Fertilization in flowering plants: an odyssey of sperm cell delivery

The MYC transcription factor PbrMYC8 negatively regulates PbrMSL5 expression to promote pollen germination in Pyrus

The successful germination of pollen is essential for double fertilization in flowering plants. Mechanosensitive channels of small conductance (MscS-like, MSL) inhibit pollen germination and maintains cellular integrity of pollen during this process. Therefore, it is vital to carefully regulate the expression of MSL to promote successful pollen germination. Despite its importance, the molecular mechanisms governing MSL expression in plants remain poorly understood. Here, we had identified 15 MSL genes in the pear, among which PbrMSL5 was expressed in pollen development. Subcellular localization experiments revealed that PbrMSL5 was located in both plasma membrane and cytoplasm. Functional investigations, including complementation experiments using the atmsl8 mutant background, demonstrated the involvement of PbrMSL5 in preserving pollen cell integrity and inhibiting germination. Antisense oligonucleotide experiments further confirmed that PbrMSL5 suppressed pear pollen germination by reducing osmotic pressure and Cl- content. Yeast one-hybrid, electrophoretic mobility shift assays, and dual luciferase reporter assay elucidated that PbrMYC8 interacts directly with the N-box element, leading to the suppression of PbrMSL5 expression and promoted pollen germination. These results represented a significant advancement in unraveling the molecular mechanisms controlling plant MSL expression. This study showed valuable contribution to advancing our comprehension of the mechanism underlying pollen germination.

An Overview on MADS Box Members in Plants: A Meta-Review

Most of the studied MADS box members are linked to flowering and fruit traits. However, higher volumes of studies on type II of the two types so far suggest that the florigenic effect of the gene members could just be the tip of the iceberg. In the current study, we used a systematic approach to obtain a general overview of the MADS box members' cross-trait and multifactor associations, and their pleiotropic potentials, based on a manually curated local reference database. While doing so, we screened for the co-occurrence of terms of interest within the title or abstract of each reference, with a threshold of three hits. The analysis results showed that our approach can retrieve multi-faceted information on the subject of study (MADS box gene members in the current case), which could otherwise have been skewed depending on the authors' expertise and/or volume of the literature reference base. Overall, our study discusses the roles of MADS box members in association with plant organs and trait-linked factors among plant species. Our assessment showed that plants with most of the MADS box member studies included tomato, apple, and rice after Arabidopsis. Furthermore, based on the degree of their multi-trait associations, FLC, SVP, and SOC1 are suggested to have relatively higher pleiotropic potential among others in plant growth, development, and flowering processes. The approach devised in this study is expected to be applicable for a basic understanding of any study subject of interest, regardless of the depth of prior knowledge.

Integration analysis of ATAC-seq and RNA-seq provides insight into fatty acid biosynthesis in Schizochytrium limacinum under nitrogen limitation stress

BACKGROUND

Schizochytrium limacinum holds significant value utilized in the industrial-scale synthesis of natural DHA. Nitrogen-limited treatment can effectively increase the content of fatty acids and DHA, but there is currently no research on chromatin accessibility during the process of transcript regulation. The objective of this research was to delve into the workings of fatty acid production in S. limacinum by examining the accessibility of promoters and profiling gene expressions.

RESULTS

Results showed that differentially accessible chromatin regions (DARs)-associated genes were enriched in fatty acid metabolism, signal transduction mechanisms, and energy production. By identifying and annotating DARs-associated motifs, the study obtained 54 target transcription factor classes, including BPC, RAMOSA1, SPI1, MYC, and MYB families. Transcriptomics results revealed that several differentially expressed genes (DEGs), including SlFAD2, SlALDH, SlCAS1, SlNSDHL, and SlDGKI, are directly related to the biosynthesis of fatty acids, meanwhile, SlRPS6KA, SlCAMK1, SlMYB3R1, and SlMYB3R5 serve as transcription factors that could potentially influence the regulation of fatty acid production. In the integration analysis of DARs and ATAC-seq, 13 genes were identified, which were shared by both DEGs and DARs-associated genes, including SlCAKM, SlRP2, SlSHOC2, SlTN, SlSGK2, SlHMP, SlOGT, SlclpB, and SlDNAAF3.

