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

Abstract

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.

A complex case of simple leaves: indeterminate leaves co-express ARP and KNOX1 genes

“…4,5,and 9), and together with SrWUS, another meristem gene expressed in the leaves of S. rexii (Mantegazza et al 2009), it seems that the expression of several meristem genes is spatially and temporally shifted in Streptocarpus compared to Arabidopsis. However, later stages of leaf development, such as acquisition of adaxial/ abaxial polarity are not changed in Streptocarpus, and accordingly, the polarity gene FIL is expressed in S. rexii in a similar pattern as in Arabidopsis (Tononi et al 2010).…”

Loss of YABBY2-Like Gene Expression May Underlie the Evolution of the Laminar Style in Canna and Contribute to Floral Morphological Diversity in the Zingiberales

“…GRAM has also been shown to be involved in the control of floral organ initiation and identity (Navarro et al, 2004 ). TmFIL in Tropaelum majus and SrGRAM in Streptocarpus rexii , both FIL -like genes, are also expressed abaxially (Gleissberg et al, 2005 ; Tononi et al, 2010 ).…”

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

“…Jong (1970) introduced new terms to describe such unique development in Streptocarpus. In unifoliates, the macrocotyledon consists of a lamina and a 'petiolode', a stem-like petiole (Jong 1970;Tononi et al 2010). Lamina and petiolode form a structure called 'phyllomorph'.…”

“…Caulescent Streptocarpus species possess a shoot and SAM (Jong 1970;Jong & Burtt 1975;Imaichi et al 2007). A SAM or groove meristem is not present in Streptocarpus embryos (Figs 1A, 1B;Imaichi et al 2000, Mantegazza et al 2007Tononi et al 2010), or for a while after germination (Figs 1C, 1D;Nishii et al 2004Nishii et al , 2007. Its development is delayed by the activity of the basal meristem, for a short while in caulescents, longer in rosulates and completely suppressed in unifoliates (Jong 1970;Hilliard & Burtt 1971;Dubuc-Lebreux 1978).…”

“…Its development is delayed by the activity of the basal meristem, for a short while in caulescents, longer in rosulates and completely suppressed in unifoliates (Jong 1970;Hilliard & Burtt 1971;Dubuc-Lebreux 1978). Likewise, anisocotyly is not predetermined during embryogenesis, because both embryonic cotyledons are equal in size in mature seeds (Figs 1A, 1B;Imaichi et al 2000Imaichi et al , 2007Mantegazza et al 2007;Tononi et al 2010), and early on after germination (Figs 1C, 1D;Nishii et al 2004Nishii et al , 2007.…”