Theobroxide inhibits stem elongation in Pharbitis nil by regulating jasmonic acid and gibberellin biosynthesis

In this study, exogenous factors affecting the elongation growth in the short day plant, Pharbitis nil, was investigated. Theobroxide inhibited stem elongation in P. nil both under short day (SD) and long day (LD) conditions. Salicylhydroxamic acid (SHAM), an inhibitor of jasmonic acid (JA) biosynthesis, and GA3 recovered the inhibitory effect of theobroxide on stem elongation. Quantitative analysis of JA showed that the level of endogenous JA increased significantly in theobroxide treated plants, while exogenously applied GA3 and SHAM suppressed JA biosynthesis stimulated by theobroxide. The activity of lipoxygenase (LOX, the key enzyme of JA biosynthesis) also was stimulated by theobroxide and this stimulation was nullified by SHAM and GA3. Quantitative analysis of GA1 showed that theobroxide suppressed GA1 biosynthesis. In non theobroxide treated P. nil, SD conditions stimulated JA biosynthesis and LOX activity, while GA1 biosynthesis was suppressed. All these results suggest that JA probably is involved negatively in the control of stem elongation, and the balance between JA and gibberellin might determine the stem growth in P. nil.