Mechanisms of Developmental Biology Series: Offshoot of a Plant
Foreword
The progression from a single-cell entity to a multicellular organism entails a series of intricate stages. This intricate journey involves the orchestrated activities of stem cells and the process of differentiation, which collectively constitute the life cycle of complex, multicellular organisms. The domain of developmental biology transcends human boundaries, encompassing studies of animals and plants alike, underscoring its extensive purview. The pursuit of unraveling the intricate mechanisms governing organismal development not only illuminates the origins of life but also provides invaluable insights into the molecular and cellular underpinnings of development across diverse species. The curriculum of this developmental biology series serves as an illuminating beacon, shedding light on the myriad manifestations of life on our planet. To gain a more profound comprehension of developmental processes, this series will traverse multiple interdisciplinary avenues, including embryology, medical science, and evolutionary biology. Through this holistic approach, we endeavor to uncover the underlying principles governing the diverse tapestry of life forms on Earth.
This series will be divided into the following sections:
4. Mechanisms of Developmental Biology Series: Offshoot of a Plant
6. Mechanisms of Developmental Biology Series: Cutting-edge Techniques in Developmental Biology
Mechanisms of Developmental Biology Series: Offshoot of a Plant
The seed, a vital propagation unit, holds the key to establishing a new individual. From the angiosperm seed's complex structure to the initiation of germination and sprouting, it unravels the physiological and biochemical intricacies of responding to environmental stimuli. Following germination, plants undergo a vegetative growth phase characterized by distinct stages—juvenile and adult. Meristems, which are responsible for cellular division and differentiation, shape the architecture of stems and roots during this phase. Transitioning from the vegetative to the reproductive phase marks a significant shift in plant growth. The reproductive system, encompassing sexual and vegetative reproduction, relies on flowers as fundamental organs facilitating seed production and dispersal. Senescence, often associated with ageing and decline, takes on a unique perspective in plant biology. Unlike animals, plants exhibit modular characteristics, challenging conventional definitions of senescence. The exploration of plant development presented in this series offers a comprehensive understanding of the intricate processes shaping the life cycle of plants, contributing to the broader tapestry of developmental biology.
Stages of Plant Development
In terrestrial plants, the progression of shoot growth unfolds through distinct developmental phases. After germination, the plant initiates a period of juvenile vegetative growth, a phase that can be brief or extend over several years followed by a transition into adult vegetative growth, known as a vegetative phase change. The shift from vegetative to reproductive phases typically occurs after this transition, which is marked by the production of essential structures for sexual reproduction, such as flowers or cones. Ultimately, to conclude its life cycle, the plant undergoes senescence and enters a state of decline leading to death. The durations of these phases exhibit significant variability both among and within plant species, with transitions influenced by a combination of intrinsic developmental pathways and external environmental signals (Doody et al., 2022).
Unlike bacterial or animal cells, plant cells do not undergo migration; instead, the shape of plants is determined by the rate of cell division and the direction of elongation. Plant development involves the formation of three primary tissue systems—dermal, ground, and vascular—yet, unlike animals, plants do not rely on gastrulation to establish this layered tissue system. Notably, plant development is a continuous process, with new organs forming throughout their lifespan through specialized clusters of embryonic cells known as meristems. Plants display developmental plasticity, allowing them to regenerate lost parts through meristems and, in some cases, entire plants can be regenerated from single cells. Moreover, external environmental factors such as light and temperature exert significant influence on the overall form of plants, underscoring their ability to adapt and respond to their surroundings.
I. Germination
The seed
Seeds serve as vital propagation units for both agricultural crops and the maintenance of species in natural environments. Positioned critically in the life cycle of higher plants, the success of establishing a new individual hinges on the physiological and biochemical characteristics of the seeds, which develop in response to their environment. The angiosperm (flowering plant) seed generally consists of the embryo (the result of the fertilization of the egg cell and one of the male pollen nuclei), the endosperm (the result of the fusion of the two polar nuclei with the second pollen nucleus) and the perisperm (the seed coat formed from the integument around the ovule). The extent to which the endosperm or perisperm persists varies between species. The embryo, representing the new individual, consists of the embryonic axis and one or two cotyledons, encompassing the radicle (the embryonic root ), hypocotyl (the stem of a germinated seedling), and plumule (the embryonic shoot) (El-Maarouf-Bouteau, 2022). The composition of diverse tissues with distinct developmental programs underscores the complexity of seeds, making the study of seeds an exploration into the coordination of these programs in time and space to achieve successful germination.
