Calcium Variously Mediates the Effect of Cytokinin on Chlorophyll and LHCPII Accumulation During Greening in Barley Leaves and Cucumber Cotyledons

Abstract

During greening, excised etiolated barley leaves and cucumber cotyledons that were depleted of exogenous Ca 2+ by a chelating agent (ethylene glycol-bis (beta aminoethyl ether)-N,N,N`N`-tetraacetic acid, EGTA) showed ~50% reduced chlorophyll (Chl) accumulation and ~30% accumulation of apoprotein of the light-harvesting chloro- phyll a/b-binding protein complex of photosystem II (LHCPII). The Ca 2+ channel blocker lanthanum chloride (LaCl 3 ) applied to cucumber cotyledons reduced LHCPII accumulation more than EGTA did. In both plant mate- rials, cytokinins enhanced chlorophyll accumulation by 50–60% and this effect was completely canceled by EGTA application. Hormones significantly increased LHCPII accumulation but EGTA application reduced that effect in barley leaves by ~30% and in cucumber cotyledons by ~80%. A similar effect was observed in LaCl 3 treated cotyledons. CaCl 2 application boosted chlorophyll accumulation in both plant materials. CaCl 2 applied together with cytokinin reduced the hormonal effect on chlorophyll accumulation by ~38% in barley leaves and 23% in cucumber cotyledons, but almost totally inhibited cytokinin-stimulated LHCPII accumulation. Our results indicate that calcium variously mediates the effect of cytokinin on chlorophyll and LHCPII accumulation. Cytokinin-induced enhancement of chlorophyll accumulation seems totally dependent on the exogenous pool of Ca 2+ , while Ca 2+ -dependent and Ca 2+ -independent pathways are involved in the hormonal effect on LHCPII accumulation. The effect of cytokinin on the increase of light-induced LHCPII accumulation appears to be sensitive to exogenously applied Ca 2+ , which almost totally blocked the hormonal effect. Our results give indirect evidence that the responses to cytokinin and light act on different events leading to Chl and LHCPII accumulation.