S1-Type Endonuclease 2 in Dedifferentiating Arabidopsis Protoplasts: Translocation to the Nucleus in Senescing Protoplasts Is Associated with De-Glycosylation

Ca2+- and Zn2+-dependent nucleases co-participate in nuclear DNA degradation during programmed cell death in secretory cavity development in Citrus fruits

Calcium (Ca2+)- and zinc Zn2+-dependent nucleases play pivotal roles in plant nuclear DNA degradation in programmed cell death (PCD). However, the mechanisms by which these two nucleases co-participate in PCD-associated nuclear DNA degradation remain unclear. Here, the spatiotemporal expression patterns of two nucleases (CrCAN and CrENDO1) were analyzed qualitatively and quantitatively during PCD in secretory cavity formation in Citrus reticulata 'Chachi' fruits. Results show that the middle and late initial cell stages and lumen-forming stages are key stages for nuclear degradation during the secretory cavity development. CAN and ENDO1 exhibited potent in vitro DNA degradation activity at pH 8.0 and pH 5.5, respectively. Quantitative real-time reverse-transcription polymerase chain reaction, in situ hybridization assays, the subcellular localization of Ca2+ and Zn2+, and immunocytochemical localization showed that CrCAN was activated at the middle and late initial cell stages, while CrENDO1 was activated at the late initial cell and lumen-forming stages. Furthermore, we used immunocytochemical double-labelling to simultaneously locate CrCAN and CrENDO1. The DNA degradation activity of the two nucleases was verified by simulating the change of intracellular pH in vitro. Our results also showed that CrCAN and CrENDO1 worked respectively and co-participated in nuclear DNA degradation during PCD of secretory cavity cells. In conclusion, we propose the model for the synergistic effect of Ca2+- and Zn2+-dependent nucleases (CrCAN and CrENDO1) in co-participating in nuclear DNA degradation during secretory cavity cell PCD in Citrus fruits. Our findings provide direct experimental evidence for exploring different ion-dependent nucleases involved in nuclear degradation during plant PCD.

Moonlight Is Perceived as a Signal Promoting Genome Reorganization, Changes in Protein and Metabolite Profiles and Plant Growth

Rhythmic exposure to moonlight has been shown to affect animal behavior, but its effects on plants, often observed in lunar agriculture, have been doubted and often regarded as myth. Consequently, lunar farming practices are not well scientifically supported, and the influence of this conspicuous environmental factor, the moon, on plant cell biology has hardly been investigated. We studied the effect of full moonlight (FML) on plant cell biology and examined changes in genome organization, protein and primary metabolite profiles in tobacco and mustard plants and the effect of FML on the post-germination growth of mustard seedlings. Exposure to FML was accompanied by a significant increase in nuclear size, changes in DNA methylation and cleavage of the histone H3 C-terminal region. Primary metabolites associated with stress were significantly increased along with the expression of stress-associated proteins and the photoreceptors phytochrome B and phototropin 2; new moon experiments disproved the light pollution effect. Exposure of mustard seedlings to FML enhanced growth. Thus, our data show that despite the low-intensity light emitted by the moon, it is an important environmental factor perceived by plants as a signal, leading to alteration in cellular activities and enhancement of plant growth.

Zn2+-Dependent Nuclease Is Involved in Nuclear Degradation during the Programmed Cell Death of Secretory Cavity Formation in Citrus grandis 'Tomentosa' Fruits

Zn2+- and Ca2+-dependent nucleases exhibit activity toward dsDNA in the four classes of cation-dependent nucleases in plants. Programmed cell death (PCD) is involved in the degradation of cells during schizolysigenous secretory cavity formation in Citrus fruits. Recently, the Ca2+-dependent DNase CgCAN was proven to play a key role in nuclear DNA degradation during the PCD of secretory cavity formation in Citrus grandis ‘Tomentosa’ fruits. However, whether Zn2+-dependent nuclease plays a role in the PCD of secretory cells remains poorly understood. Here, we identified a Zn2+-dependent nuclease gene, CgENDO1, from Citrus grandis ‘Tomentosa’, the function of which was studied using Zn2+ ions cytochemical localization, DNase activity assays, in situ hybridization, and protein immunolocalization. The full-length cDNA of CgENDO1 contains an open reading frame of 906 bp that encodes a protein 301 amino acids in length with a S1/P1-like functional domain. CgENDO1 degrades linear double-stranded DNA at acidic and neutral pH. CgENDO1 is mainly expressed in the late stage of nuclear degradation of secretory cells. Further spatiotemporal expression patterns of CgENDO1 showed that CgENDO1 is initially located on the endoplasmic reticulum and then moves into intracellular vesicles and nuclei. During the late stage of nuclear degradation, it was concentrated in the area of nuclear degradation involved in nuclear DNA degradation. Our results suggest that the Zn2+-dependent nuclease CgENDO1 plays a direct role in the late degradation stage of the nuclear DNA in the PCD of secretory cavity cells of Citrus grandis ‘Tomentosa’ fruits.