CONCLUSIONS

SlCAKM may act as a negative regulator of fatty acid and DHA synthesis, while SlSGK2 may act as a positive regulator, which requires further study in the future. These insights enhance our comprehension of the processes underlying fatty acid and DHA production in S. limacinum. They also supply a foundational theoretical framework and practical assistance for the development of strains rich in fatty acids and DHA.

The receptor-like kinase EMS1 and BRI1 coordinately regulate stamen elongation via the transcription factors BES1/BZR1 in Arabidopsis

Plants possess a large family of receptor kinase proteins to mediate cell-to-cell and cell-to-environment communication, and these regulations are essential for plant growth and development as well as resistance to biotic or abiotic stresses. EMS1 is a receptor kinase which involved in tapetum cell fate determination during anther development, while brassinosteroid (BR) receptor, BRI1, controls most aspects of plant growth and development. Although EMS1 and BRI1 are known to regulate independent biological processes, they interact with identical components of the downstream signaling pathways. However, the biological processes other than the tapetum development controlled by the EMS1 signal are not clear. Here, we report that EMS1 signaling-related mutants exhibited an insufficient stamen elongation phenotype, similar to BR signaling mutants. Transgenic expression of BRI1 restored the short filament phenotype of ems1. Conversely, co-expression of EMS1 and TPD1 also restored the short filaments of BRI1 mutants, bri1. Genetic experiments confirmed that EMS1 and BRI1 regulate filament elongation through their downstream transcription factors BES1/BZR1. Molecular analysis suggested that the decrease in BR signaling output in filaments of the ems1 mutant caused deficient filament development. Moreover, in vitro and in vivo experiments proved BES1 interacts with filament-specific transcription factor MYB21. Together, we found that the two receptor-like kinases (RLKs) EMS1 and BRI1 are cooperatively involved in the regulation of filament elongation via the transcription factors BES1/BZR1. These results indicated that the biological processes regulated by EMS1 and BRI1 in plants are both independent and interactive, which provides us with insights into multidimensional molecular control of the RLK pathway.

Transcriptome analysis of ovules offers early developmental clues after fertilization in Cicer arietinum L

Chickpea (Cicer arietinum L.) seeds are valued for their nutritional scores and limited information on the molecular mechanisms of chickpea fertilization and seed development is available. In the current work, comparative transcriptome analysis was performed on two different stages of chickpea ovules (pre- and post-fertilization) to identify key regulatory transcripts. Two-staged transcriptome sequencing was generated and over 208 million reads were mapped to quantify transcript abundance during fertilization events. Mapping to the reference genome showed that the majority (92.88%) of high-quality Illumina reads were aligned to the chickpea genome. Reference-guided genome and transcriptome assembly yielded a total of 28,783 genes. Of these, 3399 genes were differentially expressed after the fertilization event. These involve upregulated genes including a protease-like secreted in CO(2) response (LOC101500970), amino acid permease 4-like (LOC101506539), and downregulated genes MYB-related protein 305-like (LOC101493897), receptor like protein 29 (LOC101491695). WGCNA analysis and pairwise comparison of datasets, successfully constructed four co-expression modules. Transcription factor families including bHLH, MYB, MYB-related, C2H2 zinc finger, ERF, WRKY and NAC transcription factor were also found to be activated after fertilization. Activation of these genes and transcription factors results in the accumulation of carbohydrates and proteins by enhancing their trafficking and biosynthesis. Total 17 differentially expressed genes, were randomly selected for qRT-PCR for validation of transcriptome analysis and showed statistically significant correlations with the transcriptome data. Our findings provide insights into the regulatory mechanisms underlying changes in fertilized chickpea ovules. This work may come closer to a comprehensive understanding of the mechanisms that initiate developmental events in chickpea seeds after fertilization.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03599-8.

