However, the Ca flux to the xylem through the apoplastic pathway is influenced markedly by transpiration, which could lead to vagaries in the amount of Ca supplied to the shoot and the development of Ca disorders Marschner, ; McLaughlin and Wimmer, By contrast, the symplastic pathway allows the plant to control the rate and selectivity of Ca transport to the shoot Clarkson, ; White, By regulating the expression and activity of these transporters, Ca could be delivered selectively to the xylem at a rate consistent with the requirements of the shoot.
Several lines of evidence suggest that both apoplastic and symplastic pathways contribute to Ca delivery to the xylem. First, since Ca is delivered to the xylem in regions of the root where the Casparian band is fully developed and apoplastic Ca transport is restricted, some Ca might bypass the Casparian band through the cytoplasm of endodermal cells Clarkson, , ; White, When abundant Ca is present in the xylem sap, there is a close relationship between Ca distribution to the shoot and transpiration. Within the leaf, Ca follows the apoplastic route of the transpiration stream and accumulates in either the mesophyll cells, trichomes or epidermal cells adjacent to guard cells, depending on the plant species Karley et al.
A large proportion of this variation can be attributed to the phylogenetic division between eudicots and monocots Thompson et al. Eudicots generally have a higher [Ca] shoot than monocots. Within the eudicots, orders within both the rosid Cucurbitales, Rosales, Malvales and Brassicales and asterid Apiales, Asterales, Lamiales and Solanales clades have the highest [Ca] shoot.
Asparagales than in the commelinoid orders e. Although the Ca physiotype of a plant determines its ability to accumulate Ca in the shoot Kinzel and Lechner, ; Broadley et al. Historically, [Ca] shoot has been correlated with the cation exchange capacity CEC of plant roots Fig. The CEC is located in the root apoplast, and is attributed to the free carboxyl groups of galacturonic acids of cell wall pectins in the middle lamella Haynes, ; Sattelmacher, If the pectin contents of shoot and root cell walls are similar, it is unsurprising that the phylogenetic variation in CEC in monocot roots parallels that in pectin content of shoot cell walls Jarvis et al.
Haynes, Nevertheless, fixed negative charges, and charge screening, can influence both the absolute and relative concentrations of cations in the apoplast, especially at low ionic activities. Thus, root CEC could determine shoot cation content indirectly by affecting the rate and selectivity of cation uptake into the symplast in addition to cation transport through the apoplast Asher and Ozanne, ; Wacquant, The effects of CEC on cation transport may have ecological implications. It has been suggested that, in solutions of low ionic strength, plants with higher root CEC compete for divalent cations more effectively, whereas plants with lower root CEC compete for monovalent cations more effectively Smith and Wallace, ; Asher and Ozanne, Quantifying the phylogenetic impact on [Ca] shoot has several uses.
First, since [Ca] shoot correlates with ecological traits Kinzel, ; Thompson et al. Secondly, knowledge of phylogenetic variation in [Ca] shoot can be used to predict the movement of Ca and chemically similar elements such as Sr, Mg and Ba; White, ; Broadley et al. Thirdly, appreciation of phylogenetic influences on [Ca] shoot can improve the delivery of Ca to the human diet. Phylogenetic information can help identify crops that are either predisposed to higher [Ca] shoot or offer genetic potential for breeding.
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Four members of this family have been identified in the arabidopsis genome termed AtECAs 1 to 4 by Axelsen and Palmgren, Such explanations typically suggest that individual isoforms are functionally distinct and specialized to specific cellular processes requiring distinct spatial or temporal expression. Cheng et al. How, and whether, this regulatory mechanism operates in vivo has yet to be revealed. The AtCAXs have homologues in other plant species and their physiological roles have been investigated using transgenic plants Hirschi, Although each has distinct pharmacological and electrophysiological properties, all are permeable to both monovalent and divalent cations.
The dominant DACCs in protoplasts from arabidopsis tissues and carrot suspension cells appear to be controlled by cytoskeletal interactions and stabilized by the disruption of microtubules Thion et al. Mechanosensitive HACCs may orchestrate the changes in morphology induced by gravity, touch or flexure in a similar manner Pickard and Ding, ; Bibikova et al.
The Role Of Calcium And Comparable Cations In Animal Behaviour
In guard cells, HACCs have a central role in closing stomata during water stress. Cytoplasmic alkalinization or vacuolar acidification also promotes their opening under physiological conditions. Phos phorylation at one site is inhibitory, whereas phosphorylation at the other site activates the channel. Both tonoplast HACCs show spontaneous changes in their kinetic behaviour, which has been interpreted as resulting from interactions with hypothetical regulatory ligands. They are inhibited by micromolar concentrations of ruthenium red and procaine.
This is considered essential for producing a physiological response. Sanders et al. Various authors have speculated how these waves might be initiated and propagated. Antoine et al. Electrical action potentials, osmotic perturbations or chemical signals may trigger these waves. Such challenges include mechanical perturbation Knight et al. Periodicities may range from the circadian oscillations observed in various plant cells Johnson et al.
Van der Luit et al. Pauly et al. Similarly, Lecourieux et al. Blume et al. Following one brief pulse of phototropically active blue light, arabidopsis seedlings will not respond maximally again for several hours Baum et al. CaM is found in the apoplast and in the cytosol, ER and nucleus of plant cells. Calmodulins can also regulate gene expression by binding to specific transcription factors Szymanski et al. Small gene families encode CaM isoforms in plants.
