Rooting experiments carried out with isolated primary leaves of dwarf bean demonstrated the effect of GA on increasing the IAA level and IAA synthesis. Pretreatment of the lamina of isolated leaves with GA, especially when tryptophane was added at the same time, strikingly increased the rooting of petioles. Enhanced root formation on the petiole of GA-pretreated leaves can be attributed to increase of IAA level by GA in the lamina, i.e. in IAA biosynthesis the utilization of tryptophane released from the proteins of isolated leaves is promoted by GA. Application of TIBA on the upper part of petiole suppressed the stimulatory effect of GA on root initiation presumably because TIBA inhibits the transport of IAA into the petiole, which results in failure of rooting. Pretreatment of the leaves with cytokinins also results in failure of root production. These substances retard protein decomposition in isolated leaves so that the quantity of TPP available for auxin synthesis is less. Simultaneously, this treatment increases the cytokinin concentration in the leaf tissues so that the auxin/cytokinin ratio alters and mainly callus forms on the base of the petioles and differentiation of root primordia cannot take place. INTRODUCTION We have previously demonstrated that gibberellic acid (GA) treatment of bean hypocotyl tissue raises the level of endogenous indol-3yl-acetic acid (IAA). In the GA-treated stem tissues in vivo , the concentration of both the free and bound IAA increased considerably (Varga and Bito, 1968; Varga, Koves and Sirokman, 1968). According to these results, the increases in IAA arises from biosynthesis of auxin from tryptophane (TPP) rather than by decreasing auxin destruction (Varga et al ., 1968; Varga and Bito, 1967). We have also demonstrated the stimulation of the conversion of TPP into IAA by GA in vitro , using cell-free bean plant tissue extracts (Varga, 1974). It seems therefore that the utilization of the precursor in the IAA biosynthesis is increased by GA. Simple rooting experiments with isolated primary leaves of bean (Phaseolus vulgaris ) support the idea that GA increases the IAA level of IAA synthesis. It is known that locally higher IAA concentrations induce adventitious root formation on vrrious plant parts. The root-initiating effect of higher IAA levels related to the quantity of other phytohormones, especially cytokinins, plays an essential role in the phenomenon of differentiation and organization. Petioles of isolated primary leaves of young bean plants, under suitable circumstances, form roots easily without adding exogenous auxin. Rooting can be attributed to the higher local concentration of IAA formed in the leaf lamina and translocated basipetally into the petiole (Humphries, 1963; Humphries and Thorne, 1964; Humphries and Wheeler, 1964; Humphries, 1966). According to their observations on detached primary bean 1 Present address : Thrapston Kettering, Northants, England. This content downloaded from 207.46.13.51 on Sun, 19 Jun 2016 06:56:50 UTC All use subject to http://about.jstor.org/terms 8o4 Varga and Humphries Root Formation on Petioles of Dwarf Bean leaves, decomposition of the proteins to amino acids, including TPP, increases. TPP is therefore directly or with contribution of phenolic compounds (Wheeler and King, 1968) re-utilized in IAA biosynthesis. The amount of IAA translocated into the petiole is fairly exactly expressed by the number and mass of adventitious roots. According to Humphries and Thorne (1964) and Humphries (1966) all the effects that retard IAA synthesis in the lamina, simultaneously inhibit formation of roots of the petiole. Using the facts mentioned above we devised the following scheme. Since GA increases IAA synthesis it follows that if the lamina is treated with GA more IAA migrates into the petiole and root formation increases. On the other hand, 2,3,5-triiodobenzoic acid (TIBA) blocks basipetal auxin transport (Kuse, 1953; Niedergang-Kamien and Skoog, 1956; Hay, 1956; Christie and Leopold, 1965, etc), so that treating the petioles with TIBA inhibits rooting despite the stimulatory effect of GA. Similarly root formation is inhibited by cytokinins because they retard senescence and protein destruction in isolated leaves; moreover, they have an effect of net synthesis of protein (Richmond and Lang, 1957; Mothes, Engelbrecht, and Kulajeva, 1959; Mothes, i960; Parthier, 1961; Osborne, 1962, 1965; and others). Consequently, cytokinins hinder the release of TPP from proteins and its utilization in IAA synthesis.