Papilionoid inflorescences revisited (Leguminosae-Papilionoideae)

The developmental dynamics in cool season legumes with focus on chickpea

Chickpea is one of the most widely consumed grain legume world-wide. Advances in next-generation sequencing and genomics tools have led to genetic dissection and identification of potential candidate genes regulating agronomic traits in chickpea. However, the developmental particularities and its potential in reforming the yield and nutritional value remain largely unexplored. Studies in crops such as rice, maize, tomato and pea have highlighted the contribution of key regulator of developmental events in yield related traits. A comprehensive knowledge on the development aspects of a crop can pave way for new vistas to explore. Pea and Medicago are the close relatives of genus Cicer and the basic developmental events in these legumes are similar. However, there are some distinct developmental features in chickpea which hold potential for future crop improvement endeavours. The global chickpea germplasm encompasses wide range of diversities in terms of morphology at both vegetative and reproductive stages. There is an immediate need for understanding the genetic and molecular basis of this diversity and utilizing them for the yield contributing trait improvement. The review discusses some of the key developmental events which have potential in yield enhancement and the lessons which can be learnt from model legumes in this regard.

The control of compound inflorescences: insights from grasses and legumes

A major challenge in biology is to understand how organisms have increased developmental complexity during evolution. Inflorescences, with remarkable variation in branching systems, are a fitting model to understand architectural complexity. Inflorescences bear flowers that may become fruits and/or seeds, impacting crop productivity and species fitness. Great advances have been achieved in understanding the regulation of complex inflorescences, particularly in economically and ecologically important grasses and legumes. Surprisingly, a synthesis is still lacking regarding the common or distinct principles underlying the regulation of inflorescence complexity. Here, we synthesize the similarities and differences in the regulation of compound inflorescences in grasses and legumes, and propose that the emergence of novel higher-order repetitive modules is key to the evolution of inflorescence complexity.

The SINGLE FLOWER (SFL) gene encodes a MYB transcription factor that regulates the number of flowers produced by the inflorescence of chickpea

Legumes usually have compound inflorescences, where flowers/pods develop from secondary inflorescences (I2), formed laterally at the primary inflorescence (I1). Number of flowers per I2, characteristic of each legume species, has important ecological and evolutionary relevance as it determines diversity in inflorescence architecture; moreover, it is also agronomically important for its potential impact on yield. Nevertheless, the genetic network controlling the number of flowers per I2 is virtually unknown. Chickpea (Cicer arietinum) typically produces one flower per I2 but single flower (sfl) mutants produce two (double-pod phenotype). We isolated the SFL gene by mapping the sfl-d mutation and identifying and characterising a second mutant allele. We analysed the effect of sfl on chickpea inflorescence ontogeny with scanning electron microscopy and studied the expression of SFL and meristem identity genes by RNA in situ hybridisation. We show that SFL corresponds to CaRAX1/2a, which codes a MYB transcription factor specifically expressed in the I2 meristem. Our findings reveal SFL as a central factor controlling chickpea inflorescence architecture, acting in the I2 meristem to regulate the length of the period for which it remains active, and therefore determining the number of floral meristems that it can produce.

Mechanical Forces in Floral Development

Mechanical forces acting within the plant body that can mold flower shape throughout development received little attention. The palette of action of these forces ranges from mechanical pressures on organ primordia at the microscopic level up to the twisting of a peduncle that promotes resupination of a flower at the macroscopic level. Here, we argue that without these forces acting during the ontogenetic process, the actual flower phenotype would not be achieved as it is. In this review, we concentrate on mechanical forces that occur at the microscopic level and determine the fate of the flower shape by the physical constraints on meristems at an early stage of development. We thus highlight the generative role of mechanical forces over the floral phenotype and underline our general view of flower development as the sum of interactions of known physiological and genetic processes, together with physical aspects and mechanical events that are entangled towards the shaping of the mature flower.

Resolving the non-papilionaceous flower of Camoensia scandens, a papilionoid legume of the core genistoid clade: development, glands and insights into the pollination and systematics of the group

Camoensia scandens is a papilionoid legume inserted in the core genistoid clade. It has large, crepuscular, scented flowers but the corolla is non-papilionaceous, which deviates from the pattern found in the subfamily. The vexillum has a folded claw, forming a tube, which is opposed to the androecium opening; all petals have yellow-gold crinkled margins. In addition, there is a long hypanthium, which stores a translucid liquid. The goal of this study is to elucidate the ontogenetic pathways that result in such a peculiar flower and the glands responsible for the sweet fragrance of the petals. Floral buds and flowers were processed for SEM, TEM and light microscopy analyses. Five sepals arise unidirectionally followed by five petals that initiate simultaneously. After the petals, 11 stamens emerge unidirectionally; a pair of adaxial stamens is opposite to the vexillum. In the intermediate developmental stages the sepals unite basally; the two adaxial sepals unite with each other to a greater extent than with the other sepals. The filaments are basally connate, forming a tube with an adaxial opening at the base. The carpel emerges concomitantly with the two abaxial antepetalous stamens. The long hypanthium forms from the outer floral organs (base of the sepals, petals, filaments) and is attached to the base of the stipe. The corolla is noticeable in the intermediate stages of development. The crinkled golden margins house scent glands formed of a secretory epidermis with secretory trichomes and secretory subepidermal cells. The odor is composed of neutral polysaccharides, nitrogenous substances and essential oils. An extensive nectariferous region is found on the inner surface of the hypanthial tube. The nectar is translucent, viscous and released through large pores. The comparison of our data with that of other genistoid flowers enabled discussions about the pollination and systematics of the group.