Sprouting
Seed germination is considered to be the initiation of the first developmental phase in the lifecycle of higher plants, and is followed by post-germinative growth of the seedling. For a seed to start to germinate, favorable conditions (such as light, temperature, and soil components (especially nitrate) in response to environmental stimuli are required. It is a complex process, which starts with the uptake of water from the environment and is finalized by the protruding action of the radicle from the covering structures (Wolny et al., 2018). During germination, the seed transitions from dormancy to active growth, involving complex processes and interactions between the seed and its environment to ensure the successful emergence of a seedling. Critical environmental aspects that have an effect on how germination proceeds involve temperature, water, light, and oxygen. Optimal germination often occurs within a specific temperature range (e.g., 25°–30°C). Some plants such as Rudbeckia hirta, Lobelia siphilitica, Eschscholzia californica require a period of cold temperatures to break their dormancy cycle after winter. This is a survival technique termed stratification that naturally occurs when seeds are exposed to overwinter outdoors under snow or freezing temperatures. This adaptation is particularly crucial in temperate climates where winters can be harsh, with the potential for sudden frosts that could damage or kill developing seedlings (Gilbert, 2000). Plant scientists mimic this switch from winter to spring by keeping the planted seeds in a dark and cold environment overnight, making sure that all the seeds start to germinate approximately at the same time.
While germination and sprouting are related, they do not exactly refer to the same process. Germination refers to the process by which a seed begins to develop into a new plant, involving the absorption of water, activation of enzymes, and the initiation of metabolic activities within the seed. Sprouting, on the other hand, specifically refers to the emergence of a new shoot or seedling from a germinated seed. Another technique used in plant science, seed priming (Thongtip et al., 2022) entails immersing seeds in water or a chemical solution at specific temperatures and durations, aiming to initiate the early stages of germination and trigger various biochemical and metabolic processes. This process facilitates the reorganization of membranes and the repair of damaged cells and organs while the seeds are returned to optimal moisture conditions.
II. Vegetative Stage
Following germination and preceding reproduction, plants undergo a vegetative growth phase characterized by rapid increases in mass and photosynthetic capacity. This phase encompasses both a juvenile and an adult stage, each distinguished by unique growth patterns and structures. During the juvenile vegetative phase, plants typically exhibit insensitivity to photoperiod (the length of the light period over the day) and floral inducers and as the transition to the adult vegetative phase unfolds, they gradually acquire reproductive competence. Throughout vegetative growth, the vegetative phase change is marked by species-specific alterations in leaf size, shape, internode length, and trichome distribution. These changes collectively lead to a transformation in stem appearance, a phenomenon referred to as heteroblasty, and the presence of these traits at particular stages of development is essential for proper plant function (Raihan et al., 2021; Manuela et al., 2020).
Meristems
Similar to stem cells in animals, meristematic cells divide, generating one daughter cell that remains meristematic and another that undergoes differentiation. In plants, meristems are categorized into three types: apical (located at the tips of growing shoots and roots), lateral (cylindrical meristems located in both shoots and roots), and intercalary (inserted between mature tissues within the stems).
Root apical meristems (RAM) give rise to the root cap, comprised of lubricated cells that are shed as the meristem advances through the soil due to cell division and elongation in more proximal cells. Additionally, it produces daughter cells responsible for generating the three tissue systems within the root. New root apical meristems can initiate from tissue within the core of the root and emerge through the ground tissue and dermal tissue. Alternatively, root meristems can also originate secondarily from the plant's stem, with maize relying significantly on this process as a major source of root mass. The shoot apical meristem (SAM), on the other hand, is responsible for the production of stems, leaves, and reproductive structures in plants. In addition to these SAMs initiated during embryogenesis, axillary SAMs (axillary buds that originate from the surface layers of the meristem) emerge from the original meristem, forming in the angles between leaves and stems. Lateral meristems, which are cylindrical meristems located in both shoots and roots, contribute to secondary growth by leading to an expansion in stem and root diameter through the production of vascular tissues. Unlike dicot stems, monocot stems generally lack lateral meristems; however, they frequently possess intercalary meristems (Gilbert, 2000).
Root and Shoot Development
The initiation of radial and axial patterning in roots commences during embryogenesis and persists throughout development as the primary root elongates and lateral roots emerge from the pericycle cells located deep within the root. Through laser ablation experiments, where individual cells are eliminated, and clonal analyses, it has been revealed that cells exhibit plasticity, and their fate in early root development is primarily determined by their position. Axial patterning in roots appears to be influenced by morphogen-dependent mechanisms, akin to certain aspects of animal development. Numerous experiments have demonstrated that the distribution of the plant hormone auxin plays a crucial role in organizing the axial pattern. Normal axial patterning requires the perception of a peak in auxin concentration at the root tip (Scheres et al., 2002).
