GRAMINIFOLIA homolog expression in Streptocarpus rexii is associated with the basal meristems in phyllomorphs, a morphological novelty in Gesneriaceae

Streptocarpus is a genus showing great variation in vegetative plant architecture and hence provides an attractive system to study the evolution of morphological diversity. Besides species showing an orthodox caulescent plant organization, producing leaves from a conventional shoot apical meristem (SAM), there are species whose body plan is composed of units (phyllomorphs) consisting of a petiole-like structure and a lamina that has the ability of continued growth. The first of these units is the macrocotyledon, derived from the continued growth of one of the two cotyledons by the activity of a basal meristem (BM), whereas further phyllomorphs develop from a SAM-like meristem. We carried out anatomical and morphological studies on the macrocotyledon of Streptocarpus rexii showing that the lamina has a bifacial structure, whereas the petiolode is partially unifacial. YABBY transcription factors are known to be involved in organ polarity and also promote lamina growth. We characterized the expression of SrGRAM, an ortholog of the YABBY genes GRAMINIFOLIA (GRAM) and FILAMENTOUS FLOWER (FIL), in S. rexii by in situ hybridization and RT-PCR. Gene expression pattern during embryogenesis was found to be conserved between SrGRAM and FIL from Arabidopsis. During subsequent seedling development SrGRAM expression in S. rexii was closely associated with the activity of the BM of the macrocotyledon and consecutively produced phyllomorphs, whereas it was excluded from the SAM-like meristem. Our results suggest that SrGRAM acts in intercalary growth and that an altered regulation of SrGRAM may underlay the evolution of the BM in S. rexii.