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

Differential growth of the nodal regions of graviresponding Tradescantia fluminensis (Wandering Jew) was analysed with special respect to the extension-restricting epidermal cells of the opposite growing and growth-inhibited organ flanks. Gravicurvature of horizontally gravistimulated isolated nodes depends on auxin (indolyl-3-acetic acid, IAA) and shows a node-specific profile in which the third node below the tip showed the greatest response. Exogenously supplied gibberellic acid induced no gravitropic growth. Vertically oriented isolated nodes supplied with exogenous IAA showed, on an electron microscopical level, conspicuous membrane invaginations with adjacent wall depositions restricted to the outer tangential epidermal cell walls. Their number was more than doubled by exogenously supplied Ca2+, which inhibited IAA-induced growth. No such changes could be detected in water-incubated segments or inner tissues of IAA-supplied segments. Gravistimulated differential growth of nodes of intact shoots and of nodal segments was characterized by changes similar to the ones induced by exogenous IAA, with greatly increased numbers of wall depositions within the epidermal cells of the growth-inhibited upper organ flank. Similar to the gravistimulated wall depositions, an asymmetric distribution pattern of Ca2+ was detected in the epidermal cell walls employing x-ray energy spectrum analysis (EDX). The results indicate that growth of nodes of Tradescantia fluminensis is regulated via IAA-induced secretion and subsequent infiltration of wall components enabling wall extension. The data support the hypothesis that temporary differential growth during gravicurvature of Tradescantia fluminensis is mediated by the antagonistic effect of Ca(2+)-ions on the infiltration of IAA-induced wall-loosening components into the outer, extension-restricting epidermal walls thereby inhibiting growth.