Discovery of a cis-regulatory element SaeM involved in dynamic regulation of synergid-specific MYB98

MYB98 is a key regulator of the genetic network behind pollen tube attraction toward the female gametophyte. MYB98 is specifically expressed in the synergid cells (SCs), a female gametophyte component cells specialized for pollen tube attraction. However, it had not been clear how exactly MYB98 achieves this specific expression pattern. In the current study, we have determined that a normal SC-specific expression of MYB98 is dependent on a 16-bp-long cis-regulatory element, CATTTACACATTAAAA, freshly named as the "S ynergid-specific A ctivation E lement of M YB98" (SaeM). An 84 bp fragment harboring SaeM in the middle was sufficient to drive exclusively SC-specific expression. The element was present in a significantly large proportion of SC-specific gene promoters and in the promoter of MYB98 homologous genes in the Brassicaceae (pMYB98s). Significance of such family-wide SaeM-like element conservation in exclusive SC-specific expression was confirmed by the Arabidopsis-like activation feature of Brassica oleracea-derived pMYB98 and absence of such feature of pMYB98 derived from a non-Brassicaceae member Prunus persica. Additionally, the yeast-one-hybrid assay showed that the SaeM can be recognized by ANTHOCYANINLESS2 (ANL2) and DAP-seq data further suggested for additional three ANL2 homologs targeting the similar cis-element. Overall, our study has concluded that SaeM plays a crucial role in driving exclusively SC-specific expression of MYB98 and strongly suggests for the involvement of ANL2 and its homologs in its dynamic regulation in planta. Future study on the transcription factors is expected to shed more light on the mechanism behind the process.

Ovule initiation: the essential step controlling offspring number in Arabidopsis

Seed is the offspring of angiosperms. Plants produce large numbers of seeds to ensure effective reproduction and survival in varying environments. Ovule is a fundamentally important organ and is the precursor of the seed. In Arabidopsis and other plants characterized by multi-ovulate ovaries, ovule initiation determines the maximal ovule number, thus greatly affecting seed number per fruit and seed yield. Investigating the regulatory mechanism of ovule initiation has both scientific and economic significance. However, the genetic and molecular basis underlying ovule initiation remains unclear due to technological limitations. Very recently, rules governing the multiple ovules initiation from one placenta have been identified, the individual functions and crosstalk of phytohormones in regulating ovule initiation have been further characterized, and new regulators of ovule boundary are reported, therefore expanding the understanding of this field. In this review, we present an overview of current knowledge in ovule initiation and summarize the significance of ovule initiation in regulating the number of plant offspring, as well as raise insights for the future study in this field that provide potential routes for the improvement of crop yield.

Let's shape again: the concerted molecular action that builds the pollen tube

The pollen tube is being subjected to control by a complex network of communication that regulates its shape and the misfunction of a single component causes specific deformations. In flowering plants, the pollen tube is a tubular extension of the pollen grain required for successful sexual reproduction. Indeed, maintaining the unique shape of the pollen tube is essential for the pollen tube to approach the embryo sac. Many processes and molecules (such as GTPase activity, phosphoinositides, Ca²⁺ gradient, distribution of reactive oxygen species and nitric oxide, nonuniform pH values, organization of the cytoskeleton, balance between exocytosis and endocytosis, and cell wall structure) play key and coordinated roles in maintaining the cylindrical shape of pollen tubes. In addition, the above factors must also interact with each other so that the cell shape is maintained while the pollen tube follows chemical signals in the pistil that guide it to the embryo sac. Any intrinsic changes (such as erroneous signals) or extrinsic changes (such as environmental stresses) can affect the above factors and thus fertilization by altering the tube morphology. In this review, the processes and molecules that enable the development and maintenance of the unique shape of pollen tubes in angiosperms are presented emphasizing their interaction with specific tube shape. Thus, the purpose of the review is to investigate whether specific deformations in pollen tubes can help us to better understand the mechanism underlying pollen tube shape.

