Inhibition of brassinosteroid‐induced epinasty in tomato plants by aminooxyacetic acid and Co²⁺

The inhibitory effects of aminooxyacetic acid (AOA) and cobalt chloride (CoCl₂) on brassinosteroid (BR)‐induced epinasty in tomato plants (Lycopersicon esculentum Mill. cv. Heinz 1350) are evaluated. CoCl₂ dramatically decreases petiole bending and ethylene production as the concentration increases from 50 to 200 μM. The content of 1‐aminocyclopropane‐1‐carboxylic acid (ACC) in the petiole, instead of accumulating, is reduced and does not change over the concentration range tested. Inhibition of BR‐induced epinasty by AOA results from inhibition of ACC synthesis. There are dramatic reductions in petiole bending, ethylene and ACC production as the concentration of AOA is increased from 50 to 200 μM. Maximum inhibition occurs when the plants are pretreated with the inhibitors. The degree of inhibition increases as the length of pretreatment increases from 1 to 4 h. The response of BR‐treated plants to AOA and CoCl₂ is similar to the effect of auxin, indicating the integral relationship between BR and auxin.