Janine Bonmort

Early events induced by chitosan on plant cells

Chitosan (a polymer of beta-1,4-glucosamine residues) is a deacetylated derivative of chitin which presents antifungal properties and acts as a potent elicitor of plant resistance against fungal pathogens. Attention was focused in this study on the chitosan-induced early events in the elicitation chain. Thus, it was shown that chitosan triggered in a dose-dependent manner rapid membrane transient depolarization of Mimosa pudica motor cells and, correlatively, a transient rise of pH in the incubation medium of pulvinar tissues. By using plasma membrane vesicles (PMVs), it was specified that a primary site of action of the compound is the plasma membrane H(+)-ATPase as shown by its inhibitory effect on the proton pumping and the catalytic activity of the enzyme up to 250 microg ml(-1). As a consequence, chitosan treatment modified H(+)-mediated processes, in particular it inhibited the uptake of the H(+)-substrate co-transported sucrose and valine, and inhibited the light-induced H(+)/K(+)-mediated turgor reaction of motor cells. The present data also allowed the limit of the cytotoxicity of the compound to be established close to a concentration of 100 microg ml(-1) at the plasma membrane level. As a consequence, chitosan could be preferably used in plant disease control as a powerful elicitor rather than a direct antifungal agent.

PHYTOCHROME‐ and BLUE PIGMENT‐MEDIATED LEAF MOVEMENTS: EXPERIMENTAL APPROACH IN THE SIGNAL TRANSDUCTION

Abstract— Leaflet movements of Cassia fasciculata are induced by transferring leaves from light to darkness or from darkness to light. Phytochrome mediates the dark‐induced closure whereas a blue and far red light absorbing pigment (cryptochrome?) is the photoreceptor triggering the light‐induced opening. These movements are the result of reversible turgor variation driven by ionic migrations (H+, K+, Cl−) in cortical parenchyma cells of motor organs (“pulvini”) localized at the leaflet base. Calcium plays a predominant role in the regulation of the movements as shown by the inhibitory effects of chelators (EDTA, EGTA), intracellular antagonist TMB‐8 and by the promoting effect of ionophore A 23187. Compounds known as calcium channel blockers (LaCl3, verapamil and nifedipine) inhibited whereas Bay K 8644, a calcium channel activator, promoted the phytochrome‐mediated movement. In contrast, all these calcium channel modulators had no effect on the blue pigment‐mediated movement. From these results, it is suggested that calcium is not mobilized in the same manner in the two types of movements: possibly from external stores in the phytochrome‐mediated response and from internal stores in the blue pigment‐mediated response. Calcium acts possibly through calmodulin as suggested by a modification in the kinetics of the movements induced by inhibitors of calmodulin action (trifluoperazine, R 24571, W‐7). The unexpected promotion of the movements by these inhibitors shows that calmodulin action on the ion migrations is not simple and direct. Experimental observations suggested that regulation might be done through cAMP metabolism. db‐cAMP promoted the movements. Compounds known either to activate adenylate cyclase (prostaglandins, forskolin) or to inhibit phosphodiesterase (imidazolidinones, ICI 58301) induced the same modifications as db‐cAMP. By contrast, a phosphodiesterase activator (imidazole) inhibited the movements.

Effects of prostaglandins E, precursors and some inhibitors of prostaglandin biosynthesis on dark- and light-induced leaflet movements in Cassia fasciculata Michx.

Prostaglandin E1 and prostaglandin E2 speed up the dark-induced (scotonastic) and light-induced (photonastic) leaflet movements of Cassia fasciculata. The precursors of prostaglandin biosynthesis, homo γ-linolenic and arachidonic acids, and an intermediary product, prostaglandin-interm-5, act in the same manner on these movements. Inhibitors of prostaglandin biosynthesis, indomethacin and phenylbutazone, inhibited the scotonastic but promoted the photonastic movements in an unexpected way. Since the pulvinar movements are mediated by water and ion migrations, the observed modifications of these movements indicate that prostaglandins and their precursors may affect, as in animal cells, processes linked to a variation of membrane permeability.

Effect of 2,4-dichlorophenoxyacetic acid on the dark- and light-induced pulvinar movements in Cassia fasciculata Michx

Abstract2,4-dichlorophenoxyacetic (2,4-D) applied to excised leaves of Cassia fasciculata modified the dark-induced (scotonasty) and light-induced (photonasty) leaflet movements, showing that this compound acts on rapid turgor variation and the concomitant ion migrations, in particular K+. 2,4-D inhibited the scotonastic closure in a dose-dependent manner from 10−8M to 10−5M and promoted the photonastic opening in the same range of concentrations. The compound acted rapidly since a treatment as short as 5 min gave an obvious effect on the motile reaction; however, a lag period of 45–60 min was needed to observe its effect. Although 2,4-D is a weak acid, its greatest physiological efficiency was obtained with pH values close to neutrality. The physiological results are discussed in relation to the chemical properties and the characteristics of transport of the molecule.

Effects of Methylxanthines on Seismonastic Reaction, Proton Flux, Membrane Potential, Respiration and Calcium Uptake in Motor Cells of Mimosa pudica Pulvinus

Summary Methylxanthines (theophylline, aminophylline, caffeine) inhibit the seismonastic reaction of primary pulvini of Mimosa pudica leaves, showing that these compounds act on rapid turgor variations and the concomitant ion migrations. Compared with theophylline, aminophylline is more effective and caffeine is less effective in triggering the inhibitory effect. These compounds alter H + -ATPase activity as a rapid dose-dependent proton influx was noted about 4 min after application, indicating that an early impact may be at the plasmalemma. However, they have almost no effect on the motor cell membrane potential. Moreover, the compounds hinder the calcium transport. Not all of these inhibitory effects are mediated through an action on general cell metabolism since respiration was not affected even by rather long treatment (about 2 h).

Inhibition of Phytochrome- and Blue Light Absorbing Pigment-mediated Pulvinar Movements in Cassia fasciculata by Xanthine Derivatives

Summary Theophylline and aminophylline, applied on excised leaves of Cassia fasciculata inhibited the phytochrome-mediated movements induced by a light-off signal and the blue light absorbing pigment-mediated movements induced by a light-on signal. Biological activity was given at the xanthine molecule by methyl groups substituted in positions 1 and 3. Aminophylline presented a better efficiency in triggering the inhibitory effect, probably due to its higher solubility. These methylxanthines acted without destroying the phytochrome effect for the retarded and reduced response. Theophylline acted very rapidly since treatment as short as 5 min gave a very obvious inhibition of the movement. In both types of movements the inhibitory action was not modified by addition of KCl but was partially reversed by CaCl 2 , suggesting that a major influence of methylxanthine on cellular turgor changes is mediated by the effect on Ca 2+ mobilization.