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

Abstract

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.

One‐leaf plants in the Gesneriaceae: Natural mutants of the typical shoot system

“…Contrary to the effect of cytokinin, exogeneous application of GA to Streptocarpus (both unifoliate and rosulate species) inhibits the growth of cotyledons, and thus the size of the two cotyledons remains the same (Nishii et al., ; Rosenblum & Basile, ). In typical plants, it is known that accumulation of GA is spatially restricted to the incipient lateral organ primordia due to the spatially limited expression of the GA‐biosynthesis gene, GA 20‐oxidase ( GA20ox ), in the differentiating primordia.…”

“…When GA is exogenously applied to unifoliate species of Streptocarpus before the emergence of anisocotyly, new organs are produced between the two cotyledons, suggesting activation of potential GM function (Nishii et al., ; Rosenblum & Basile, ). Tsukaya () proposed that competitive organogenesis between meristems occurs in the one‐leaf plant M. horsfieldii so that the growth of one cotyledon can repress the growth of other organs.…”

“…For example, in both Monophyllaea and Streptocarpus, the application of exogeneous cytokinin causes both of the cotyledons to grow (Mantegazza et al, 2007(Mantegazza et al, , 2009Nishii, Kuwabara, & Nagata, 2004;Nishii, Wang, Spada, Nagata, & Möller, 2012;Rosenblum & Basile, 1984). This result can be interpreted as the activation of BMs in both cotyledons.…”

“…Contrary to the effect of cytokinin, exogeneous application of GA to Streptocarpus (both unifoliate and rosulate species) inhibits the growth of cotyledons, and thus the size of the two cotyledons remains the same (Nishii et al, 2012;Rosenblum & Basile, 1984).…”

“…It is notable that GA20ox is negatively regulated, and GA2ox is positively regulated by KNOX I genes in model plants (Jasinski et al, 2005;Sakamoto et al, 2001;Hay et al, 2002;Chen et al, 2004;reviewed in Shani et al, 2006). When GA is exogenously applied to unifoliate species of Streptocarpus before the emergence of anisocotyly, new organs are produced between the two cotyledons, suggesting activation of potential GM function (Nishii et al, 2012;Rosenblum & Basile, 1984). Tsukaya (1997) proposed that competitive organogenesis between meristems occurs in the one-leaf plant M. horsfieldii so that the growth of one cotyledon can repress the growth of other organs.…”