Polyamines and pectins. II. Modulation of pectic-signal transduction.

A previous study had shown that polyamines adsorb selectively on plant cell walls according to the valence of the polyamine (Messiaen et al. 1997, Plant Physiol. 113: 387-395). In this study, the adsorption of polyamines onto isolated carrot cell walls and onto pure polygalacturonic acid was investigated in the presence of competing mono- and divalent cations (Na+ and Ca2+). Putrescine (Put2+) was unable to remove all the calcium (Ca2+) from cell walls or from polygalacturonic acid. Spermidine (Spd3+) and spermine (Spm4+) adsorbed on all galacturonates and were able to remove Ca2+ completely from both the walls and the pure polygalacturonates. Therefore, Spd3+ and Spm4+, unlike Put2+, prevented polygalacturonic acid from adopting the Ca(2+)-induced supramolecular conformation recognized by the 2F4 anti-pectin monoclonal antibody. We show that the signal transduction cascade otherwise initiated in plant cells by Ca(2+)-bound alpha-1,4-oligogalacturonides was indeed blocked by both Spd3+ and Spm4+, but not by Put2+. The mobilization of cytosolic free Ca2+ and the cytosolic acidification usually observed after treatment with pectic fragments did not occur and the subsequent activation of phenylalanine ammonia-lyase was suppressed. It is hypothesized that the disruption by Spd3+ and Spm4+ of the Ca(2+)-induced supramolecular conformation of pectic fragments was the cause of the inhibition of the pectic signal. We conclude that polyamines can act on plant cell physiology by modulating the transduction of the pectic signal.

Polyamines and Pectins (I. Ion Exchange and Selectivity).

The ion-binding and -exchange properties of putrescine, spermidine, and spermine on purified walls of carrot (Daucus carota L.) cell suspensions were investigated by producing ion-exchange isotherms and comparing them with the behavior of Na+, Mg2+, and Ca2+. The cation exchange capacity of the carrot cell walls was 0.8 equivalent kg-1 dry matter, and the ionic selectivity sequence of the walls for polyamines followed the sequence spermine4+ > spermidine3+ [almost equal to] Ca2+ > putrescine2+. The polyamines were subjected to only electroselectivity and probably did not induce any favorable supramolecular conformation of pectin like the one induced by Ca2+. Triangular ion exchanges were also performed with three diamines: ethanediamine, butanediamine, and octanediamine. The shorter the diamine, the higher the total adsorption and selectivity of the exchange. The lower selectivity of the cell wall for putrescine was partly attributed to its inability to access and displace Ca2+ from higher affinity sites within dimerized pectic sequences. The polyamine adsorption and exchange on pectic sequences could result in pectic signal modulation in pathogenesis and in differentiation.

Callose synthesis in spirostanol treated carrot cells is not triggered by cytosolic calcium, cytosolic pH or membrane potential changes.

Carrot (Daucus carota L.) cell suspensions were treated with a spirostanol saponin from Yucca. This saponin is an elicitor of callose synthesis. Irrespectively of the mode of action of spirostanol on the callose synthase activity itself, the spirostanol-induced callose synthesis in carrot is not preceded by changes in membrane potential, cytosolic free calcium or cytosolic pH. The inability of modulators of cytosolic free calcium content (verapamil, nifedipine and Br-A23187), EGTA and a proton pump inhibitor (vandate) to inhibit or induce callose formation is consistent with a calcium- and pH-independent mechanism for callose deposition.