The YABBY Genes of Leaf and Leaf-Like Organ Polarity in Leafless Plant Monotropa hypopitys

DWARF AND ROBUST PLANT regulates plant height via modulating gibberellin biosynthesis in chrysanthemum

YABBY (YAB) genes are specifically expressed in abaxial cells of lateral organs and determine abaxial cell fate. However, most studies have focused on few model plants, and the molecular mechanisms of YAB genes are not well understood. Here, we identified a YAB transcription factor in chrysanthemum (Chrysanthemum morifolium), Dwarf and Robust Plant (CmDRP), that belongs to a distinct FILAMENTOUS FLOWER (FlL)/YAB3 sub-clade lost in Brassicaceae. CmDRP was expressed in various tissues but did not show any polar distribution in chrysanthemum. Overexpression of CmDRP resulted in a semi-dwarf phenotype with a significantly decreased active GA3 content, while reduced expression generated the opposite phenotype. Furthermore, plant height of transgenic plants was partially rescued through the exogenous application of GA3 and Paclobutrazol, and expression of the GA biosynthesis gene CmGA3ox1 was significantly altered in transgenic plants. Yeast one-hybrid, luciferase, and chromatin immunoprecipitation-qPCR analyses showed that CmDRP could directly bind to the CmGA3ox1 promoter and suppress its expression. Our research reveals a nonpolar expression pattern of a YAB family gene in dicots and demonstrates it regulates plant height through the GA pathway, which will deepen the understanding of the genetic and molecular mechanisms of YAB genes.

The Arabidopsis HDZIP class II transcription factor ABA INSENSITIVE TO GROWTH 1 functions in leaf development

Leaf laminar growth and adaxial-abaxial boundary formation are fundamental outcomes of plant development. Boundary and laminar growth coordinate the further patterning and growth of the leaf, directing the differentiation of cell types within the top and bottom domains and promoting initiation of lateral organs along their adaxial or abaxial axis. Leaf adaxial-abaxial polarity specification and laminar outgrowth are regulated by two transcription factors, REVOLUTA (REV) and KANADI (KAN). ABA INSENSITIVE TO GROWTH 1 (ABIG1) encodes a HOMEODOMAIN-LEUCINE ZIPPER (HD-ZIP) class II transcription factor and is a direct target of the adaxial-abaxial regulators REV and KAN. To investigate the role of ABIG1 in leaf development and in the establishment of polarity, we examined the phenotypes of both gain-of-function and loss-of-function mutants. Through genetic interaction analysis with REV and KAN mutants, we determined that ABIG1 plays a role in leaf laminar growth as well as in adaxial-abaxial polarity establishment. Genetic and physical interaction assays showed that ABIG1 interacts with the transcriptional TOPLESS corepressor. This study provides new evidence that ABIG1, another HD-ZIP II, facilitates growth through the corepressor TOPLESS.

YABBY Genes in the Development and Evolution of Land Plants

Mounting evidence from genomic and transcriptomic studies suggests that most genetic networks regulating the morphogenesis of land plant sporophytes were co-opted and modified from those already present in streptophyte algae and gametophytes of bryophytes sensu lato. However, thus far, no candidate genes have been identified that could be responsible for "planation", a conversion from a three-dimensional to a two-dimensional growth pattern. According to the telome theory, "planation" was required for the genesis of the leaf blade in the course of leaf evolution. The key transcription factors responsible for leaf blade development in angiosperms are YABBY proteins, which until recently were thought to be unique for seed plants. Yet, identification of a YABBY homologue in a green alga and the recent findings of YABBY homologues in lycophytes and hornworts suggest that YABBY proteins were already present in the last common ancestor of land plants. Thus, these transcriptional factors could have been involved in "planation", which fosters our understanding of the origin of leaves. Here, we summarise the current data on functions of YABBY proteins in the vegetative and reproductive development of diverse angiosperms and gymnosperms as well as in the development of lycophytes. Furthermore, we discuss a putative role of YABBY proteins in the genesis of multicellular shoot apical meristems and in the evolution of leaves in early divergent terrestrial plants.

Roles of YABBY transcription factors in the modulation of morphogenesis, development, and phytohormone and stress responses in plants

The YABBY family is a class of plant-specific transcription factors comprising a typical N-terminal C2C2-type zinc finger domain and a C-terminal helix-loop-helix YABBY domain. YABBY transcription factors play important roles in multiple biological processes, including polarity establishment in plant leaves, the formation and development of reproductive organs, the response to plant hormone signals, resistance to stress, crop breeding and agricultural production. The aim of this review is to summarize our current understanding of the roles, functions and value of the YABBY family in plants, with particular emphasis on new insights into the molecular and physiological mechanisms involved in the YABBY-mediated modulation of polarity establishment, morphogenesis and development, and phytohormone and stress responses in plants. In addition, we propose that this transcription factor family presents great value and potential for research, application and development in crop breeding and agricultural production in the future.

Transcription factors MhyFIL1 and MhyFIL3 <i>(Monotropa hypopitys)</i> determine the asymmetric development of above-ground lateral organs in plants

It is believed that the complete mycoheterotroph pinesap Monotropa hypopitys adaptively evolved from a photosynthetic mycorrhizal ancestor, which had lost its photosynthetic apparatus and vegetative organs (stem and leaves). The aerial part of the plant is a reproductive axis with sterile bracts and inflorescence with a flower type canonical for higher plants. The origin of leaves and leaf-like lateral organs is associated, among other factors, with the evolution of the YABBY genes, which are divided into“vegetative” and evolutionarily recent“reproductive” genes, with regard to their expression profiles. The study of the vegetative YABBY genes in pinesap will determine whether their functions (identification of cell identity on the abaxial surface of the lateral organs) are preserved in the leafless plant. In this study, the structural and phylogenetic analysis of the pinesap vegetative genes MhyFIL1 and MhyFIL3 is performed, the main conserved domains and motifs of the encoded proteins are characterized, and it is confirmed that the genes belong to the vegetative clade YABBY3/FIL. The effect of heterologous ectopic expression of the MhyFIL1 and MhyFIL3 genes on the phenotype of transgenic tobacco Nicotiana tabacum is evaluated. The leaves formed by both types of plants, 35S::MhyFIL1 and 35S::MhyFIL3, were narrower than in control plants and were twisted due to the changed identity of adaxial surface cells. Also, changes in the architecture of the aerial part and the root system of transgenic plants, including aberrant phyllotaxis and arrest of the shoot and root apical meristem development, were noted. Some of the 35S::MhyFIL1 and 35S::MhyFIL3 plants died as early as the stage of the formation of the first leaves, others did not bloom, and still others had a greatly prolonged vegetation period and formed fewer flowers than normal ones. The flowers had no visible differences from the control except for fragile pedicles. Thus, the absence of structural changes from the M. hypopitys flower in comparison to autotrophic species and the effect of MhyFIL1/3 heterologous expression on the development of tobacco plants indicate the preservation of the functions of the vegetative YABBY genes by the MhyFIL1/3 genes in pinesap. Moreover, the activity of YABBY transcription factors of the FIL clade in M. hypopitys is not directly related to the loss of the ability of pinesap to form leaves during the evolutionary transition from autotrophic nutrition to heterotrophy.