The role of arbuscular mycorrhiza in reducing Cd stress was investigated in three genotypes of Pisum sativum L. (cv. Frisson, VIR4788, VIR7128), grown in soil/sand pot cultures in the presence and absence of 2-3 mg kg(-1) bioavailable Cd, and inoculated or not with the arbuscular mycorrhizal fungus Glomus intraradices. Shoot, root and pod biomass were decreased by Cd in non-mycorrhizal plants. The presence of mycorrhiza attenuated the negative effect of Cd so that shoot biomass and activity of photosystem II, based on chlorophyll a fluorescence, were not significantly different between mycorrhizal plants growing in the presence or absence of the heavy metal (HM). Total P concentrations were not significantly different between mycorrhizal and non-mycorrhizal plants treated with Cd. From 20-50-fold more Cd accumulated in roots than in shoots of Cd-treated plants, and overall levels were comparable to other metal-accumulating plants. Genetic variability in Cd accumulation existed between the pea genotypes. Concentration of the HM was lowest in roots of VIR4788 and in pods of VIR4788 and VIR7128. G. intraradices inoculation decreased Cd accumulation in roots and pods of cv. Frisson, whilst high concentrations were maintained in roots and pods of mycorrhizal VIR7128. Shoot concentrations of Cd increased in mycorrhizal cv. Frisson and VIR4788. Sequestration of Cd in root cell walls and/or cytoplasm, measured by EDS/SEM, was comparable between non-mycorrhizal pea genotypes but considerably decreased in mycorrhizal cv. Frisson and VIR7128. Possible mechanisms for mycorrhiza buffering of Cd-induced stress in the pea genotypes are discussed.
Molecular responses to cadmium (Cd) stress were studied in mycorrhizal and non-mycorrhizal Pisum sativum L. cv. Frisson inoculated with Glomus intraradices. Biomass decreases caused by the heavy metal were significantly less in mycorrhizal than in non-mycorrhizal plants. Real-time reverse transcriptase-polymerase chain reaction showed that genes implicated in pathways of Cd detoxification varied in response to mycorrhiza development or Cd application. Expression of a metallothionein-encoding gene increased strongly in roots of Cd-treated non-mycorrhizal plants. Genes encoding gamma-glutamylcysteine synthetase and glutathione (GSH) synthetase, responsible for the synthesis of the phytochelatin (PC) precursor GSH, were activated by Cd in mycorrhizal and non-mycorrhizal plants. Cd stress decreased accumulation of GSH/homoglutathione (hGSH) and increased thiol groups in pea roots, whether mycorrhizal or not, suggesting synthesis of PCs and/or homophytochelatins. An hGSH synthetase gene, involved in hGSH synthesis, did not respond to Cd alone but was activated by mycorrhizal development in the presence of Cd. Transcript levels of a glutathione reductase gene were only increased in non-mycorrhizal roots treated with Cd. Studies of three stress-related genes showed that a heat-shock protein gene was activated in mycorrhizal roots or by Cd and chitinase gene transcripts increased under Cd stress to a greater extent in mycorrhizal roots, whilst a chalcone isomerase gene was only up-regulated by Cd. Results indicate that although heavy metal chelation pathways contribute to Cd stress responses in pea, they may not make a major contribution to Cd tolerance strategies operating in the arbuscular mycorrhizal symbiosis.
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