Hokuto Nakayama

Regulation of the KNOX-GA Gene Module Induces Heterophyllic Alteration in North American Lake Cress

A gene module responsible for morphological diversification of leaf form within a species may have been co-opted during plant evolution to regulate morphological variation among species. Plants show leaf form alteration in response to changes in the surrounding environment, and this phenomenon is called heterophylly. Although heterophylly is seen across plant species, the regulatory mechanisms involved are largely unknown. Here, we investigated the mechanism underlying heterophylly in Rorippa aquatica (Brassicaceae), also known as North American lake cress. R. aquatica develops pinnately dissected leaves in submerged conditions, whereas it forms simple leaves with serrated margins in terrestrial conditions. We found that the expression levels of KNOTTED1-LIKE HOMEOBOX (KNOX1) orthologs changed in response to changes in the surrounding environment (e.g., change of ambient temperature; below or above water) and that the accumulation of gibberellin (GA), which is thought to be regulated by KNOX1 genes, also changed in the leaf primordia. We further demonstrated that exogenous GA affects the complexity of leaf form in this species. Moreover, RNA-seq revealed a relationship between light intensity and leaf form. These results suggest that regulation of GA level via KNOX1 genes is involved in regulating heterophylly in R. aquatica. The mechanism responsible for morphological diversification of leaf form among species may also govern the variation of leaf form within a species in response to environmental changes.

Expression patterns of AaDL, a CRABS CLAW ortholog in Asparagus asparagoides (Asparagaceae), demonstrate a stepwise evolution of CRC/DL subfamily of YABBY genes

Understanding the evolutionary path of transcription factors is essential for the elucidation of plant evolution. The CRC/DL subfamily of the YABBY gene family are functionally diverse, plant-specific, putative transcription factors. In Arabidopsis thaliana, CRABS CLAW (CRC) is expressed in the abaxial region of carpel primordia and in floral nectaries, where it regulates carpel morphology and nectary development. By contrast, in Oryza sativa, DROOPING LEAF (DL) is expressed in the entire carpel primordium and in the central undifferentiated cells of leaves, where it regulates carpel identity and midrib development. Recent studies suggest that abaxial expression and functional roles in the carpel are ancestral characters, although when and how neo-functionalizations occurred remains unclear. To elucidate the evolutionary processes of the CRC/DL subfamily, we examined in situ expression patterns of a CRC ortholog (AaDL) in Asparagus asparagoides (Asparagales). Like CRC in Arabidopsis thaliana, AaDL was clearly expressed in the abaxial region of the ovary wall. Expression was also detected in the phloem of leaves, but not in the septal nectary. Thus, expression in the entire carpel primordium might have been acquired after the divergence of Asparagus, with expression competence in the leaves acquired before the divergence of Asparagus in monocots. Our data indicate that the evolution of CRC/DL subfamily genes occurred in a stepwise manner.

A Developmental Model for Branching Morphogenesis of Lake Cress Compound Leaf

Lake cress, Rorippa aquatica (Brassicaceae), is a semi-aquatic plant that exhibits a variety of leaf shapes, from simple leaves to highly branched compound leaves, depending on the environment. Leaf shape can vary within a single plant, suggesting that the variation can be explained by a simple model. In order to simulate the branched structure in the compound leaves of R. aquatica, we implemented reaction-diffusion (RD) patterning onto a theoretical framework that had been developed for serration distribution in the leaves of Arabidopsis thaliana, with the modification of the one-dimensional reaction-diffusion domain being deformed with the spatial periodicity of the RD pattern while expanding. This simple method using an iterative pattern could create regular and nested branching patterns. Subsequently, we verified the plausibility of our theoretical model by comparing it with the experimentally observed branching patterns. The results suggested that our model successfully predicted both the qualitative and quantitative aspects of the timing and positioning of branching in growing R. aquatica leaves.

Leaves may function as temperature sensors in the heterophylly of Rorippa aquatica (Brassicaceae)

Many plants show heterophylly, which is variation in leaf form within a plant owing to environmental change. The molecular mechanisms underlying heterophylly have recently been investigated in several plant species. However, little is known about how plants exhibiting heterophylly sense environmental cues. Here, we used Rorippa aquatica (Brassicaceae), which shows heterophylly, to investigate whether a single leaf can sense and transit changes in ambient temperature. The morphology of newly developed leaves after single-leaf warming treatment was significantly different from that of mock-treated control leaves, suggesting that leaves are sensing organs that mediate the responses to changes in ambient temperature in R. aquatica.

Modification and co-option of leaf developmental programs for the acquisition of flat structures in monocots: unifacial leaves in Juncus and cladodes in Asparagus

It has been suggested that modification and co-option of existing gene regulatory networks (GRNs) play an important role in the morphological diversity. In plants, leaf development is one of active research areas, and the basic GRN for leaf development is beginning to be understood. Moreover, leaves show wide variation in their form, and some of this variation is thought to be the result of adaptation. Thus, leaves and leaf-like organs are an emerging and interesting model to reveal how existing GRNs give rise to novel forms and architectures during evolution. In this review, we highlight recent findings in evo-devo studies, especially on Juncus unifacial leaves, which are composed of lamina with abaxialized identities, and Asparagus cladodes, which are leaf-like organs at the axils of scale leaves. Based on these studies, we discuss how flat structures have evolved and morphologically diversified in shoot systems of monocot species, focusing on the modification and co-option of GRN for leaf development.

