DiSUMO-LIKE Interacts with RNA-Binding Proteins and Affects Cell-Cycle Progression during Maize Embryogenesis

Functional characterization of three maize SIZ/PIAS-type SUMO E3 ligases

SUMOylation is a critical post-translational modification that regulates the nature and activity of protein substrates. The reaction is usually enhanced by a SIZ/PIAS-type of SUMO E3 ligase, but the functions of its homologs in maize have not yet been reported. In this study, we functionally characterized three members of this family of SUMO ligases, ZmSIZ1a, ZmSIZ1b, and ZmSIZ1c, from Zea mays. These maize SIZ1 homologs harbor conserved domains and structures with AtSIZ1, suggesting that they are potential functional SUMO ligases, which is supported by further biochemical data. The expression of these maize SIZ1 genes was detectable ubiquitously in different maize tissues and was usually induced by abiotic stresses. Expression of ZmSIZ1 members complements the leaf developmental defects of the AtSIZ1 mutant, suggesting their conserved function in development regulation. Interestingly, overexpression of ZmSIZ1c, but not ZmSIZ1a or ZmSIZ1b, in the wild-type Arabidopsis resulted in early flowering, implying that these members differ in terms of flowering control. Besides, overexpression of these ZmSIZ1 genes also improved salt tolerance in Arabidopsis. Collectively, our functional characterization of the ZmSIZ1 members provides hints for further investigation on the functions of SUMOylation in the development and stress responses in maize.

Comparative Embryogenesis in Angiosperms: Activation and Patterning of Embryonic Cell Lineages

Following fertilization in flowering plants (angiosperms), egg and sperm cells unite to form the zygote, which generates an entire new organism through a process called embryogenesis. In this review, we provide a comparative perspective on early zygotic embryogenesis in flowering plants by using the Poaceae maize and rice as monocot grass and crop models as well as Arabidopsis as a eudicot model of the Brassicaceae family. Beginning with the activation of the egg cell, we summarize and discuss the process of maternal-to-zygotic transition in plants, also taking recent work on parthenogenesis and haploid induction into consideration. Aspects like imprinting, which is mainly associated with endosperm development and somatic embryogenesis, are not considered. Controversial findings about the timing of zygotic genome activation as well as maternal versus paternal contribution to zygote and early embryo development are highlighted. The establishment of zygotic polarity, asymmetric division, and apical and basal cell lineages represents another chapter in which we also examine and compare the role of major signaling pathways, cell fate genes, and hormones in early embryogenesis. Except for the model Arabidopsis, little is known about embryopatterning and the establishment of the basic body plan in angiosperms. Using available in situ hybridization, RNA-sequencing, and marker data, we try to compare how and when stem cell niches are established. Finally, evolutionary aspects of plant embryo development are discussed.

Giant cells: Linking McClintock's heredity to early embryogenesis and tumor origin throughout millennia of evolution on Earth

The "life code" theory postulates that egg cells, which are giant, are the first cells in reproduction and that damaged or aged giant somatic cells are the first cells in tumorigenesis. However, the hereditary basis for giant cells remains undefined. Here I propose that stress-induced genomic reorganization proposed by Nobel Laureate Barbara McClintock may represent the underlying heredity for giant cells, referred to as McClintock's heredity. Increase in cell size may serve as a response to environmental stress via switching proliferative mitosis to intranuclear replication for reproduction. Intranuclear replication activates McClintock's heredity to reset the genome following fertilization for reproduction or restructures the somatic genome for neoplastic transformation via formation of polyploid giant cancer cells (PGCCs). The genome-based McClintock heredity functions together with gene-based Mendel's heredity to regulate the genomic stability at two different stages of life cycle or tumorigenesis. Thus, giant cells link McClintock's heredity to both early embryogenesis and tumor origin. Cycling change in cell size together with ploidy number switch may represent the most fundamental mechanism on how both germ and soma for coping with environmental stresses for the survival across the tree of life which evolved over millions of years on Earth.

Plant zygote development: recent insights and applications to clonal seeds

In flowering plants, haploid gametes - an egg cell and a sperm cell fuse to form the first diploid cell - the zygote. The zygote is the progenitor stem cell that gives rise to all the embryonic and post embryonic tissues and organs. Unlike animals, both maternal and paternal gene products participate in the initial development of zygotes in plants. Here, we discuss recent advances in understanding of the zygotic transition and embryo initiation in angiosperms, including the role of parental contributions to gene expression in the zygote. We further discuss utilization of this knowledge in agricultural biotechnology through synthetic apomixis. Parthenogenesis obtained by manipulation of embryogenic factors, combined with mutations that bypass meiosis, enables clonal propagation of hybrid crops through seeds.

