CaEGTA uncompetitively inhibits calcium activation of whorl morphogenesis inAcetabularia

Abstract

Summary.The spacing between adjacent hairs in vegetative whorls of Acetabularia acetabulum (formerly A. mediterranea) was earlier reported as being quantitatively responsive to calcium ion concentration in the culture medium. We here report a quantitative response to the concentration of the calcium-chelator EGTA, in the opposite sense to the effect of calcium. (Increasing [Ca 2+] diminishes the spacing; increasing [EGTA] increases it.) The earlier work was interpreted in terms of control of the spacing by a putative reaction-diffusion mechanism in the cell membrane, in which a receptor R was activated by calcium-binding to initiate the process. We extend this interpretation by treating CaEGTA as an uncompetitive inhibitor of the effect of calcium on R. This leads to thermodynamic constants for CaEGTA binding to the CaR complex: A/-/~ =-250 _+ 60 kJ/mol; AS~ = -820 -+ 200 J/mol 9 K. Consistency of the concentration and temperature dependences reported here with the postulated dynamic mechanism increases the probability that this mechanism is correct.

Algal morphogenesis: modelling interspecific variation inMicrasteriaswith reaction–diffusion patterned catalysis of cell surface growth

“…We suppose X to be a membrane protein, associated with vesicle fusion (as in Kiermayer & Meindl 1989). We speculate (as also Harrison et al (1997), for a similar morphogen system that may be at work in Acetabularia), that X might be some sort of autophosphorylating kinase, with A its unphosphorylated form. This protein may be Ca-binding (Kiermayer & Meindl 1989), with this binding perhaps activating the phosphorylation process (Harrison et al 1997).…”

“…We speculate (as also Harrison et al (1997), for a similar morphogen system that may be at work in Acetabularia), that X might be some sort of autophosphorylating kinase, with A its unphosphorylated form. This protein may be Ca-binding (Kiermayer & Meindl 1989), with this binding perhaps activating the phosphorylation process (Harrison et al 1997). In such a scenario, the feedback of the membrane on the genome (represented in the model by X th ) may be through some type of signal transduction pathway.…”

The role of chemical dynamics in plant morphogenesis

“…(This giant unicellular green alga, sketched in Figure 1, allowed for the study of complex morphogenesis in the absence of cellcell signalling, tissue layering, etc.). In a series of experimental papers, Harrison's group discovered a number of points suggesting that RD dynamics controlled the spacing of the hairs: temperature shifts changed spacing as expected for an RD mechanism [25,26]; altered Ca 2+ concentration and Ca 2+ inhibitors changed spacing as expected for a membrane-bound Ca 2+ -activated Turing morphogen [27,28] (thermodynamic parameters for these processes were also determined); direct imaging indicated Ca 2+ patterning in the cell membrane as expected from the RD model [29], and not in the cytoplasm [30] as suggested by other models [31]. Recently, molecular biology techniques have been used to study Turing patterning more directly in plants: a large collaborative project [32] quantitatively matched a model of the GLABRA/TRIPTYCHON reaction network underlying leaf trichome patterning (which has RD dynamics) to experimental manipulations, especially overexpression.…”

Two-stage patterning dynamics in conifer cotyledon whorl morphogenesis

“…While RD can be used to study de novo pattern formation from an unpatterned state, the more common occurrence in development is for patterns to form on prior patterns, as with the P1/P2 stages studied here. Harrison et al (1981) first proposed such hierarchical patterning for whorl formation in the alga Acetabularia, with subsequent experiments indicating that RD was involved in circumferential spacing in the whorl (Harrison et al, , 1997. Turing identified RD parameter conditions for spatial patterns to grow (amplify) from initially uniform concentration states (PAT models can be analysed similarly).…”