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.

C4 Photosynthetic Gene Expression in Light- and Dark-Grown Amaranth Cotyledons

“…Although cotyledons differ from leaves in origin, pattern of development, and many aspects of gene expression (Scott and Possingham, 1982;Meinke, 1992), it is known that cotyledon development can be altered in such a way that the cotyledons acquire many leaf-like characteristics. In unifoliate species of Streptocarpus, a single cotyledon enlarges during vegetative growth and is transformed into the single leaf of the mature plant (Hill, 1938;Rosenblum and Basile, 1984). In Arabidopsis, a mutation in a single homeotic gene, leafy cotyledon (LEC), causes the cotyledons to acquire many leaflike characteristics (Meinke, 1992 Under normal illumination conditions, amaranth cotyledons emerge from the seed coat within 2 d after planting and usually appear above the soil at the tip of an extended hypocotyl by the end of d 3.…”

The Role ofKNOXGenes in the Evolution of Morphological Novelty in Streptocarpus

“…In crosses between rosulate and unifoliate species, inheritance of the rosulate character was suggested to involve dominant alleles at two loci (Oehlkers, 1938(Oehlkers, , 1942(Oehlkers, , 1964. In some acaulescent species, application of gibberellic acid (GA) resulted in either the formation of extra phyllomorphs or induction of a vegetative shoot (Rosenblum and Basile, 1984).…”

“…These properties are manifest as ectopic leaf outgrowths, and occasionally, SAMs (reviewed in Tsiantis and Hay, 2003), and therefore mimic the morphologies of acaulescent Streptocarpus. A further suggestion that altered KNOX activity might be involved in the novel morphology of Streptocarpus is that KNOX activity is able to repress GA biosynthesis (Sakamoto et al, 2001;Hay et al, 2002), and unifoliate Streptocarpus can phenocopy caulescent Streptocarpus when they are treated with GA, suggesting that acaulescent morphology might involve reduction of GA levels by KNOX activity (Rosenblum and Basile, 1984).…”

Ecotypic divergence in Crepis tectorum (Asteraceae): inferring trait lability and correlational constraints from hormonally manipulated phenotypes

“…Much of this lability can be expected to involve growth regulators or ‘plant hormones’, acting singly or, more commonly, interacting with ontogenetic and environmental influences (Sachs , Heschel and Hausmann , Weyers and Paterson , Farnsworth , Kurepin et al , Busov et al 2008, Ross and Reid , Brock et al , Luo et al 2015). In accordance with this view, numerous cases have been reported in which the exogenous application of a specific plant hormone (or a substance inhibiting its effects) results in a phyletic phenocopy – an altered phenotype with one or several features normally found in some related lineage (Rosenblum and Basile , Stebbins and Basile , Sachs , van Hinsberg 1997).…”

“…Understanding how novel phenotypes arise and become established remains a central goal for evolutionary biologists. While traditional models view trait evolution as a slow, gradual accumulation of genetic factors with individually small effects on the phenotype (Fisher , Coyne and Lande ), it has long been known that evolutionary change can result from genetically simple changes of basic developmental processes at the cell or tissue level, especially in plants because of their open, modular organization (Stebbins , Bachmann , Gottlieb , Doebley and Lukens , Busov et al 2008). Much of this lability can be expected to involve growth regulators or ‘plant hormones’, acting singly or, more commonly, interacting with ontogenetic and environmental influences (Sachs , Heschel and Hausmann , Weyers and Paterson , Farnsworth , Kurepin et al , Busov et al 2008, Ross and Reid , Brock et al , Luo et al 2015).…”