Akitoshi Iwamoto

Transcriptional repression by MYB3R proteins regulates plant organ growth

In multicellular organisms, temporal and spatial regulation of cell proliferation is central for generating organs with defined sizes and morphologies. For establishing and maintaining the post‐mitotic quiescent state during cell differentiation, it is important to repress genes with mitotic functions. We found that three of the Arabidopsis MYB3R transcription factors synergistically maintain G2/M‐specific genes repressed in post‐mitotic cells and restrict the time window of mitotic gene expression in proliferating cells. The combined mutants of the three repressor‐type MYB3R genes displayed long roots, enlarged leaves, embryos, and seeds. Genome‐wide chromatin immunoprecipitation revealed that MYB3R3 binds to the promoters of G2/M‐specific genes and to E2F target genes. MYB3R3 associates with the repressor‐type E2F, E2FC, and the RETINOBLASTOMA RELATED proteins. In contrast, the activator MYB3R4 was in complex with E2FB in proliferating cells. With mass spectrometry and pairwise interaction assays, we identified some of the other conserved components of the multiprotein complexes, known as DREAM/dREAM in human and flies. In plants, these repressor complexes are important for periodic expression during cell cycle and to establish a post‐mitotic quiescent state determining organ size.

Kinematic study of root elongation in Arabidopsis thaliana with a novel image-analysis program

The measurement of the spatial profile of root elongation needs to determine matching points between time-lapse images and calculate their displacement. These data have been obtained by laborious manual methods in the past. Some computer-based programs have been developed to improve the measurement, but they require many time-series digital images or sprinkling graphite particles on the root prior to image capture. Here, we have developed GrowthTracer, a new image-analysis program for the kinematic study of root elongation. GrowthTracer employs a multiresolution image matching method with a nonlinear filter called the critical point filter (CPF), which extracts critical points from images at each resolution and can determine precise matching points by analysis of only two intact images, without pre-marking by graphite particles. This program calculates the displacement of each matching point and determines the displacement velocity profile along the medial axis of the root. In addition, a manual input of distinct matching points increases the matching accuracy. We show a successful application of this novel program for the kinematic analysis of root growth in Arabidopsis thaliana.

Floral anatomy and vegetative development in Ceratophyllum demersum: A morphological picture of an “unsolved” plant

PREMISE OF THE STUDY The phylogenetic position of Ceratophyllum is still controversial in recent molecular analyses of angiosperms, with various suggestions of a sister group relation to all other angiosperms, eudicots, monocots, eudicots + monocots, and magnoliids. Therefore, the morphological characters of Ceratophyllum are important for resolving the phylogeny of angiosperms. In this study, we observed the detailed developmental anatomy of all lateral organs and their configurations to elucidate the floral development and phyllotactic pattern of Ceratophyllum demersum. METHODS We observed fixed shoots of C. demersum with scanning electron microscopy and serial sections of the samples with light microscopy. KEY RESULTS Bract primordia arise first, followed by the stamen primordia in staminate flowers. Both bracts and stamens initiate unidirectionally, first on the abaxial side of the floral apex and later on the adaxial side, most likely due to the contact pressure imposed by the leaf primordium at the superior node. In pistillate flowers, bract primordia on the abaxial side were also initiated first. The configuration of buds at one node showed six patterns and each pattern included at least one vegetative bud, and flower buds were always accompanied by vegetative buds at the same node. CONCLUSIONS The initiation pattern of organs in the outer whorls of C. demersum flowers is distorted by mechanical pressure, resulting in the phyllotactic variation of staminate flowers. Vegetative buds are the main axillary buds with floral buds as accessory buds, which suggests that the shoot of C. demersum has been modified from a decussate phyllotaxis.

Correction to: Floral development of petaloid Alismatales as an insight into the origin of the trimerous Bauplan in monocot flowers

The caption of Figure 5 was published incorrectly in the original publication of the article.

Floral development: re-evaluation of its importance

Floral development, a fundamental issue in plant morphology, provides an understanding of how flower organs arise in the plant body and offers linkages to many fields of plant biology, such as developmental genetics, systematics, biophysics, crop science, physiology, and so on. Although plant morphologists have been interested in floral development for a long time with the aid of light microscopy, more studies on floral development in various species have been published from the early 1980s in particular since scanning electron microscopy became widespread. However, interest in floral development as a core part of plant morphology has recently declined in the context of plant science as a whole. For example, in JPR, the journal that gathers and disseminates fundamental knowledge in all areas of plant sciences, only two papers relevant to floral development were published in 2017 out of more than 90 regular papers and reviews: one focused on the flower structure and development of Smallanthus (Ibañez et al. 2017) and the other on the embryology of Pera (de Olivera Franca and De-Paula 2017). In contrast, this century has witnessed several attempts at reconsidering one of the oldest disciplines of plant science, namely, plant morphology, introduced by Goethe (1790) in the late eighteenth century (Claßen-Bockhoff 2001). From the late twentieth century to the early twenty-first century, several prominent plant morphologists appealed for the need to re-evaluate plant morphology. Sattler and Rutishauser (1997) elucidated the potential impact of plant morphology on future plant science research, and Kaplan (2001) expressed concern that plant morphology is regarded as a provider of morphological characters for systematics research due to a decline in the interest in plant morphology and insisted that re-evaluating the importance of plant morphology is necessary. Tobe (2003) also claimed that the importance of plant morphology has increased and that further morphological research is essential to appropriately evaluate the increasing data from molecular phylogenetic research and to understand plant evolution. In the field of floral development, a core part of plant morphology, attempts have also been made to re-evaluate its importance and to promote future studies. Ronse De Craene (2010) published a landmark book integrating the results of many studies on floral development to expatiate flower morphology, and this promoted further studies in this field for several years. Bull-Hereñu et al. (2016) also presented a collection of papers on flower morphology and biology, including floral development, which renewed interest in floral development. This special issue of JPR entitled “Floral development: Re-evaluation of its importance” aims to accelerate this movement. One review and five regular papers in this issue re-evaluate the importance of floral development and shed light on its essential linkage with the evolution of flower morphology. Ronse De Craene (2018) reviews floral development from two perspectives: a historic and physicodynamic one. The former focuses on the phylogenetic aspect of floral development, clarifying how flower morphology is the result of change over time. The latter focuses on the physical factors that affect floral development, clarifying how changes in the physical environment of floral meristems affect development, e.g., shift in space, time of organ initiation, mechanical pressures of organs, and changes in the size of the floral meristem. This review presents developmental events as drivers of evolutionary change in flower morphology. Iwamoto et al. (2018) observe and compare the floral development of five members of the “petaloid” Alismatales. As the Alismatales is placed in the second most basal position and represents the highest diversity in floral Floral development –Re‐evaluation of its importance–