The aboveground structures seen in various plant species trace their origins back to SAMs. The architecture of shoots is influenced by the extent of outgrowth from axillary buds, and the regulation of branching patterns is linked to a phenomenon known as apical dominance, where the shoot tip plays a crucial role. Plant hormones, particularly auxin, produced by young leaves and transported toward the leaf base, can suppress axillary bud outgrowth. Factors such as grazing and flowering can release buds from apical dominance, leading to branching. Cytokinins also have the potential to release buds from apical dominance, and the initiation of axillary buds by existing ones adds complexity to branching patterns. Environmental signals further contribute to the regulation of branching patterns, ensuring optimal light capture in open areas. Shoot architecture displays both environmental plasticity and genetic regulation, with identified genes in various species controlling branching patterns.
Leaf primordia, groups of cells giving rise to leaves, are initiated at the periphery of the shoot meristem. The connection between a leaf and the stem is termed a node, and the stem section between nodes is referred to as an internode. The mature sporophyte essentially consists of stacked node/internode units. Phyllotaxy, the arrangement of leaves on the stem, involves communication among existing and newly forming leaf primordia, leading to diverse patterns such as spirals and whorls of three or more leaves at a node. Various mechanisms, including chemical and physical interactions, maintain regular leaf spacing, but the initiation of a specific phyllotactic pattern remains unclear (Gilbert, 2000).
Leaf Development
Leaf development reveals leaf identity, the establishment of leaf axes, and morphogenesis, leading to a vast diversity of leaf shapes. Culture experiments have been conducted to determine when leaf primordia become committed to leaf development, with findings in ferns and angiosperms suggesting that the youngest visible leaf primordia are not inherently committed to leaf fate; instead, they can develop as shoots in culture. The initial radial symmetry of leaf primordia transforms into dorsal-ventral orientation or flattens in all leaves, establishing two additional axes—proximal-distal and lateral. The diverse shapes of leaves result from the regulation of cell division and cell expansion during the development of the leaf blade. While selective cell death (apoptosis) is involved in shaping certain leaves, differential cell growth appears to be a more common mechanism (Gilbert, 2000). Leaves are broadly categorized into two types: simple and compound. Simple leaves exhibit various shapes, from smooth-edged to deeply lobed, such as oak leaves. Compound leaves, on the other hand, are composed of individual leaflets (and sometimes tendrils) rather than a single leaf blade. The developmental mechanism governing simple and compound leaf formation remains an open question.
The transition from the vegetative to the reproductive phase in plants involves a shift in growth, with the vegetative shoot apical meristem transforming into an inflorescence meristem (reproductive shoot apical meristem). In annual plants (plants that complete their lifecycle within a single season), the changes associated with this transition are typically unidirectional; once they enter the adult vegetative phase, they proceed into the reproductive phase. In contrast, perennial plants (plants that persist for several years) follow a cyclical pattern, alternating between the adult vegetative and reproductive phases. Many perennial species exhibit a polycarpic growth habit, undergoing multiple reproductive cycles throughout their lifetimes. This means that the plant sets seeds many times before its lifetime is over. Within a perennial plant, different meristems demonstrate distinct behaviors, with some undergoing floral transition while others remain in the vegetative state (Raihan et al., 2021).
III. Reproductive Stage
In contrast to certain animal systems where the germ line is established early in embryonic development, the plant germ line is set up only after the transition from vegetative to reproductive development, specifically during the flowering phase. The shoot's vegetative and reproductive structures all originate from the shoot meristem that forms during embryogenesis (Gilbert, 2000). The plant reproductive system can be used either to reproduce sexually (sexual reproduction) or asexually (vegetative reproduction). Sexual reproduction in plants typically relies on pollinating agents, and the flower serves as the fundamental reproductive organ facilitating the production of seeds. Seeds, in most plants, become the primary means through which individuals of the species are dispersed across the environment, promoting reproduction. In contrast, vegetative reproduction is a natural form of asexual reproduction that operates independently of sex cells or gametes. At the end of this process, new plant individuals emerge without the need for seed or spore production. The contrasting strategies of sexual and vegetative reproduction highlight the diverse mechanisms employed by plants to ensure their survival and propagation. Unlike higher animals where genetic mechanisms determine sex, most higher plant species possess both male and female structures within the same flower, allowing for self-fertilization (Boavida et al., 2005).
