Inflorescence development in petunia: through the maze of botanical terminology

Abstract

Flowering plants have developed many ways to arrange their flowers. A flower-bearing branch or system of branches is called an inflorescence. The number of flowers that an inflorescence contains ranges from a single flower to endless flower-clusters. Over the past centuries, botanists have classified inflorescences based on their morphology, which has led to an unfortunate maze of complex botanical terminology. With the rise of molecular developmental biology, research has become increasingly focused on how inflorescences develop, rather than on their morphology. It is the decisions taken by groups of stem cells at the growing tips of shoots, called meristems, on when and where to produce a flower or a shoot that specify the course of inflorescence development. Modelling is a helpful aid to follow the consequences of these decisions for inflorescence development. The so-called transient model can produce the broad inflorescence types: cyme, raceme, and panicle, into which most inflorescences found in nature can be classified. The analysis of several inflorescence branching mutants has led to a solid understanding of cymose inflorescence development in petunia (Petunia hybrida). The cyme of petunia is a distinct body plan compared with the well-studied racemes of Arabidopsis and Antirrhinum, which provides an excellent opportunity to study evolutionary developmental biology (evo-devo) related questions. However, thus far, limited use has been made of this opportunity, which may, at least in part, be due to researchers getting lost in the terminology. Some general issues are discussed here, while focusing on inflorescence development in petunia.

Changes in cis-regulatory elements of a key floral regulator are associated with divergence of inflorescence architectures

“…Many plant families contain species with inflorescences described as racemes and cymes (Watson and Dalwitz, 2007), suggesting that these structures evolved multiple times independently. However, the details are hard to reconstruct with certainty because inflorescence architecture is (often) misclassified for a variety of reasons (Castel et al, 2010). In addition, assessing when the regulatory differences in pDOT and pUFO arose during evolution requires data on the regulation of DOT/UFO homologs in many more (related) species than currently available.…”

“…The slightly delayed expression of endogenous ALF in SIMs compared with the apical FM (Souer et al, 1998) was not observed with the limited resolution of GUS assays. This is not surprising because (i) the SIM emerges as a very small region between the bract and the apical FM, which both express ALF, and because (ii) ALF expression in the SIM is only briefly delayed (Souer et al, 1998;Castel et al, 2010). In Arabidopsis inflorescences, however, expression of pLFY: GUS and pALF:GUS was restricted to lateral FMs (Fig.…”

“…Flowering plants (Angiosperms) display an enormous morphological diversity and, because many species are amenable to genetic analysis and transgenesis, they offer excellent possibilities to study the evolution of developmental mechanisms and morphological change (Benlloch et al, 2007;Castel et al, 2010;Moyroud et al, 2010;Della Pina et al, 2014). Angiosperms differ widely with regard to the moment (i.e.…”

“…Angiosperms differ widely with regard to the moment (i.e. the season and age of the plant) they switch from vegetative growth to flowering, as well as the number of flowers that are formed and their position on the plant body (Weberling, 1989;Castel et al, 2010). Distinct species may form solitary flowers or inflorescences that bear many flowers in a variety of different patterns (Rickett, 1954).…”

“…In cymes, flowers are formed from apical meristems, and inflorescence growth continues from lateral meristems, called sympodial inflorescence meristems (SIMs), which ultimately will also acquire floral identity after having formed a subsequent lateral SIM. Panicles go through several rounds of producing lateral meristems before each meristem ends in a flower (Rickett, 1954;Prusinkiewicz et al, 2007;Castel et al, 2010).…”