Hormonal crosstalk in the regulation of meristem activity and the phyllomorph architecture in Streptocarpus (Gesneriaceae): a review

Plants belonging to the family Gesneriaceae exhibit great diversity in shoot architecture. One genus within the family, Streptocarpus, encompasses species with different body plans that do not conform to the standard bauplan of angiosperms. These include features such as ‘anisocotyly’, the unequal cotyledon morphogenesis, and the ‘phyllomorph’ a leaf/shoot construct of which the development is governed by three meristems (groove meristem, petiolode meristem, basal meristem). In the extreme case, the plants only consist of one hugely enlarged cotyledon (‘unifoliate’ habit). Modification in the position and activities of the meristem are responsible for the morphological flexibility of the genus. This review summarises the interactions between hormones and developmental genes and compares these to model plants. Some mechanisms controlled by class 1 KNOX (KNOX1) genes appear to be conserved between plants with ordinary shoots and the Streptocarpus phyllomorph, while others have diverged. In particular, cytokinins and gibberellins appear to be important for meristem regulation to establish anisocotyly and the development of phyllomorphs in Streptocarpus through KNOX1 regulation. This is supported by expression patterns from hormone metabolism genes. The establishment of anisocotyly is based on an imbalance between cytokinins and gibberellins, causing a shift from apical to lateral dominance that involves the suppression of the microcotyledon and groove meristem and promotion of the basal meristem. We point out future perspectives in the study of Streptocarpus organogenesis. Streptocarpusmay provide a system to study the functional evolution of plant form in relation to adaptation to diverse environmental conditions.