The plant sexual life cycle encompasses two primary stages: the gametophyte stage and the sporophyte stage. In the gametophyte stage, the haploid gametophyte generates distinct multicellular structures, namely male and female gametes. Fertilization involves the fusion of male and female gametes, resulting in the formation of a diploid zygote. The male gametophyte, pollen, is released from the anthers and adheres to, grows along, and interacts with various tissues of the female organs, collectively forming the pistil. Pollen, released as bi- or tricellular and highly dehydrated, carries all necessary biochemical components and transcripts for germination. Upon hydration on female tissues, it forms a rapidly growing cytoplasmic extension known as the pollen tube. This process culminates in the fertilization of the female gametophyte, and the embryo sac, and results in the formation of a zygote (Boavida et al., 2005).
The zygote undergoes development into a still-diploid sporophyte, marking the transition to the sporophyte stage of the plant life cycle. The diploid sporophyte produces spores through meiosis. These spores undergo mitotic division, giving rise once again to haploid gametophytes. The spores are stored within structures called sporangium and with the dispersal of them, new organisms can arise without the need for fertilization. This cyclical process repeats as the gametophytes produce gametes, perpetuating the plant life cycle.
IV. Senescence
Senescence, derived from the Latin word senēscere meaning to grow weak, become exhausted, and be in decline, typically refers to the aging process associated with decay, mortality, or decreased fertility. However, the concept of senescence in plants challenges the classical definition due to specific characteristics that deviate from the conventional understanding. Unlike animals, plants are modular, featuring architecture composed of repetitive units that enable rejuvenation and their cellular division does not always result in shorter telomeres. Some plant species such as bristlecone pine and perennial herbs like Borderea pyrenaica even defy the concept of senescence altogether. Other research on perennial herbs indicated no age-dependent signs of oxidative stress, challenging the notion that age-induced senescence is a universal feature in aging perennial plants (Miryeganeh, 2021).
Throughout the process of senescence, plants incorporate various internal and external signals related to their developmental age through complex regulatory pathways. These integrated features enable the plant to achieve optimal fitness within a specific ecological niche by precisely regulating the timing of initiation, progression rate, and the characteristics of senescence. Despite its significant biological relevance, the molecular mechanisms governing plant senescence remain largely elusive due to their extreme complexity. In their article, Woo et al. (2018) introduce six major research themes that try to understand when and how a plant knows to die: "(1) changes in the photosynthetic apparatus during leaf senescence, (2) the interaction of endogenous and exogenous signals with senescence pathways, (3) integrative, multi-layered, spatiotemporal networks in leaf senescence, (4) the evolutionary basis of leaf senescence, (5) extended applications of multi-omics technologies to explore genetic elements and their networks that control plant senescence, and integration of multiple types of omics and genetic/physiological data, and (6) source–sink interactions for enhancing crop yield." (p. 715).
V. Dormancy
Seeds undergo continuous evolution from their inception, adapting to detect environmental changes and respond to prevailing conditions. Dormancy, a crucial adaptation to unfavorable environmental conditions, enables the coordinated timing of seed germination and plant establishment with the environment. In ecological and evolutionary terms, this property holds importance and is essential for ensuring species continuity and preserving biodiversity Serving as one of the earliest features in the plant life cycle, seed dormancy becomes a critical determinant influencing the colonization and distribution of a species. The environmental conditions experienced by plants during seed maturation, often referred to as the parental or maternal effect of the environment/induced by the environment, have a significant impact on seed dormancy levels and germination timing. Lower temperatures influencing the maternal plant tend to deepen seed dormancy as well as water stress or nutrient availability, particularly nitrate, also influences dormancy depth.
Following harvest, dormant seeds may maintain their dormancy for an extended period in which, during this period, they continually adjust their dormancy states by detecting and integrating various environmental signals. Soil temperature and humidity play key roles in controlling dormancy depth in mature seeds, serving as fundamental factors responsible for dormancy cycling. The rate of dormancy increases and decreases throughout the year and is regulated by seasonal changes in soil parameters. In temperate soils, temperature and humidity signals manifest as gradual seasonal changes, indicating the optimal time of year for germination and seedling establishment. To modulate dormancy depth, these signals are integrated over time, subsequently altering seed sensitivity to the second set of signals that alleviate dormancy and facilitate germination completion. The second set of signals more directly indicates the suitability of conditions for ending the dormancy period and completing germination. If the appropriate spatial window does not manifest (i.e., favorable habitat conditions), the time window will close for the subsequent year. Dormancy aligns with the seasons, determining the optimal time for plant establishment and facilitating the spread of a population's germination events over time (Klupczyńska & Pawłowski, 2021).
