Auxin-induced delay of abscission: The involvement of calcium ions and protein phosphorylation in bean explants

Treatment with Ca²⁺ channel blockers such as lanthanum chloride and verapamil promoted abscission in pulvinar explants of bean ( Phaseolus vulgaris L. ev. Pirate). In addition, auxin-induced delay of abscission was markedly reduced in the presence of Ca²⁺ channel blockers. In vitro phosphorylation studies were performed using membrane preparations (130000 g pellet) from freshly excised as well as auxintreated and control (minus auxin) pulvinar sections. Auxin-treated sections showed a 66 kDa phosphoprotein as well as Ca²⁺ -dependent phosphorylation that were not observed in control explants.
Coomassie blue stammg of soluble proteins (130000 g supernatent) separated on SDS-PAGE revealed the presence of 62. 55 and 47 kDa polypeptides only in the freshly excised pulvini. However. no distinct changes were observed in soluble protein profile between auxin-treated and control explants. When soluble proteins were phosphorylated in vitro, Ca²⁺ promoted the phosphorylation of 92, 55. 40 and I7 kDa polypeptides only in freshly excised pulvmi. Ca²⁺-dependent phosphorylation of soluble proteins was not observed in either the control or auxin-treated explants. In addition. in vivo phosphorylation studies were performed using freshly excised. auxrn-treated and control explants. Freshly excused segments and auxin-treated ex-plants showed similar phosphoproteins, which were different from those observed mcontrol.