Autophagy is required for self-incompatible pollen rejection in two transgenic Arabidopsis thaliana accessions

Successful reproduction in the Brassicaceae is mediated by a complex series of interactions between the pollen and the pistil, and some species have an additional layer of regulation with the self-incompatibility trait. While the initial activation of the self-incompatibility pathway by the pollen S-locus protein 11/S locus cysteine-rich protein and the stigma S Receptor Kinase is well characterized, the downstream mechanisms causing self-pollen rejection are still not fully understood. In previous studies, we detected the presence of autophagic bodies with self-incompatible (SI) pollinations in Arabidopsis lyrata and transgenic Arabidopsis thaliana lines, but whether autophagy was essential for self-pollen rejection was unknown. Here, we investigated the requirement of autophagy in this response by crossing mutations in the essential AUTOPHAGY7 (ATG7) and ATG5 genes into two different transgenic SI A. thaliana lines in the Col-0 and C24 accessions. By using these previously characterized transgenic lines that express A. lyrata and Arabidopsis halleri self-incompatibility genes, we demonstrated that disrupting autophagy weakened their SI responses in the stigma. When the atg7 or atg5 mutations were present, an increased number of SI pollen was found to hydrate and form pollen tubes that successfully fertilized the SI pistils. Additionally, we confirmed the presence of GFP-ATG8a-labeled autophagosomes in the stigmatic papillae following SI pollinations. Together, these findings support the requirement of autophagy in the self-incompatibility response and add to the growing understanding of the intracellular mechanisms employed in the transgenic A. thaliana stigmas to reject self-pollen.

Reproduction Multitasking: The Male Gametophyte

The gametophyte represents the sexual phase in the alternation of generations in plants; the other, nonsexual phase is the sporophyte. Here, we review the evolutionary origins of the male gametophyte among land plants and, in particular, its ontogenesis in flowering plants. The highly reduced male gametophyte of angiosperm plants is a two- or three-celled pollen grain. Its task is the production of two male gametes and their transport to the female gametophyte, the embryo sac, where double fertilization takes place. We describe two phases of pollen ontogenesis-a developmental phase leading to the differentiation of the male germline and the formation of a mature pollen grain and a functional phase representing the pollen tube growth, beginning with the landing of the pollen grain on the stigma and ending with double fertilization. We highlight recent advances in the complex regulatory mechanisms involved, including posttranscriptional regulation and transcript storage, intracellular metabolic signaling, pollen cell wall structure and synthesis, protein secretion, and phased cell-cell communication within the reproductive tissues.

Pollen-Pistil Interactions as Reproductive Barriers

Pollen-pistil interactions serve as important prezygotic reproductive barriers that play a critical role in mate selection in plants. Here, we highlight recent progress toward understanding the molecular basis of pollen-pistil interactions as reproductive isolating barriers. These barriers can be active systems of pollen rejection, or they can result from a mismatch of required male and female factors. In some cases, the barriers are mechanistically linked to self-incompatibility systems, while others represent completely independent processes. Pollen-pistil reproductive barriers can act as soon as pollen is deposited on a stigma, where penetration of heterospecific pollen tubes is blocked by the stigma papillae. As pollen tubes extend, the female transmitting tissue can selectively limit growth by producing cell wall-modifying enzymes and cytotoxins that interact with the growing pollen tube. At ovules, differential pollen tube attraction and inhibition of sperm cell release can act as barriers to heterospecific pollen tubes.