Emerging Roles of RNA-Binding Proteins in Seed Development and Performance

Seed development, dormancy, and germination are key physiological events that are not only important for seed generation, survival, and dispersal, but also contribute to agricultural production. RNA-binding proteins (RBPs) directly interact with target mRNAs and fine-tune mRNA metabolism by governing post-transcriptional regulation, including RNA processing, intron splicing, nuclear export, trafficking, stability/decay, and translational control. Recent studies have functionally characterized increasing numbers of diverse RBPs and shown that they participate in seed development and performance, providing significant insight into the role of RBP-mRNA interactions in seed processes. In this review, we discuss recent research progress on newly defined RBPs that have crucial roles in RNA metabolism and affect seed development, dormancy, and germination.

Intracellular dynamics and transcriptional regulations in plant zygotes: a case study of Arabidopsis

Recent understandings ofArabidopsiszygote. Body axis formation is essential for the proper development of multicellular organisms. The apical-basal axis in Arabidopsis thaliana is determined by the asymmetric division of the zygote, following its cellular polarization. However, the regulatory mechanism of zygote polarization is unclear due to technical issues. The zygote is located deep in the seed (ovule) in flowers, which prevents the living dynamics of zygotes from being observed. In addition, elucidation of molecular pathways by conventional forward genetic screens was not enough because of high gene redundancy in early development. Here, we present a review introducing two new methods, which have been developed to overcome these problems. Method 1: the two-photon live-cell imaging method provides a new system to visualize the dynamics of intracellular structures in Arabidopsis zygotes, such as cytoskeletons and vacuoles. Microtubules form transverse rings and control zygote elongation, while vacuoles dynamically change their shapes along longitudinal actin filaments and support polar nuclear migration. Method 2: the transcriptome method uses isolated Arabidopsis zygotes and egg cells to reveal the gene expression profiles before and after fertilization. This approach revealed that de novo transcription occurs extensively and immediately after fertilization. Moreover, inhibition of the de novo transcription was shown to sufficiently block the zygotic division, thus indicating a strong possibility that yet unidentified zygote regulators can be found using this transcriptome approach. These new strategies in Arabidopsis will help to further our understanding of the fundamental principles regarding the proper formation of plant bodies from unicellular zygotes.

Imaging of Embryo Sac and Early Seed Development in Maize after Feulgen Staining

Compared with small model plants like Arabidopsis containing ovules with few cell layers, embryo sac and embryo development of model crop plants such as maize and other grasses are difficult to image. Multiple layers of tissue usually surround the deeply embedded embryo sac and developing embryo. Moreover, reliable cell biological marker lines labeling, for example, nuclei, plasma membrane, cell walls, or cells of a specific identity are often not available. The introduction of markers to study mutants is difficult and time-consuming and may require several generations of backcrosses. In this chapter, we therefore present an easy protocol to image maize ovaries and developing embryo sacs before and after fertilization allowing also high-throughput mutant analysis. The laborious embedding of samples and preparation of thin sections are omitted in this fixing-Feulgen staining-clearing (FFC) method. Optical sectioning through multiple layers of tissue is possible allowing 3D reconstructions of the whole embryo sac if necessary. The advantage of staining cell nuclei using the FFC method described here compared, for example, with DAPI staining is a wide range of Schiff's type reagents available for the Feulgen reaction. Depending on the reagent of choice, various conditions such as different excitation/emission filters or even white light can be applied for imaging. Moreover, in order to better visualize cell division, nuclei polarity as well as cell extent and integrity, periodic acid staining (PAS) of cell walls can be combined with Feulgen staining.

