Auxin-dependent cell wall depositions in the epidermal periplasmic space of graviresponding nodes of Tradescantia fluminensis

Differential growth of the nodal regions of gravires-ponding Tradescantia f luminensis (Wandering Jew) was analysed with special respect to the extensi on-restricting epidermal cells of the opposite growing and growth-inhibited organ flanks. Gravicurvature of horizontally gravistimulated isolated node s depends on auxin (indolyl-3-acetic acid, IAA) and shows a node-specific p rofile in which the third node below the tip showed the greatest response. Exogenously supplied gibberellic acid induced no gravitropic growth. Vertic ally oriented isolated nodes supplied with exogenous IAA showed, on an elec tron microscopical level, conspicuous membrane invaginations with adjacent wall depositions restricted to the outer tangential epidermal cell walls. T heir number was more than doubled by exogenously supplied Ca2+, which inhib ited IAA-induced growth. No such changes could be detected in water-incubat ed segments or inner tissues of IAA-supplied segments. Gravistimulated diff erential growth of nodes of intact shoots and of nodal segments was charact erized by changes similar to the ones induced by exogenous IAA, with greatl y increased numbers of wall depositions within the epidermal cells of the g rowth-inhibited upper organ flank. Similar to the gravistimulated wall depo sitions, an asymmetric distribution pattern of Ca2+ was detected in the epi dermal cell walls employing x-ray energy spectrum analysis (EDX). The resul ts indicate that growth of nodes of Tradescantia fluminensis is regulated v ia IAA-induced secretion and subsequent infiltration of wall components ena bling wall extension. The data support the hypothesis that temporary differ ential growth during gravicurvature of Tradescantia fluminensis is mediated by the antagonistic effect of Ca2+-ions on the infiltration of IAA-induced wall-loosening components into the outer, extension-restricting epidermal walls thereby inhibiting growth.