IMPERFECTIVE EXINE FORMATION (IEF) is required for exine formation and male fertility in Arabidopsis

KEY MESSAGE

IEF, a novel plasma plasma membrane protein, is important for exine formation in Arabidopsis. Exine, an important part of pollen wall, is crucial for male fertility. The major component of exine is sporopollenin which are synthesized and secreted by tapetum. Although sporopollenin synthesis has been well studied, the transportation of it remains elusive. To understand it, we analyzed the gene expression pattern in tapetal microdissection data, and investigated the potential transporter genes that are putatively regulated by ABORTED MICROSPORES (AMS). Among these genes, we identified IMPERFECTIVE EXINE FORMATION (IEF) that is important for exine formation. Compared to the wild type, ief mutants exhibit severe male sterility and pollen abortion, suggesting IEF is crucial for pollen development and male fertility. Using both scanning and transmission electron microscopes, we showed that exine structure was not well defined in ief mutant. The transient expression of IEF-GFP driven by the 35S promoter indicated that IEF-GFP was localized in plasma membrane. Furthermore, AMS can specifically activate the expression of promoterIEF:LUC in vitro, which suggesting AMS regulates IEF for exine formation. The expression of ATP-BINDING CASSETTE TRANSPORTER G26 (AGCB26) was not affected in ief mutants. In addition, SEM and TEM data showed that the sporopollenin deposition is more defective in abcg26/ief-2 than that of in abcg26, which suggesting that IEF is involved in an independent sporopollenin transportation pathway. This work reveal a novel gene, IEF regulated by AMS that is essential for exine formation.

Two aquaporins, SIP1;1 and PIP1;2, mediate water transport for pollen hydration in the Arabidopsis pistil

Pollination is the crucial initial step that brings together the male and female gametophytes, and occurs at the surface of the stigmatic papilla cell in Arabidopsis thaliana. After pollen recognition, pollen hydration is initiated as a second critical step to activate desiccated mature pollen grains for germination, and thus water transport from pistil to pollen is essential for this process. In this study, we report a novel aquaporin-mediated water transport process in the papilla cell as a control mechanism for pollen hydration. Coupled with a time-series imaging analysis of pollination and a reverse genetic analysis using T-DNA insertion Arabidopsis mutants, we found that two aquaporins, the ER-bound SIP1;1 and the plasma membrane-bound PIP1;2, are key players in water transport from papilla cell to pollen during pollination. In wild type plant, hydration speed reached its maximal value within 5 min after pollination, remained high until 10-15 min. In contrast, sip1;1 and pip1;2 mutants showed no rapid increase of hydration speed, but instead a moderate increase during ∼25 min after pollination. Pollen of sip1;1 and pip1;2 mutants had normal viability without any functional defects for pollination, indicating that decelerated pollen hydration is due to a functional defect on the female side in sip1;1 and pip1;2 mutants. In addition, sip1;1 pip1;2 double knockout mutant showed a similar impairment of pollen hydration to individual single mutants, suggesting that their coordinated regulation is critical for proper water transport, in terms of speed and amount, in the pistil to accomplish successful pollen hydration.

Microscopic and Transcriptomic Analysis of Pollination Processes in Self-Incompatible Taraxacum koksaghyz

The transition of the Russian dandelion Taraxacum koksaghyz (Asteraceae) to a profitable, alternative crop producing natural rubber and inulin requires the optimization of several agronomic traits, cultivation conditions and harvesting procedures to improve the yield. However, efficient breeding is hindered by the obligatory sexual outcrossing of this species. Several other asters have been investigated to determine the mechanism of self-incompatibility, but the underlying molecular basis remains unclear. We therefore investigated the self-pollination and cross-pollination of two compatible T. koksaghyz varieties (TkMS2 and TkMS3) by microscopy and transcriptomic analysis to shed light on the pollination process. Self-pollination showed typical sporophytic self-incompatibility characteristics, with the rare pollen swelling at the pollen tube apex. In contrast, cross-pollination was characterized by pollen germination and penetration of the stigma by the growing pollen tubes. RNA-Seq was used to profile gene expression in the floret tissue during self-pollination and cross-pollination, and the differentially expressed genes were identified. This revealed three candidates for the early regulation of pollination in T. koksaghyz, which can be used to examine self-incompatibility mechanisms in more detail and to facilitate breeding programs.