Cell type specific transcriptional reprogramming of maize leaves during Ustilago maydis induced tumor formation

Ustilago maydis is a biotrophic pathogen and well-established genetic model to understand the molecular basis of biotrophic interactions. U. maydis suppresses plant defense and induces tumors on all aerial parts of its host plant maize. In a previous study we found that U. maydis induced leaf tumor formation builds on two major processes: the induction of hypertrophy in the mesophyll and the induction of cell division (hyperplasia) in the bundle sheath. In this study we analyzed the cell-type specific transcriptome of maize leaves 4 days post infection. This analysis allowed identification of key features underlying the hypertrophic and hyperplasic cell identities derived from mesophyll and bundle sheath cells, respectively. We examined the differentially expressed (DE) genes with particular focus on maize cell cycle genes and found that three A-type cyclins, one B-, D- and T-type are upregulated in the hyperplasic tumorous cells, in which the U. maydis effector protein See1 promotes cell division. Additionally, most of the proteins involved in the formation of the pre-replication complex (pre-RC, that assure that each daughter cell receives identic DNA copies), the transcription factors E2F and DPa as well as several D-type cyclins are deregulated in the hypertrophic cells.

Exploring the regulatory levels of SUMOylation to increase crop productivity

SUMOylation is an essential post-translational modification that affects several cellular processes, from gene replication to stress response. Studies using the SUMO (de)conjugation machinery have provided evidence regarding its potential to improve crop performance and productivity under normal and adverse conditions. However, the pleiotropic effect of SUMOylation can be a disadvantage in both situations, especially when considering unpredictable environmental conditions caused by climate changes. Here, we discuss the pleiotropic effects caused by disrupting the SUMOylation machinery, and new strategies that may help to overcome pleiotropy. We propose exploring the several regulatory levels of SUMOylation recently revealed, including transcriptional, post-transcriptional regulation by alternative splicing, and post-translational modifications. These new findings may provide valuable tools to increase crop productivity.

The Scope, Functions, and Dynamics of Posttranslational Protein Modifications

Assessing posttranslational modification (PTM) patterns within protein molecules and reading their functional implications present grand challenges for plant biology. We combine four perspectives on PTMs and their roles by considering five classes of PTMs as examples of the broader context of PTMs. These include modifications of the N terminus, glycosylation, phosphorylation, oxidation, and N-terminal and protein modifiers linked to protein degradation. We consider the spatial distribution of PTMs, the subcellular distribution of modifying enzymes, and their targets throughout the cell, and we outline the complexity of compartmentation in understanding of PTM function. We also consider PTMs temporally in the context of the lifetime of a protein molecule and the need for different PTMs for assembly, localization, function, and degradation. Finally, we consider the combined action of PTMs on the same proteins, their interactions, and the challenge ahead of integrating PTMs into an understanding of protein function in plants.

Overexpression of SUMO1 located predominately to euchromatin of dividing cells affects reproductive development in maize

Post-translational modification of proteins by small ubiquitin-like modifier (SUMO) plays essential roles in a large variety of cellular and developmental processes. While SUMO conjugation to target proteins has been reported in numerous studies in animals and human, and partly also in the model plant Arabidopsis, little is known about the specific roles of SUMO in crop plants. Here, we report about the maize SUMO family and show that the highly conserved core isoform SUMO1 predominately locates to the nucleus where it marks euchromatin rather than heterochromatin. Moreover, SUMO1 is especially present in nuclei of small dividing cells. Strong overexpression of SUMO1 caused a severe dwarf phenotype and abnormalities in floral organ structures. Defects in anther development and female gametogenesis occurred similar to null-mutant phenotypes reported in Arabidopsis. Taken together, these studies imply that precise and fine-tuned conjugation of the highly conserved plant SUMO1 isoform to target proteins is required for vegetative and reproductive development. Mis-regulation by overexpression or knock-out is deleterious, strongly affecting fertility in both dicots and monocots, including the crop plant maize.

SUMOylation: re-wiring the plant nucleus during stress and development

Conjugation of small ubiquitin-related modifier (SUMO) to intracellular proteins provides a dynamic regulatory mechanism that enables plants to rapidly defend against environmental challenges. SUMOylation of mostly nuclear proteins is among the fastest stress responses observed but precisely how this post-translational modification provides stress resilience remains unclear. Here, we describe the plant SUMO system and its expanding target catalog, which implicates this modification in DNA repair, chromatin modification/remodeling, transcriptional activation/repression, epigenetics, and RNA metabolism, with a likely outcome being extensive nuclear re-wiring to withstand stress. In parallel, studies have linked SUMO to developmental programs such as gametogenesis and gene silencing. The accumulating data support the notion that SUMOylation substantially influences the transcriptional and epigenetic landscapes to promote stress tolerance and developmental progression.