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Heavy Metal Stress and Root Exudates in Plants

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Heavy metal contamination poses a significant threat to plant health, with elements like cadmium (Cd), lead (Pb), and aluminum (Al³⁺) inducing toxic effects. Cd, a non-essential element, exhibits high toxicity and non-biodegradability, triggering reactive oxygen species (ROS) production in plant cells, membrane lipid peroxidation, and stunted growth. Pb, similarly toxic and non-degradable, harms plants even at trace levels, while excess Al³⁺ accumulates in cell walls or organelles, disrupting development.

Root exudates are diverse compounds secreted by plant roots into the growth medium, including organic acids, amino acids, and soluble sugars. Their composition and quantity shift under environmental stresses like heavy metal exposure, playing a crucial role in plant adaptation.

Contents hide
1 Altered Root Exudates Under Heavy Metal Stress
1.1 1. Organic Acids: The Frontline of Plant Defense
1.2 2. Amino Acids: Adaptive Signaling Molecules
1.3 3. Soluble Sugars: Energy Reservoirs Under Stress

Altered Root Exudates Under Heavy Metal Stress

1. Organic Acids: The Frontline of Plant Defense
Organic acids, vital components of root exudates, are synthesized via the tricarboxylic acid (TCA) cycle in mitochondria. Classified by molecular weight into high-molecular-weight (HMWOAs) and low-molecular-weight organic acids (LMWOAs), they exhibit dynamic changes under heavy metal stress:

 

  • Cd Stress Responses:
    • Sedum secretes tartaric acid, lactic acid, acetic acid, citric acid, and oxalic acid, with tartaric and citric acids dominating. Tartaric, acetic, and oxalic acid secretion initially decreases then increases with Cd concentration, while lactic and citric acids rise proportionally.
    • Salix variegata prioritizes oxalic acid, peaking at 5 mg·L⁻¹ Cd, alongside trace tartaric, succinic, and acetic acids.
    • Maize relies on citric acid, sorghum on malic acid, and Eichhornia crassipes on oxalic and tartaric acids (secretion inhibited by prolonged stress). Rice shows a significant positive correlation between organic acid content and Cd accumulation.
  • Pb Stress Responses:
    • Pogonatherum crinitum secretes oxalic, citric, and malic acids; Sedum and peanuts favor oxalic and citric acids.
    • Tea plants show positive correlations between Pb concentration and malic, oxalic, and succinic acid secretion, but a negative correlation with citric acid.
    • Arenaria orbiculata increases LMWOA secretion, with citric acid content rising significantly with Pb concentration. Carex elevates total oxalic, malic, and citric acid levels as Pb increases, but citric acid’s proportion declines.
  • Cu and Al Stress Responses:
    • Typha latifolia secretes oxalic, malic, acetic, and tartaric acids under Cu stress; castor bean prioritizes tartaric acid.
    • Willows in polluted areas secrete abundant organic acids to avoid necrosis. Under Al stress, Setaria viridis and soybeans increase citric acid secretion, while poplar and tea plants release oxalic and citric acids (total organic acid content first decreases, then increases).
2. Amino Acids: Adaptive Signaling Molecules
Amino acids, the building blocks of proteins, show stress-dependent secretion patterns:

 

  • Cd stress promotes alanine and valine secretion in Sedum, while rice prioritizes glutamic acid.
  • Maize seedlings increase glycine, alanine, glutamic acid, valine, and threonine secretion at Cd >10 mg·L⁻¹.
  • Eichhornia crassipes maintains amino acid diversity under Cd stress but shows concentration-dependent total secretion (low promotion, high inhibition).
  • Composite Cd-Pb stress in Athyrium filix-femina enhances amino acid secretion, with glutamic acid, glutamine, and aspartic acid dominating in two Asteraceae species.
3. Soluble Sugars: Energy Reservoirs Under Stress
Soluble sugars, key metabolites in carbohydrate metabolism, exhibit dynamic shifts:

 

  • Eichhornia crassipes shows initially increased then decreased soluble sugar secretion under Cd stress, always exceeding controls.
  • Poplar seedlings boost fructose and glucose under NaCl and CdCl₂ stress; camphor leaves accumulate more soluble sugars, while low Cd enhances glucose, sucrose, and galactose in spinach and maize.
  • Sedum secretes the most soluble sugars at 400 μmol·L⁻¹ Cd. Arenaria orbiculata shows a positive correlation between soluble sugar content and Pb concentration, while Sedum’s galactose, glucose, and maltose first increase then decrease with rising Pb.

Heavy metal stress triggers precise adjustments in root exudate composition, with organic acids chelating metals, amino acids modulating physiological processes, and soluble sugars providing energy. Understanding these mechanisms enhances strategies for phytoremediation and crop improvement in contaminated soils.

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