Gibberellin as a suppressor of lateral dominance and inducer of apical growth in the unifoliate Streptocarpus wendlandii (Gesneriaceae)

We report on the effects of exogenously applied hormones on the lateral and apical dominance that governs morphogenesis in the unifoliate Streptocarpus wendlandii. In this phenotype, lateral dominance is extreme as the plants only retain a macrocotyledon that develops into a leaf-like phyllomorph by means of a basal meristem and do not show apical growth. Gibberellin applications suppressed the basal meristem activity of the macrocotyledon resulting in an isocotylous seedling with two microcotyledons and caused the formation of a primary phyllomorph, which suggests that the groove meristem, a shoot apical meristem equivalent, is released from apical suppression by the basal meristem. Interestingly, uniconazol, a gibberellin biosynthesis inhibitor, also caused a reduction in basal meristem activity, but without primary phyllomorph formation, suggesting that some gibberellin is required for proper function of the basal meristem. Co-application of gibberellin and cytokinin resulted in two macrocotyledons also without phyllomorph formation, which is similar to previous results for cytokinin-only applications. Thus, cytokinin may act downstream in the regulatory pathway of the basal meristem. Our results suggest that the balance between gibberellin and cytokinin in the cotyledons appears thus as key factor in the regulation of lateral and apical dominance in Streptocarpus. Their interplay may well be the primary explanation for the great diversity in growth form exhibited in species of this genus. Our work shows that small imbalances of hormones in early stages of plant development can have major effects on the final phenotype.