Plant Science Research
Arabidopsis stands out among plants with one of the smallest genomes, consisting of approximately 125 million nucleotide pairs, a scale comparable to the genomes of C. elegans and Drosophila. The complete DNA sequence of Arabidopsis is now well-documented, revealing around 26,000 genes within its genome including numerous duplicates, but the actual number of distinct protein types may be fewer due to recent duplications. The establishment of cell culture and genetic transformation techniques, coupled with extensive libraries of mutated seeds generated through random insertions of mobile genetic elements, facilitates the targeted acquisition of plants with mutations in specific genes. This arsenal of tools empowers researchers to delve into gene functions. Although only a fraction of the total gene set has undergone experimental characterization, functions can be tentatively attributed to approximately 18,000 genes by drawing parallels with well-characterized genes in Arabidopsis and other organisms.
The diversity within the plant genome is evident in the significant expansion of certain families of animal gene regulatory proteins. Conversely, some gene families found in animals, like nuclear hormone receptors, appear to be entirely absent in Arabidopsis. Additionally, Arabidopsis boasts large families of gene regulatory proteins that lack counterparts in the animal kingdom, highlighting the unique and complex landscape of gene regulation in plants. This genomic architecture provides a fertile ground for exploring the diverse regulatory mechanisms that govern plant development and responses to environmental cues (Alberts et al., 2002).
Future Perspectives
The urgent need for rapid advancements in crop improvement and plant conservation due to factors, such as the growing global population, climate change, pollution, habitat and agricultural land loss, and increasing pathogen pressures, leads to a gradual shift in research focus from Arabidopsis to non-model organisms, such as crops and rare plant species. In order to facilitate the study of crops and other non-models, there is a demand for robust computational pipelines that can generate high-quality genome assemblies from a combination of short- and long-read sequences. Once the sequences of the genomes and high-quality assemblies are available, the next step involves testing orthologous genes, previously studied only in reference organisms, for their function in candidate processes in non-model species. This helps determine the conservation and divergence of their functions in the species of interest. More targeted gene edits through base- and prime-editing or homologous-recombination-based methods are techniques that enable precise manipulation of genes of interest (Stepanova, 2021). The merged knowledge acquired from both model and non-model organisms can then be applied by plant biologists and environmentalists in crop improvement and plant conservation efforts.
Future efforts to advance the understanding of plant growth and development should take into account several critical considerations. When initiating genome projects, careful consideration is needed in selecting organisms. Factors such as genome size, ploidy level, genetic model system viability, and ease of experimental manipulation are crucial. The chosen organisms should serve as effective models for genetic studies. In the context of changing ecological and climatological conditions, it is vital to identify organisms that provide insights into how plants grow, develop, and adapt. Our ability to modify plants for food, feed, and fuel is intricately linked to understanding how these organisms respond to their environments during growth and development (Sinha, 2011). While automated measurements for growth at a basic level are achievable, microscopic measurements remain challenging. Additionally, capturing the temporal aspects of development—the ability to observe developmental processes over time—has been achieved in only a few systems. Advancements in these areas are fundamental for unlocking the complexities of plant biology and addressing the challenges posed by a changing world.
Conclusion
The journey through the mechanisms of developmental biology, particularly as manifested in the life cycle of plants, reveals a captivating interplay of various processes that define the very essence of life on Earth. From the miraculous initiation of germination in seeds, a testament to adaptation and environmental responsiveness, to the dynamic phases of vegetative and reproductive growth, each stage unfolds with its unique complexities. With the diverse manifestations of life, from the microscopic world of stem cells to the flourishing landscapes of mature plants, the developmental biology series serves as a guiding agent, illuminating the underlying principles that govern the captivating tapestry of existence. This exploration not only deepens our comprehension of the origins of life but also enriches our perspectives on the molecular and cellular intricacies that unite all living organisms in a shared journey of development and evolution. Through this interdisciplinary lens, it can be continued to unravel the secrets of life, weaving together the threads of embryology, medical science, and evolutionary biology, embracing the profound interconnectedness that defines the diverse forms of life.
Bibliographical References
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Visual Sources
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Figure 6: How well do you know shoots? ugaoo. From: https://www.ugaoo.com/blogs/gardening-basics/how-well-do-you-know-shoots
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Figure 13: Plant research. Universität Basel. From: https://www.unibas.ch/de/Studium/Studienangebot/Studiengaenge-faecher.html?study=Plant-Science
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