Morphological diversity and evolution of Centrolepidaceae (Poales), a species-poor clade with diverse body plans and developmental patterns

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PREMISE OF THE STUDY

The small primarily Australian commelinid monocot family Centrolepidaceae displays remarkably high structural diversity that has been hitherto relatively poorly explored. Data on Centrolepidaceae are important for comparison with other Poales, including grasses and sedges.•

METHODS

We examined vegetative and reproductive morphology in a global survey of Centrolepidaceae based on light and scanning electron microscopy of 18 species, representing all three genera. We used these data to perform a cladistic analysis to assess character evolution.•

KEY RESULTS

Each of the three genera is monophyletic; Centrolepis is sister to Aphelia. Some Centrolepidaceae show a change from spiral to distichous phyllotaxy on inflorescence transition. In Aphelia and most species of Centrolepis, several morphologically distinct leaf types develop along the primary shoot axis and flowers are confined to dorsiventral lateral spikelets. Centrolepis racemosa displays secondary unification of programs of leaf development, absence of the leaf hyperphyll and loss of shoot dimorphism. Presence or absence of a leaf ligule and features of inflorescence and flower morphology are useful as phylogenetic characters in Centrolepidaceae.•

CONCLUSIONS

Ontogenetic changes in phyllotaxy differ fundamentally between some Centrolepidaceae and many grasses. Inferred evolutionary transformations of phyllotaxy in Centrolepidaceae inflorescences also differ from those in grasses. In contrast with grasses, some Centrolepidaceae possess ligulate leaves where the ligule represents the boundary between the bifacial hypophyll and unifacial hyperphyll. All the highly unusual features of the morphological-misfit species Centrolepis racemosa could result from the same saltational event. Centrolepidaceae offer good perspectives for studies of evolutionary developmental biology.

Genetic control of inflorescence architecture in legumes

The architecture of the inflorescence, the shoot system that bears the flowers, is a main component of the huge diversity of forms found in flowering plants. Inflorescence architecture has also a strong impact on the production of fruits and seeds, and on crop management, two highly relevant agronomical traits. Elucidating the genetic networks that control inflorescence development, and how they vary between different species, is essential to understanding the evolution of plant form and to being able to breed key architectural traits in crop species. Inflorescence architecture depends on the identity and activity of the meristems in the inflorescence apex, which determines when flowers are formed, how many are produced and their relative position in the inflorescence axis. Arabidopsis thaliana, where the genetic control of inflorescence development is best known, has a simple inflorescence, where the primary inflorescence meristem directly produces the flowers, which are thus borne in the main inflorescence axis. In contrast, legumes represent a more complex inflorescence type, the compound inflorescence, where flowers are not directly borne in the main inflorescence axis but, instead, they are formed by secondary or higher order inflorescence meristems. Studies in model legumes such as pea (Pisum sativum) or Medicago truncatula have led to a rather good knowledge of the genetic control of the development of the legume compound inflorescence. In addition, the increasing availability of genetic and genomic tools for legumes is allowing to rapidly extending this knowledge to other grain legume crops. This review aims to describe the current knowledge of the genetic network controlling inflorescence development in legumes. It also discusses how the combination of this knowledge with the use of emerging genomic tools and resources may allow rapid advances in the breeding of grain legume crops.

Inflorescences: concepts, function, development and evolution

BACKGROUND

Inflorescences are complex structures with many functions. At anthesis they present the flowers in ways that allow for the transfer of pollen and optimization of the plant's reproductive success. During flower and fruit development they provide nutrients to the developing flowers and fruits. At fruit maturity they support the fruits prior to dispersal, and facilitate effective fruit and seed dispersal. From a structural point of view, inflorescences have played important roles in systematic and phylogenetic studies. As functional units they facilitate reproduction, and are largely shaped by natural selection.

SCOPE

The papers in this Special Issue bridge the gap between structural and functional approaches to inflorescence evolution. They include a literature review of inflorescence function, an experimental study of inflorescences as essential contributors to the display of flowers, and two papers that present new methods and concepts for understanding inflorescence diversity and for dealing with terminological problems. The transient model of inflorescence development is evaluated in an ontogenetic study, and partially supported. Four papers present morphological and ontogenetic studies of inflorescence development in monophyletic groups, and two of these evaluate the usefulness of Hofmeister's Rule and inhibitory fields to predict inflorescence structure. In the final two papers, Bayesian and Monte-Carlo methods are used to elucidate inflorescence evolution in the Panicoid grasses, and a candidate gene approach is used in an attempt to understand the evolutionary genetics of inflorescence evolution in the genus Cornus (Cornaceae). Taken as a whole, the papers in this issue provide a glimpse of contemporary approaches to the study of the structure, development, and evolution of inflorescences, and suggest fruitful new directions for research.