Plant Histone HTB (H2B) Variants in Regulating Chromatin Structure and Function

Besides chemical modification of histone proteins, chromatin dynamics can be modulated by histone variants. Most organisms possess multiple genes encoding for core histone proteins, which are highly similar in amino acid sequence. The Arabidopsis thaliana genome contains 11 genes encoding for histone H2B (HTBs), 13 for H2A (HTAs), 15 for H3 (HTRs), and 8 genes encoding for histone H4 (HFOs). The finding that histone variants may be expressed in specific tissues and/or during specific developmental stages, often displaying specific nuclear localization and involvement in specific nuclear processes suggests that histone variants have evolved to carry out specific functions in regulating chromatin structure and function and might be important for better understanding of growth and development and particularly the response to stress. In this review, we will elaborate on a group of core histone proteins in Arabidopsis, namely histone H2B, summarize existing data, and illuminate the potential function of H2B variants in regulating chromatin structure and function in Arabidopsis thaliana.

From Single Cell to Plants: Mesophyll Protoplasts as a Versatile System for Investigating Plant Cell Reprogramming

Plants are sessile organisms that have a remarkable developmental plasticity, which ensures their optimal adaptation to environmental stresses. Plant cell totipotency is an extreme example of such plasticity, whereby somatic cells have the potential to form plants via direct shoot organogenesis or somatic embryogenesis in response to various exogenous and/or endogenous signals. Protoplasts provide one of the most suitable systems for investigating molecular mechanisms of totipotency, because they are effectively single cell populations. In this review, we consider the current state of knowledge of the mechanisms that induce cell proliferation from individual, differentiated somatic plant cells. We highlight initial explant metabolic status, ploidy level and isolation procedure as determinants of successful cell reprogramming. We also discuss the importance of auxin signalling and its interaction with stress-regulated pathways in governing cell cycle induction and further stages of plant cell totipotency.

The Dead Can Nurture: Novel Insights into the Function of Dead Organs Enclosing Embryos

Plants have evolved a variety of dispersal units whereby the embryo is enclosed by various dead protective layers derived from maternal organs of the reproductive system including seed coats (integuments), pericarps (ovary wall, e.g., indehiscent dry fruits) as well as floral bracts (e.g., glumes) in grasses. Commonly, dead organs enclosing embryos (DOEEs) are assumed to provide a physical shield for embryo protection and means for dispersal in the ecosystem. In this review article, we highlight recent studies showing that DOEEs of various species across families also have the capability for long-term storage of various substances including active proteins (hydrolases and ROS detoxifying enzymes), nutrients and metabolites that have the potential to support the embryo during storage in the soil and assist in germination and seedling establishment. We discuss a possible role for DOEEs as natural coatings capable of "engineering" the seed microenvironment for the benefit of the embryo, the seedling and the growing plant.

Dead Pericarps of Dry Fruits Function as Long-Term Storage for Active Hydrolytic Enzymes and Other Substances That Affect Germination and Microbial Growth

It is commonly assumed that dead pericarps of dry indehiscent fruits have evolved to provide an additional physical layer for embryo protection and as a means for long distance dispersal. The pericarps of dry fruits undergo programmed cell death (PCD) during maturation whereby most macromolecules such DNA, RNA, and proteins are thought to be degraded and their constituents remobilized to filial tissues such as embryo and endosperm. We wanted to test the hypothesis that the dead pericarp represents an elaborated layer that is capable of storing active proteins and other substances for increasing survival rate of germinating seeds. Using in gel assays we found that dead pericarps of both dehiscent and indehiscent dry fruits of various plant species including Arabidopsis thaliana and Sinapis alba release upon hydration multiple active hydrolytic enzymes that can persist in an active form for decades, including nucleases, proteases, and chitinases. Proteomic analysis of indehiscent pericarp of S. alba revealed multiple proteins released upon hydration, among them proteases and chitinases, as well as proteins involved in reactive oxygen species (ROS) detoxification and cell wall modification. Pericarps appear to function also as a nutritional element-rich storage for nitrate, potassium, phosphorus, sulfur, and others. Sinapis alba dehiscent and indehiscent pericarps possess germination inhibitory substances as well as substances that promote microbial growth. Collectively, our study explored previously unknown features of the dead pericarp acting also as a reservoir of biological active proteins, and other substances capable of “engineering” the microenvironment for the benefit of the embryo.

The dead seed coat functions as a long-term storage for active hydrolytic enzymes

Seed development culminates in programmed cell death (PCD) and hardening of organs enclosing the embryo (e.g., pericarp, seed coat) providing essentially a physical shield for protection during storage in the soil. We examined the proposal that dead organs enclosing embryos are unique entities that store and release upon hydration active proteins that might increase seed persistence in soil, germination and seedling establishment. Proteome analyses of dead seed coats of Brassicaceae species revealed hundreds of proteins being stored in the seed coat and released upon hydration, many are stress-associated proteins such as nucleases, proteases and chitinases. Functional analysis revealed that dead seed coats function as long-term storage for multiple active hydrolytic enzymes (e.g., nucleases) that can persist in active forms for decades. Substances released from the dead seed coat of the annual desert plant Anastatica hierochuntica displayed strong antimicrobial activity. Our data highlighted a previously unrecognized feature of dead organs enclosing embryos (e.g., seed coat) functioning not only as a physical shield for embryo protection but also as a long-term storage for active proteins and other substances that are released upon hydration to the “seedsphere” and could contribute to seed persistence in the soil, germination and seedling establishment.