Hormonal Regulation of Morphogenesis in Streptocarpus and Its Relevance to Evolutionary History of the Gesneriaceae

Abstract

Two morphogenetic patterns have contributed to phylogenetic diversification within the Gesneriaceae: accrescence of one of the paired cotyledons (anisocotyly), which serves to differentiate the subfamily Cyrtandroideae; sustained growth of the accrescent cotyledon accompanied by prolonged suppression and displacement of the embryonic apical meristem, which gives rise to an acaulescent, dorsiventral vegetative plant body (phyllomorph) and further serves to differentiate species of Cyrtandroideae found in two tribes and several genera including Streptocarpus. It was possible to prevent cotyledonary accrescence and induce caulescence at will, either by supplying exogenous GA3 or inhibiting auxin transport in species of Streptocarpus that normally manifest an extreme, phyllomorphic morphology. It was also possible to induce sustained, phyllomorphic development of cotyledons that are normally non‐accrescent with exogenous cytokinin. Therefore morphogenetic capacities previously thought to be “lost” or “lacking” in subgenus Streptocarpus and, with respect to isocotyly, the tribe Cyrtandroideae, are, in fact, present but suppressed. An hypothesis regarding the role of hormones with respect to morphogenesis and phylogeny of Streptocarpus is suggested.

WUS and STM homologs are linked to the expression of lateral dominance in the acaulescent Streptocarpus rexii (Gesneriaceae)

“…In accordance with Rosenblum and Basile's (1984) observations, our results demonstrate that the exogenous hormone applications are able to change the morphogenetic pattern in S. rexii. In particular, changes in the endogenous hormone balance by direct GA 3 or CK administration, or by inhibiting gibberellin biosynthesis (via PAC) were able to interfere with the cotyledonary accrescence, inducing isocotyly (regardless of the size of the cotyledons; GA !…”

“…The development of the phyllomorph, a speciWc term introduced for leaves in Streptocarpus (Jong and Burtt 1975), is governed by three meristems in S. rexii: the basal meristem (BM) and the petiolode meristem (PM), both located at the proximal end of the lamina and responsible for the expansion growth of the phyllomorph, and the groove meristem (GM) positioned on the petiolode near the base of the lamina, from which further phyllomorphs are produced (Jong and Burtt 1975;Mantegazza et al 2007;Nishii and Nagata 2007). Previous studies showed how the morphology of Streptocarpus species can be modulated experimentally by exogenous application of hormones (Rosenblum and Basile 1984;Nishii and Nagata 2007). In particular, the application of cytokinins (CK) to the acaulescent species S. rexii was found capable to promote isocotyly and a re-establishment of a central post-embryonic SAM (Mantegazza et al 2007).…”

Anisocotyly and meristem initiation in an unorthodox plant, Streptocarpus rexii (Gesneriaceae)

“…They claimed that exogenous hormones re-establish morphogenetic capacities previously thought to be "lost" in the rosulate and unifoliate species, such as isocotyly and caulescence. In particular, cytokinins were found capable of inducing phyllomorphic development of the normally non-accrescent microcotyledon, i.e., the development of basal meristems in both cotyledons (Rosenblum and Basile 1984;Nishii et al 2004). It would thus be interesting to investigate the interaction of cytokinin and SrSTM1 expression in the light of the hormone eVect on the fate of the SAM.…”

“…In this respect, it is interesting to note Rosenblum and Basile (1984) who examined the eVect of Jong and Burtt (1975). a Cotyledonary phyllomorph.…”

“…They exhibit such an unorthodox bauplan to necessitate the introduction of new botanical terms to describe their unusual morphological features, such as anisocotyly, petiolode and phyllomorph (Jong 1970). The unequal development of the two cotyledons after germination (anisocotyly) is common to most members of the Old World Gesneriaceae (Jong 1970;Cronk and Möller 1997;Nishii et al 2004) and likely constitutes the prerequisite condition for the development of the extraordinary growth forms of acaulescent species of Streptocarpus (Hilliard and Burtt 1971;Rosenblum and Basile 1984;Möller and Cronk 2001). Anisocotyly is thought to advantage the growth in typically extreme habitats of Streptocarpus in deep shade, thus to boost the production of photosynthetic tissue soon after germination from the endospermless seeds (Burtt 1970).…”