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Purification and Characterization

Na+ Tolerance and Na+ Transport in Higher Plants

Abscisic acid (ABA) regulates many agronomically important aspects of plant development, including the synthesis of seed storage proteins and lipids, the promotion of seed desiccation tolerance and dormancy, and the inhibition of the phase transitions from embryonic to germinative growth and from

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Abscisic Acid Signaling in Seeds and Seedlings

The level of abscisic acid (ABAabscisic acid) in any particular tissue in a plant is determined by the rate of biosynthesis and catabolism of the hormone. Therefore, identifying all the genes involved in the metabolism is essential for a complete understanding of how this hormone directs plant growth and development. To date, almost all the biosynthetic genes have been identified through the isolation of auxotrophic mutants. On the other hand, among several ABA catabolic pathways, current genomic approaches revealed that Arabidopsis CYP707A genes encode ABA 8′-hydroxylases, which catalyze the first committed step in the predominant ABA catabolic pathway. Identification of ABA metabolic genes has revealed that multiple metabolic steps are differentially regulated to fine-tune the ABA level at both transcriptional and post-transcriptional levels. Furthermore, recent ongoing studies have given new insights into the regulation and site of ABA metabolism in relation to its physiological roles.

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Abscisic acid biosynthesis and catabolism

Jasmonic acid and its derivatives can modulate aspects of fruit ripening, production of viable pollen, root growth, tendril coiling, and plant resistance to insects and pathogens. Jasmonate activates genes involved in pathogen and insect resistance, and genes encoding vegetative storage proteins, but represses genes encoding proteins involved in photosynthesis. Jasmonic acid is derived from linolenic acid, and most of the enzymes in the biosynthetic pathway have been extensively characterized. Modulation of lipoxygenase and allene oxide synthase gene expression in transgenic plants raises new questions about the compartmentation of the biosynthetic pathway and its regulation. The activation of jasmonic acid biosynthesis by cell wall elicitors, the peptide systemin, and other compounds will be related to the function of jasmonates in plants. Jasmonate modulates gene expression at the level of translation, RNA processing, and transcription. Promoter elements that mediate responses to jasmonate have been isolated. This review covers recent advances in our understanding of how jasmonate biosynthesis is regulated and relates this information to knowledge of jasmonate modulated gene expression.

Biosynthesis and action of jasmonates in plants

shown the alterations of root architecture and structure induced by a variety of stressful conditions (Neumann A municipal solid-waste bottom slag was used to grow et al., 1994; Shannon et al., 1994; Tsegaye and Mullins, maize plants under various abiotic stresses (high pH, 1994; Plaut et al., 1997). For instance, it is known that high salt and high heavy metal content) and to analyse plants respond to salt stress by a reduction in root growth the structural and chemical adaptations of the cell and length. A decrease in the diameter of wheat roots walls of various root tissues. When compared with and of the transectional area of cortical cells was observed roots of control plants, more intensive wall thickenings with heavy metal treatment (Setia and Bala, 1994). were detected in the inner tangential wall of the endo- Besides changes in external root morphology there are dermis. In addition, phi thickenings in the rhizodermis also alterations in the fine structure of the root. It has in the oldest part of the seminal root were induced been shown (Shannon et al., 1994) that salinity promotes when plants were grown in the slag. The role of the suberization of the hypodermis and endodermis and that phi thickenings may not be a barrier for solutes as an the Casparian strip is developed closer to the root tip apoplastic dye could freely diffuse through them. The than in non saline roots. In cotton seedling roots, the chemical composition of cell walls from endodermis formation of an exodermis can be induced by salinity and hypodermis was analysed. Slag-grown plants had (Reinhardt and Rost, 1995). In addition, some plant higher amounts of lignin in endodermal cell walls when species develop cell wall thickenings in different root compared to control plants and a higher proportion of tissues. These thickenings are modifications of the midH-type lignin in the cell walls of the hypodermis. portion of the radial cell walls and are therefore called Finally, the amount of aliphatic suberin in both endo- phi thickenings. They can either form a uniseriate or a and hypodermal cell walls was not affected by growing multiseriate layer (Guttenberg, 1968; Peterson et al., 1981; the plants on slag. The role of these changes in relation Eschrich, 1995). Phi thickenings are formed on the to the increase in mechanical strengthening of the walls of certain cell layers in the root cortex of several root is discussed. species of gymnosperms ( Taxaceae, Podocarpaceae, Cupressaceae) (Guttenberg, 1968; Haas et al., 1976) and

Cell wall adaptations to multiple environmental stresses in maize roots

The Distribution of Abscisic Acid between Chloroplasts and Cytoplasm of Leaf Cells and the Permeability of the Chloroplast Envelope for Abscisic Acid

The distribution of the phytohormone, abscisic acid (ABA), within the phylum of Phycophyta was investigated by an enzyme-linked immunoassay (ELISA) Of 64 algal species tested (originating from 9 divisions, 20 classes and 36 orders, including procaryotes) all species contained ABA, whereas no ABA could be detected in the bacteria Escherichia coli, Rhodospirillum rubrum, and Halobacterium halobium It is concluded that ABA is universally distributed within the algal kingdom and is not restricted to cormophytes The ability to synthesize ABA must have been developed even within the procaryotes The physiological role of ABA in some selected algae was studied by investigating 1 the distribution of ABA between the cells and the culture medium, 2 the responses of endogenous ABA to stress, 3 the synthesis of 14C-ABA from externally applied 14C-mevalonic acid, 4 the metabolism of ABA, 5 the effect of externally applied ABA on various physiological reactions of the algae, and the effect of norflurazon on ABA content 14C-mevalonic acid served as precursor of 14C-ABA synthesis in Dunaliella cells and ABA was metabolised to the same products which have been observed in higher plants In D parva the internal ABA level increased upon hyperosmotic salt shocks, and in D acidophila upon alkalization of the medium Norflurazon caused an increase of ABA content in Dunaliella Externally applied ABA did not affect photosynthesis, respiration and K+ content of the cells The permeability of the plasma membrane of D acidophila to water was slightly decreased by ABA The possible physiological function of ABA in algae is discussed

Abscisic Acid Content of Algae under Stress

Do chloroplasts play a role in abscisic acid synthesis

For thermodynamic reasons algae growing in media of both high salinity and high alkalinity require active export of sodium. However, experimental evidence for an active Na+-dependent cycle was scarce until recently, in contrast to the situation in marine bacteria (including cyanobacteria), fungi and animals. However, a review of literature reveals that some progress has been made in this respect, recently: data demonstrate that at least in two marine algae, Tetraselmis (Platymonas) viridis and Heterosigma akashiwo (syn. Olisthodiscus luteus), active Na+-export is carried out by means of a plasma membrane localized Na+-pump (apparent molecular mass 100-140 kDa). Biochemical characteristics of this vanadate-sensitive, but ouabain-resistant primary P-type Na+-ATPase are described and compared with the corresponding properties of Na+-ATPase from prokaryotes and animals. Alternative mechanisms for Na+-pumping are discussed.

Hartmut Gimmler

Papers

Cell wall adaptations to multiple environmental stresses in maize roots

The Distribution of Abscisic Acid between Chloroplasts and Cytoplasm of Leaf Cells and the Permeability of the Chloroplast Envelope for Abscisic Acid

Abscisic Acid Content of Algae under Stress

Do chloroplasts play a role in abscisic acid synthesis

Primary sodium plasma membrane ATPases in salt‐tolerant algae: facts and fictions