Involvement of Ethylene in Chromosaponin-Induced Stimulation of Growth in Lettuce Roots

To elucidate the mode of action of chromosaponin I (CSI) in stimulating the growth of lettuce roots (Lactuca sativa L. cv. Grand Rapids), the possible involvement of ethylene was examined. Lettuce seedlings evolved ethylene at a rate of 0.7 nl 10 seeds-1 h-1. The growth of lettuce roots was stimulated by 2-aminoethoxyvinyl-glycine (AVG), an inhibitor of ethylene synthesis, and 2,5-norbornadiene (NBD), an inhibitor of ethylene action, as well as by CSI. In contrast to ethylene, treatments with CSI, AVG and NBD promoted longitudinal elongation of cortical cells of roots and inhibited their lateral expansion. Application of CSI slightly reduced ethylene production from lettuce, but this reduction was not sufficient to account for the CSI-induced stimulation of growth. The maximal promotive effects of AVG and NBD were obtained at 3 micromolar and 150 microliters, respectively. The growth promotion by CSI disappeared in the presence of the optimum levels of AVG or NBD; a further addition of ethylene caused the stimulatory effects of CSI to increase, depending on the concentration of ethylene. Thus, CSI reduced both the sensitivity of the roots to ethylene and the maximal effects of ethylene. The CSI-induced stimulation of growth was ascribed to the reduction of the response to ethylene in the lettuce roots.