The Role of Anaphase-Promoting Complex/Cyclosome (APC/C) in Plant Reproduction

Most eukaryotic species propagate through sexual reproduction that requires male and female gametes. In flowering plants, it starts through a single round of DNA replication (S phase) and two consecutive chromosome segregation (meiosis I and II). Subsequently, haploid mitotic divisions occur, which results in a male gametophyte (pollen grain) and a female gametophyte (embryo sac) formation. In order to obtain viable gametophytes, accurate chromosome segregation is crucial to ensure ploidy stability. A precise gametogenesis progression is tightly regulated in plants and is controlled by multiple mechanisms to guarantee a correct evolution through meiotic cell division and sexual differentiation. In the past years, research in the field has shown an important role of the conserved E3-ubiquitin ligase complex, Anaphase-Promoting Complex/Cyclosome (APC/C), in this process. The APC/C is a multi-subunit complex that targets proteins for degradation via proteasome 26S. The functional characterization of APC/C subunits in Arabidopsis, which is one of the main E3 ubiquitin ligase that controls cell cycle, has revealed that all subunits investigated so far are essential for gametophytic development and/or embryogenesis.

Plant Biology: Gynoecium Development with Style

The gynoecium is the female reproductive part of the flower and is essential for plant sexual reproduction. A new study shows a novel angiosperm-specific gene family that fine tunes the architecture of the stigma and style in Arabidopsis.

A Complex Journey: Cell Wall Remodeling, Interactions, and Integrity During Pollen Tube Growth

In flowering plants, pollen tubes undergo a journey that starts in the stigma and ends in the ovule with the delivery of the sperm cells to achieve double fertilization. The pollen cell wall plays an essential role to accomplish all the steps required for the successful delivery of the male gametes. This extended path involves female tissue recognition, rapid hydration and germination, polar growth, and a tight regulation of cell wall synthesis and modification, as its properties change not only along the pollen tube but also in response to guidance cues inside the pistil. In this review, we focus on the most recent advances in elucidating the molecular mechanisms involved in the regulation of cell wall synthesis and modification during pollen germination, pollen tube growth, and rupture.

Mechanics of Pollen Tube Elongation: A Perspective

Pollen tube (PT) serves as a vehicle that delivers male gametes (sperm cells) to a female gametophyte during double fertilization, which eventually leads to the seed formation. It is one of the fastest elongating structures in plants. Normally, PTs traverse through the extracellular matrix at the transmitting tract after penetrating the stigma. While the endeavor may appear simple, the molecular processes and mechanics of the PT elongation is yet to be fully resolved. Although it is the most studied "tip-growing" structure in plants, several features of the structure (e.g., Membrane dynamics, growth behavior, mechanosensing etc.) are only partially understood. In many aspects, PTs are still considered as a tissue rather than a "unique cell." In this review, we have attempted to discuss mainly on the mechanics behind PT-elongation and briefly on the molecular players involved in the process. Four aspects of PTs are particularly discussed: the PT as a cell, its membrane dynamics, mechanics of its elongation, and the potential mechanosensors involved in its elongation based on relevant findings in both plant and non-plant models.

Genetic activity during early plant embryogenesis

Seeds are essential for human civilization, so understanding the molecular events underpinning seed development and the zygotic embryo it contains is important. In addition, the approach of somatic embryogenesis is a critical propagation and regeneration strategy to increase desirable genotypes, to develop new genetically modified plants to meet agricultural challenges, and at a basic science level, to test gene function. We briefly review some of the transcription factors (TFs) involved in establishing primary and apical meristems during zygotic embryogenesis, as well as TFs necessary and/or sufficient to drive somatic embryo programs. We focus on the model plant Arabidopsis for which many tools are available, and review as well as speculate about comparisons and contrasts between zygotic and somatic embryo processes.

