Is membrane potential involved in calmodulin gene expression after external stimulation in plants?

In Bidens pilosa (cv. radiata), a non-injurious stimulus induces a local and transient change in membrane potential, and an injurious stimulus induces a transmitted electrical signal described as the combination of an action potential and a slow wave. We have studied calmodulin gene expression after these stimuli. When the stimulus is non-injurious, calmodulin mRNA accumulation is only increased in the stimulated region. In contrast, when the stimulus is injurious, mRNA accumulation takes place in both wounded and distant, unwounded tissue. We propose that the slow wave plays a role in the long-distance transmission of a wound-induced information in plants.

Control of a symmetry-breaking process in the course of the morphogenesis of plantlets of Bidens pilosa L.

A mechanism involving transport, storage and retrieval of a symmetry-breaking message controls the relative growth rate of the cotyledonary buds of plantlets of Bidens pilosa L. The asymmetry was induced by administering a few needle pricks to one cotyledon of each plant. The storage of the symmetry-breaking message was independent of the number of pricks ("all or nothing" process) and irreversible. However, various treatments could render the plants either able to retrieve the stored symmetry-breaking message (in which case, the bud opposite to the pricked cotyledon began to elongate statistically sooner than the one associated with the stimulated cotyledon) or not (both buds then had an equal chance to be the first to start to grow). The retrieval process was also associated with a temporal oscillation. At the level of the whole plants, bud growth was observed only after the removal of apical dominance, and its degree of asymmetry was expressed by use of a parameter g ranging from zero (symmetrical case) to ± 1 (full asymmetry in favor of one of the cotyledonary buds). The highest g-values observed in the present contribution were of the order of 0.5. At the cellular level, the pricking of one cotyledon caused a number of cells, which were within the meristem of the bud associated with the pricked cotyledon and were in cell-cycle phases S or G2, to undergo cellular division and then be blocked in phase G1, whereas the cells of the opposite bud were practically unchanged.

[The role of mineral ions in controlling morphogenesis in plants: a case of the inhibition of growth of the hypocotyle of Bidens pilosa L]. / Intervention d'ions minéraux dans le contrôle de la morphogenèse des végétaux: cas de l'inhibition de croissance de l'hypocotyle de Bidens pilosa L.

Mineral ions are implicated in various events occurring in the transduction of messages in plants, from the reception of the initial signal to the final morphogenetic expression. Ions may be involved also in the possible migration, storage and retrieval of the message. In a number of cases, cellular exchange of Ca2+ have been shown to occur at the reception of a signal. This is the reason why Ca2+ has often been considered as a "second messenger". For the possible message migration, cellular exchanger of Cl-, K+, H+ and Ca2+ are involved in the propagation of a wave of electric depolarization. The mechanisms underlying the possible storage of the message in a plant are still not clearly understood. However, ions such as K+, Na+ and Ca2+ interfere with the retrieval of the stored information. There are some indications that protons are involved in the metabolic reactions responsible for the final morphogenetic expression of the original signal. Moreover, the addition of Li+ ions inhibits, or shifts the latter effects, while the action of Li+ is counterbalanced by increasing the concentration of K+. The particular case of the inhibition of the growth of Bidens hypocotyles following delivering a few needle pricks to the cotyledons has been examined in more details.

Memorization and delayed expression of regulatory messages in plants.

Plantlets of Bidens pilosus L., considered to be basically symmetrical, can be "lateralized" (A/B) by being administered a "symmetry-breaking" signal such as puncturing one of the plant cotyledons. The induced asymmetry remains latent as long as the plants have not been made "permissive", i.e. as long as the plant apex is left functioning. When the apex has been removed (plant "decapitation"), the latent asymmetry is expressed by one of the cotyledonary buds (a/b) statistically beginning to elongate before the other. The interval of time between delivering the "symmetry-breaking" signal and making the plant "permissive" is the "memorization-time", Δt. Memorization can be quantified by using a "precedence index", q, the values of which range from 0 (no detectable asymmetry with regard to bud growth) to ±1 (bud growth perfectly asymmetric in favour of either bud b or a). Even for memorization times, Δt, up to 14 d, q-values up to 0.4 (or even larger) are observed. Various experimental characteristics (e.g. light, temperature, presence or absence of the root system) but not the plant age can affect the q-values, at the moment when the treatments are performed, at least in the range of 6 to 25 d. Combining several puncturing treatments either increases or decreases the q-values, depending on the nature of these treatments and the time-intervals, δt, between them. Symmetrically removing both cotyledons in the minutes following the puncturing of one of them does not significantly alter the results, which means that the "symmetry-breaking" message is rapidly transported and memorized within the plant. Non-traumatic asymmetrical treatments (droplets of saline solutions, light-gradients) can also act as symmetry-breaking signals and be memorized. Plants other than Bidens are likely to possess similar memorization ability, although the q-values observed up to now have not been very large.

Effect of Lithium on Thigmomorphogenesis in Bryonia dioica Ethylene Production and Sensitivity.

Rubbing internodes of Bryonia dioica plants reduced their ethylene production but increased their capacity to convert 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene. These results were explained by the previously shown rubbing-induced decrease of indoleacetic acid, which controls the level of ACC synthase, and by the increase of membrane-associated peroxidases which would participate in the conversion of ACC-ethylene. Pretreatment of the plants with Li had no significant effect on control plants but counteracted the rubbing-induced decrease of ethylene production and diminished the capacity of the internodes to convert ACC to ethylene. Exogenously applied ethylene induced an increase of peroxidase activity similar to that caused by rubbing. Inasmuch as both effects were reduced by Li, it was concluded that Li inhibition of thigmomorphogenetic processes was essentially due to a Li inhibition of the effect of ethylene formed in response to mechanical stimuli. The decreased ethylene production and ACC conversion capacity in the presence of Li were explained by a cellular redistribution of peroxidases.

Hypocotyl Growth and Peroxidases of Bidens pilosus: EFFECT OF COTYLEDONARY PRICKINGS AND LITHIUM PRETREATMENT.

Pricking one cotyledon of young Bidens pilosus plants induces rapid inhibition of hypocotyl growth, essentially in its middle portion. Analysis of soluble peroxidases indicates rapid changes (increase of activity) in basic isoenzymes followed by more progressive enhancement of the acidic ones. Pretreatment of the plants with lithium prevents the inhibition of elongation due to pricking as well as the peroxidase changes. The phenomenon is similar to the previously described thigmomorphogenetic process in Bryonia dioica.

[Growth rate of the axillary buds and variation of the type of correlation between the cotyledon and its axillary bud in Bidens pilosus L]. / Vitesse de croissance des bourgeons axillaires et variation du sens des corrélations entre le cotylédon et son bourgeon axillaire chez Bidens pilosus L.

Application of various growth substances to seedlings of Bidens with an inhibitory cotyledon causes a cotyledonary correlative stimulation. At the same time, if we measure the influence of these substances on the growth rate of the buds during the experiment, we see that the cotyledonary correlative stimulation is always associated with a decrease of this growth rate. If the control had an indifferent or stimulating cotyledon, the application of the same various substances either causes an increase of this cotyledonary correlative stimulation or brings about a cotyledonary correlative inhibition. In the first case a decrease of the growth rate is always noted, while the cotyledonary correlative inhibition is associated with an increase in the growth rate.