteracts oxidative stress. Different pathways could lead
to upregulating genes in the antioxidant defense systems, some of which include the utilization of
plant hormones. In addition to the five classical plant hormones, i.e., gibberellins (GAs), cytokinins
(CKs), auxins, abscisic acid (ABA), and ethylene, jasmonate (JA), brassinosteroids (BR), and sali-
cylic acid (SA) are also well known for regulating many physiological processes and heavy metal
stress tolerance. Furthermore, it is also expected that some more growth hormones are yet to be dis-
covered in future.
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