Vacuoles sequestration

Many plants detoxify heavy metals by sequestering them, either as phytochelatin complexes or without specific ligands, in the vacuoles (for reviews see e.g., [73,74]). It makes sense for hyperaccumulating plants to store metal in the vacuoles as well because this organelle only contains enzymes like phosphatases, lipases, and proteinases [75] that were never identified as targets of heavy metal toxicity. Vacuole sequestration is driven to an extreme form in hyperaccumulators, where... 

ALFENITO, M.R., SOUER, E GOODMAN, C.D., BUELL, R., MOL, J., KOES, R., WALBOT, V., Functional complementation of anthocyanin sequestration in the vacuole by widely divergent glutathione 5-transferases, Plant Cell, 1998,10,1135-1149. 

M., The TRANSPARENT TESTA12 gene of Arabidopsis encodes a multidrug secondary transporter-like protein required for flavonoid sequestration in vacuoles of the seed coat endothelium, Plant Cell, 2001,13, 853-871. 

In the lower eukaryotes, cation sequestration and storage are observed in vacuoles. Vacuoles of yeast accumulate amino acids (Wiemken and Durr, 1974), K, Mg2+ and Mn2+ (Okorokov et al, 1980 Lichko et al, 1982) (Table 7.1), and Ca2+ (Ohsumi and Anraku, 1983 Dunn et al, 1994). PolyP, which is able to confine different cations in an osmotic inert form, was also found in these storage organelles (Indge, 1968a,b,c Westenberg et al, 1989). 

M. Diirr, K. Urech, T. Boiler, A. Wiemken, J. Schwencke and M. Nagy (1979). Sequestration of arginine by polyphosphate in vacuoles of yeast Saccharomyces cerevisiae. Arch. Microbiol., 121, 169-175. 

Several plants produce milk juice sequestered in laticifers in several plant genera alkaloids are mainly stored in latex vesicles, such as isoquinoline alkaloids in Papaver and Chdidonium, or piperidine alkaloids in Lobelia. If herbivores wound such a plant, the latex will spill out and the herbivore will immediately be confronted with alkaloids. Since most of them are strong poisons, a deterrent effect is usually achieved. Another strategic way to store alkaloids is their sequestration in epidermal vacuoles or in trichomes. These tissues have to ward off not only herbivores (especially small ones) but also microorganisms in the first place. Several classes of alkaloids have been found in epidermal tissues, such as quinohzidine and tropane alkaloids [2,3]. 

Intracellular distribution of essential transition metals is mediated by specific metallochaperones and transporters localized in endomembranes. In other words, the major processes involved in hyperaccumulation of trace metals from the contaminated medium to the shoots by hyperaccumulators as proposed by Yang et al. (2005) include bioactivation of metals in the rhizosphere through root-microbial interaction enhanced uptake by metal transporters in the plasma membranes detoxification of metals by distributing metals to the apoplasts such as binding to cell walls and chelation of metals in the cytoplasm with various ligands (such as PCs, metallothioneins, metal-binding proteins) and sequestration of metals into the vacuole by tonoplast-located transporters. 

Plants as well as other organisms have evolved several adaptive strategies to counter these types of abiotic stresses (Csonka, 1989 Bohnert et al., 1995). At the cellular level, the most common type of osmotic adaptation is the accumulation of compatible solutes in the cytoplasm and the sequestration of NaCl into the vacuole (Rhodes and Hanson 1993 Bohnert et al., 1995). Compatible solutes arc small molecules that can act as nontoxic cytoplasmic osmolytes to raise osmotic pressure, and stabilize enzymes and membranes against damage by high salt levels (Wyn Jones, 1984). 

One facile method for investigation of cellular interactions of liposomes, and more precisely, their ability to accumulate within cells, is the fluorescent microscopy. To meet this objective, liposomes are loaded with fluorescent marker (calcein or other suitable dye) at high concentration, whereby its fluorescence is self-quenched. Upon cellular fusion and internalization the dye is released from the carrier, diluted in the environment so the selfquenching effect is lost and the increased fluorescence is detected by fluorescent microscopy. In the case of effective cellular internalization of pH-sensitive liposomes, that is, without endosomal sequestration, calcein would have been diluted several-hundred-fold and the cells will display uniform cytosolic fluorescence. If the liposomes have been taken up by cells by endocytosis, punctuate fluorescence will be restricted to the secondary lysosomal and endocytic vacuoles. In contrast, adsorbed liposomes should... 

As we saw in Chapter 7, there are several plasma membrane zinc uptake transporters in yeast. Within the cell, a number of other proteins are involved in zinc transport within the cell. S. cerevisiae is unusual in that it does not appear to have any plasma membrane zinc efflux transporters. This is to a large extent compensated by the capacity of the vacuole to serve as a major site of zinc sequestration and detoxification, enabling wild-type cells to tolerate exogenous zinc concentrations as high as 5 mM. The zinc stored in the vacuole can attain millimolar levels, and can be mobilised under zinc-deficient conditions for use by the cell. Vacuolar zinc uptake is mediated by two members of the cation diffusion facility CDF family, Zrcl and Cotl (Fig. 8.16). 

Diclofop-methyl is a herbicide which, upon entry into the plant, undergoes minimal acropetal and basipetal transport (8). In our experiments transport to the roots or to the leaves of [14C] diclofop-methyl applied to the axils of two-leaved susceptible and resistant Lolium plants does not appear to differ. No data are presently available as to whether proplastids or chloroplasts from the two biotypes exhibit differential permeability to diclofop-methyl or to the active acid derivative diclofop. Similarly, there is no evidence for a differential capacity to convert diclofop-methyl to diclofop nor differential sequestration of the ester or the acid in some secondary compartment such as the vacuole or within membranes. 

ADNTs with sugars, followed by their sequestration into the cell vacuole or incorporation into bound residues with the cell wall are likely endpoints. The explosives RDX and HMX are also taken up by aquatic vascular plants, but their initial biotransformation products and endpoints are not known at this time. Fundamental studies on RDX and HMX metabolism by aquatic vascular plants and nonvascular macroalgae are needed. 

Intensive use of the herbicide paraquat has resulted in the evolution of resistance in various weed species. Intensive research on the resistance mechanisms was mainly carried out with resistant biotypes from Hordeum spp. and Conyza spp., and altered distribution of the herbicide in the resistant weeds was suggested as the cause - or at least the partial cause - of resistance. In resistant Conyza canadensis it was supposed that a paraquat inducible protein may function by carrying paraquat to a metabolically inactive compartment, either the cell wall or the vacuole. This sequestration process would prevent the herbicide from getting in sufficient amounts into the chloroplasts as the cellular site of paraquat action. Inhibitors of membrane transport systems, e.g., N,N-dicyclohexylcarbodii-mide (DCCD), caused a delay in the recovery of photosynthetic functions of the paraquat-resistant biotype, when given after the herbicide. These transport inhibitor experiments supported the involvement of a membrane transporter in paraquat resistance [75]. 

At the present time, this type of regulation is not clearly understood and must be examined in the future. In this way, the special role of the large vacuole, which contains high concentrations of reactants has to be studied this compartment may play a central role not only in the sequestration of metabolites and inhibitors, but, also in supplying substrates and cofactors. 

Recent studies indicated that the rate of sequestration of substrate proteins into the autophagic vacuoles may be the rate-limiting step in the process of protein degradation, and that lysosomal proteinases, especially thiol proteinases, exist in excess and may not be the rate-limiting step. 

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