Module 1: Phyto-route
Hyperaccumulator was coined for plants that, differently from the excluder plants, actively take up exceedingly large amounts of one or more heavy metals from the soil. Moreover, the heavy metals are not retained in the roots but are translocated to the shoot and accumulated in aboveground organs, especially leaves and stem at concentrations 100–1000-fold higher than those found in non-hyperaccumulating species.
The major processes involved in hyperaccumulation of trace metals from the soil to the shoots by hyperaccumulators include: (a) bioactivation of metals in the rhizosphere through root–microbe interaction; (b) enhanced uptake by metal transporters in the plasma membranes; (c) detoxification of metals by distributing to the apoplasts like binding to cell walls and chelation of metals in the cytoplasm with various ligands, such as phytochelatins, metallothioneins, metal-binding proteins; (d) sequestration of metals into the vacuole by tonoplast-located transporters.
Fig. 1 Hyperaccumulator- Sedum alfredii Hance grown in the growth chamber with a 16/8 h (22/18 ℃) day/night regimes at 120 molm-2 s-1 irradiation during our phyto-experiment.(Picture by FAFU-CHINA team 2017).
Ethylene is a gaseous plant growth hormone produced endogenously by almost all plants. Apart from being a plant growth regulator, ethylene has also been established as a stress hormone. Under stress conditions like those generated by salinity, drought, waterlogging, heavy metals and pathogenicity, the endogenous production of ethylene is accelerated substantially which adversely affects the root growth and consequently the growth of the plant as a whole.
1-aminocyclopropane-1-carboxylate (ACC) deaminase(BBa_K2384005), which regulates ethylene production by metabolizing ACC (an immediate precursor of ethylene biosynthesis in higher plants) into α-ketobutyrate and ammonia，simultaneously enhances plant growth and biomass by P solubilization and uptake particularly under stress condition by heavy metals.
Fig. 2 Plants growth promoting parameters. Abbreviations: indole-3-acetic acid (IAA), methionine-S-adenosylmethionine (SAM), 1-aminocyclopropane-1-carboxylate (ACC).
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