How Do Plants and Phytohormones Accomplish Heterophylly, Leaf Phenotypic Plasticity, in Response to Environmental Cues

Plant species are known to respond to variations in environmental conditions. Many plant species have the ability to alter their leaf morphology in response to such changes. This phenomenon is termed heterophylly and is widespread among land plants. In some cases, heterophylly is thought to be an adaptive mechanism that allows plants to optimally respond to environmental heterogeneity. Recently, many research studies have investigated the occurrence of heterophylly in a wide variety of plants. Several studies have suggested that heterophylly in plants is regulated by phytohormones. Herein, we reviewed the existing knowledge on the relationship and role of phytohormones, especially abscisic acid, ethylene, gibberellins, and auxins (IAA), in regulating heterophylly and attempted to elucidate the mechanisms that regulate heterophylly.

A Decrease in Ambient Temperature Induces Post-Mitotic Enlargement of Palisade Cells in North American Lake Cress

In order to maintain organs and structures at their appropriate sizes, multicellular organisms orchestrate cell proliferation and post-mitotic cell expansion during morphogenesis. Recent studies using Arabidopsis leaves have shown that compensation, which is defined as post-mitotic cell expansion induced by a decrease in the number of cells during lateral organ development, is one example of such orchestration. Some of the basic molecular mechanisms underlying compensation have been revealed by genetic and chimeric analyses. However, to date, compensation had been observed only in mutants, transgenics, and γ-ray–treated plants, and it was unclear whether it occurs in plants under natural conditions. Here, we illustrate that a shift in ambient temperature could induce compensation in Rorippa aquatica (Brassicaceae), a semi-aquatic plant found in North America. The results suggest that compensation is a universal phenomenon among angiosperms and that the mechanism underlying compensation is shared, in part, between Arabidopsis and R. aquatica.

Comparative transcriptomics with self-organizing map reveals cryptic photosynthetic differences between two accessions of North American Lake cress

Because natural variation in wild species is likely the result of local adaptation, it provides a valuable resource for understanding plant-environmental interactions. Rorippa aquatica (Brassicaceae) is a semi-aquatic North American plant with morphological differences between several accessions, but little information available on any physiological differences. Here, we surveyed the transcriptomes of two R. aquatica accessions and identified cryptic physiological differences between them. We first reconstructed a Rorippa phylogeny to confirm relationships between the accessions. We performed large-scale RNA-seq and de novo assembly; the resulting 87,754 unigenes were then annotated via comparisons to different databases. Between-accession physiological variation was identified with transcriptomes from both accessions. Transcriptome data were analyzed with principal component analysis and self-organizing map. Results of analyses suggested that photosynthetic capability differs between the accessions. Indeed, physiological experiments revealed between-accession variation in electron transport rate and the redox state of the plastoquinone pool. These results indicated that one accession may have adapted to differences in temperature or length of the growing season.

A GLABRA1 ortholog on LG A9 controls trichome number in the Japanese leafy vegetables Mizuna and Mibuna (Brassica rapa L. subsp. nipposinica L.H. Bailey): evidence from QTL analysis

Brassica rapa show a wide range of morphological variations. In particular, the leaf morphologies of the Japanese traditional leafy vegetables Mizuna and Mibuna (Brassica rapa L. subsp. nipposinica L. H. Bailey) are distinctly different, even though they are closely related cultivars that are easy to cross. In addition to the differences in the gross morphology of leaves, some cultivars of Mibuna (Kyo-nishiki) have many trichomes on its leaves, whereas Mizuna (Kyo-mizore) does not. To identify the genes responsible for the different number of trichomes, we performed a quantitative trait loci (QTL) analysis of Mizuna and Mibuna. To construct linkage maps for these cultivars, we used RNA-seq data to develop cleaved amplified polymorphic sequence (CAPS) markers. We also performed a restriction site-associated DNA sequencing (RAD-seq) analysis to detect single-nucleotide polymorphisms (SNPs). Two QTL analyses were performed in different years, and both analyses indicated that the largest effect was found on LG A9. Expression analyses showed that a gene homologous to GLABRA1 (GL1), a transcription factor implicated in trichome development in Arabidopsis thaliana, and the sequences 3′-flanking (downstream) of BrGL1, differed considerably between Mizuna (Kyo-mizore) and Mibuna (Kyo-nishiki). These results indicate that BrGL1 on LG A9 is one of the candidate genes responsible for the difference in trichome number between Mizuna and Mibuna. Detecting genes that are responsible for morphological variations allows us to better understand the breeding history of Mizuna and Mibuna.