Many species possess several CaM genes encoding identical proteins as well as other genes encoding divergent isoforms Zielinski, ; Snedden and Fromm, Indeed, it has been suggested that particular CaM isoforms transduce specific environmental or developmental signals. These proteins have been implicated in cellular responses to diverse environmental, developmental and pathological challenges.
It has been proposed that myristoylation of CBLs alters their cellular location and intracellular interactions Luan et al. It is thought that particular CBLs transduce specific environmental or developmental signals. Consistent with this hypothesis is the induction of AtCBL1 expression by drought, salinity, cold and wounding Kudla et al.
Guo et al. The CDPKs are ubiquitous in plants. They act as monomers and many may be autoinhibited by autophosphorylation of a pseudosubstrate domain S. No CDPKs appear to be integral membrane proteins, but many are associated with the cytoskeleton, nucleus, plasma membrane and ER Reddy, ; Sanders et al. In various plant species specific CDPKs are induced by cold, drought, salinity, annoxia, mechanical stress, wounding and pathogen elicitors Urao et al.
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Orthologues of these are present in many plant species. They are expressed highly in rapidly growing cells and tissues of the root and flower Zhang and Lu, Chimeric CCaMKs are expressed in the anthers of several plant species, including lily and tobacco Liu et al. Liu et al.
These biochemical modulators of PLD activity are the substrates or products of phospholipase C, which generates IP 3 and diacylglycerol, phospholipase A 2 and diacylglycerol kinase, both of which are regulated by CaM. This often appears only in the first quarter of plant annexins, although animal annexins generally possess four such motifs.
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In plants, annexins are encoded by small gene families. The expression of calreticulin is increased in reproductive tissues. Several CaM isoforms have been located in the nuclei of plant cells. It has long been known that Ca is an essential element for plants, and that plant species differ in both the amounts of Ca they require and their tolerance of Ca in the rhizosphere. These differences between plant species not only influence the natural flora of calcareous soils but also have consequences for breeding crops to improve the delivery of Ca to the human diet. Recent research in this area has concentrated on elucidating the physiology and biochemistry underpinning contrasting Ca physiotypes and in developing a phylogenetic framework to describe the variation in [Ca] shoot within the angiosperms.
This work will be used to inform the modelling of responses of plant communities to environmental change, of nutrient and contaminant partitioning within an ecosystem, and of the delivery of Ca to the food chain. More recently, research on Ca in plants has been focused at the cellular level. Genes encoding these transport proteins have been identified, and their spatial and temporal expression patterns, subcellular locations, functional properties and physiological roles are being studied using a variety of biochemical, physiological, molecular biological and genetic techniques.
Undoubtedly, the application of functional genomics will provide further insights to the biology of Ca in plants. This will provide a unique insight into the physiology of Ca accumulation. These responses could not be predicted a priori.
In parallel, genes that impact on Ca accumulation in the shoot might be identified through the resolution of quantitative trait loci QTL detected using mapping populations that show variation in their ability to accumulate Ca in the shoot. These are available for several plant species. Such approaches could ultimately lead to the development of crops that accumulate less of contaminants, such as radiostrontium, or deliver more Ca to the food chain.
Calcium disorders in horticultural crops: a cracking in tomato fruit; b tipburn in lettuce; c calcium deficiency in celery; d blossom end rot in immature tomato fruit; e bitter pit in apples; f gold spot in tomato fruit with calcium oxalate crystals inset. Plants were grown for approx.
The relationships between root CEC, derived from a literature survey, and shoot calcium concentration, derived from both literature and experimental data, for angiosperm orders Table 1. Circles represent shoot Ca content data from a literature survey. Triangles represent shoot Ca content estimated in a phylogenetically balanced experiment.
The mean relative shoot calcium concentration of angiosperm orders derived from both a literature survey and a phylogenetically balanced experiment, and their mean relative root cation exchange capacity CEC. Data for shoot Ca concentration were taken from Broadley et al. All literature data sets were subjected to a residual maximum likelihood REML analysis, using a procedure described previously Broadley et al. Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide.
Sign In or Create an Account. Sign In. Advanced Search. Article Navigation. Close mobile search navigation Article Navigation. Volume Article Contents. Zhang et al. This resulted in an overall significant decrease of synaptic spike frequency 1. They also display comparable amplitude 6. In nca gf mutants, HSN cell bodies also displayed trains of calcium spikes similar to those in wild-type animals in their frequency 3. To identify proteins that modulate NCA activities, we performed a genetic suppressor screen for mutations that reverted locomotion defects of nca gf mutants see Materials and Methods.
We identified two extragenic suppressors that reverted nca gf coilers to fainters and fully suppressed their synaptic morphology defects Figure S1. One suppressor, hp , corresponds to unc , a gene encoding a large, novel protein [ 24 ]. Another suppressor, hp , failed to complement unc , an uncloned mutant previously isolated by its locomotion phenotype [ 32 ] and later shown to confer hypersensitivity to halothane [ 30 ].
We found that nca lf ;unc triple mutants are indistinguishable from either nca lf double mutants or unc single mutants in behavior Video S7. Furthermore, all nca gf ;unc double mutants display the same fainter phenotype as unc single mutants Video S8.