Signalling Pinpointed to the Tip: The Complex Regulatory Network That Allows Pollen Tube Growth

Plants display a complex life cycle, alternating between haploid and diploid generations. During fertilisation, the haploid sperm cells are delivered to the female gametophyte by pollen tubes, specialised structures elongating by tip growth, which is based on an equilibrium between cell wall-reinforcing processes and turgor-driven expansion. One important factor of this equilibrium is the rate of pectin secretion mediated and regulated by factors including the exocyst complex and small G proteins. Critically important are also non-proteinaceous molecules comprising protons, calcium ions, reactive oxygen species (ROS), and signalling lipids. Among the latter, phosphatidylinositol 4,5-bisphosphate and the kinases involved in its formation have been assigned important functions. The negatively charged headgroup of this lipid serves as an interaction point at the apical plasma membrane for partners such as the exocyst complex, thereby polarising the cell and its secretion processes. Another important signalling lipid is phosphatidic acid (PA), that can either be formed by the combination of phospholipases C and diacylglycerol kinases or by phospholipases D. It further fine-tunes pollen tube growth, for example by regulating ROS formation. How the individual signalling cues are intertwined or how external guidance cues are integrated to facilitate directional growth remain open questions.

Fertilization-Defective Gametophytic Mutant Screening: A Novel Approach

Gametophytic mutants share very small proportion of the total mutants generated by any mutagenic approach; even rarer are the fertilization-defective gametophytic mutants. They require an efficient and targeted strategy instead of 'brute force' screening approach. The classical gametophyte mutant screening method, mainly based on the segregation distortion, can distinguish gametophytic mutants from the others. However, the mutants pooled after the screening constitute both fertilization-defective and developmental-defective gametophytic mutants. Until recently, there has not been any straightforward way to screen the former from the latter. Additionally, most of the mutations affecting both gametes are lost during the screening process. The novel gametophyte screening approach tends to circumvent those shortcomings. This review discusses on the classical approach of gametophytic mutant screening and focuses on the novel approach on distinguishing fertilization-/developmental-defective gametophytic mutants (both male and female). It offers an empirical basis of screening such mutants by taking in the consideration of earlier studies on fertilization failure, initiation of seed coat formation, and fertilization recovery system in plants.

Why so Complex? The Intricacy of Genome Structure and Gene Expression, Associated with Angiosperm Mitochondria, May Relate to the Regulation of Embryo Quiescence or Dormancy-Intrinsic Blocks to Early Plant Life

Mitochondria play key roles in cellular-energy metabolism and are vital for plant-life, such as for successful germination and early-seedling establishment. Most mitochondria contain their own genetic system (mtDNA, mitogenome), with an intrinsic protein-synthesis machinery. Although the challenges of maintaining prokaryotic-type structures and functions are common to Eukarya, land plants possess some of the most complex organelle composition of all known organisms. Angiosperms mtDNAs are characteristically the largest and least gene-dense among the eukaryotes. They often contain highly-variable intergenic regions of endogenous or foreign origins and undergo frequent recombination events, which result in different mtDNA configurations, even between closely-related species. The expression of the mitogenome in angiosperms involves extensive mtRNA processing steps, including numerous editing and splicing events. Why do land-plant's mitochondria have to be so complex? The answer to this remains a matter of speculation. We propose that this complexity may have arisen throughout the terrestrialization of plants, as a means to control embryonic mitochondrial functions -a critical adaptive trait to optimize seed germination. The unique characteristics of plant mtDNA may play pivotal roles in the nuclear-regulation of organellar biogenesis and metabolism, possibly to control embryos quiescence or dormancy, essential determinants for the establishment of viable plantlets that can survive post-germination.