BOTANY VOLUME 2 - PLANT PHYSIOLOGY - 2007

7. DEVELOPMENTAL PHYSIOLOGY

Development is defined as the sum total of all processes involving changes in form and function throughout The life cycle of a unicellular or multicellular Organism. Developmental processes can take place at the level of molecules, compartments, Cells, Tissues, or Organs, and they are inextricably linked with the METABOLISM discussed in Chapter 6. Developmental physiology is concerned with analyzing the mechanisms that control development. Its goal is to understand the molecular processes whereby, through the interaction of Nucleic Acids, Proteins, and low-molecular-weight compounds, as well as the environment, Genetic information (genotype) is translated during organismal development into a specific set of traits (phenotype). In the offspring, this leads both to the precise reproduction of the characteristic properties of the parent generation's species and to individual Variability in phenotypic expression within the Structure/21.html">Limits of the reaction norm defined by the genotype. Such developmental plasticity serves plants primarily to adapt the organism to various environmental conditions. Examples of plant life cycles from various systematic groups are presented in Section 11.2.

7.1. Basic Principles of Developmental Physiology

Organismal development comprises the processes of growth and differentiation. Growth is understood as the irreversible increase in volume (size), whereas differentiation refers to the qualitative change in form or, correspondingly, function of a Cell, tissue, or organ.

We will refer primarily to growth processes when describing, for example, The Development of a potato tuber from the Swelling of the apical part of the stolon until the tuber reaches its final size, the elongation of a coleoptile that occurs at a constant cell number solely due to cell expansion, or tissue proliferation in an in vitro cell culture. Differentiation is the predominant process, for example, in the transformation of an epidermal cell into a guard cell (see Fig. 3.13) or the transformation of procambial strands into various elements of a vascular bundle (see 3.2.4, Fig. 3.22, F — L).

Plant regeneration from cell cultures (Fig. 7.1) makes it possible to experimentally separate the processes of growth and differentiation to a significant degree. However, growth and differentiation generally follow one another sequentially. Their separate Discussion is primarily for didactic reasons. At the same time, The ability to regenerate an entire plant from a differentiated cell (for example, from a mesophyll cell or a SHOOT pith cell) demonstrates the totipotency of living plant cells—that is, the retention of complete genetic information even after Cell Differentiation is complete, since all Organelles containing hereditary information are present (see 7.2.1). In sieve elements, the Cell Nucleus is lost during differentiation, and therefore regeneration from such cells is fundamentally impossible.

Class="center">Fig. 7.1. Development of a complete plant organism from isolated cells using Daucus carota as an example. Individual cells from phloem explants, as well as from immature embryos, develop via the stage of embryoids (also referred to as somatic embryos) into young plants that grow tall, flowering and fruiting. Growth in the form of cell suspension and the regeneration of whole plants from individual cells in in vitro culture are regulated by the composition of phytohormones in the nutrient medium (see 7.6.2.3). Many species can be vegetatively propagated in a similar manner.

During ontogeny, a multicellular organism passes through various levels of differentiation, illustrated in Fig. 7.2 using the development of a flowering plant as an example.

During Embryogenesis, alongside the embryo axis (hypocotyl and radicle) and 1 — 2 cotyledons, both primary Meristems (shoot and ROOT meristems) are established, from which all other plant organs develop. This event in organismal development can be viewed as a hierarchical process during which, first, the type and position of the meristem¹ are determined; then, within the meristem, the type, number, and position of organs are specified; and finally, within the designated organ-forming regions (e.g., in a leaf primordium), the number, arrangement, and differentiation of cells take place, leading to tissue formation and determining the shape and size of the organ.

¹ Foreign literature uses the term "identity," which implies the determination of an organ's type, position, and Anatomical Structure. In our Translation, we provide an explanatory definition because in Russian [and Ukrainian] "identity" carries a different semantic nuance. — Ed. note.

Fig. 7.2. Stages of differentiation during higher plant development

Even the First Division of the zygote is unequal, producing two cells: a basal cell, which gives rise to the suspensor, the quiescent center, and part of the root cap, and an apical cell, which forms the rest of the embryo. Zygotic polarity already occurs in lower plants, but cellular polarity is also found in other cell types (see 7.3.3). Substance gradients are considered potential causes of cell polarization. At the same time, they are important for establishing positional information in a multicellular system (see 7.4). Fate determination during plant cell differentiation (determination) is only partially regulated by autonomous intracellular processes (exclusively determined by The Genome); for example, meristematic initials largely determine the future differentiation pathway of their daughter cells (in accordance with The Cell-Lineage model)¹. However, a cell's position within a multicellular organism also exerts a significant influence on its future fate—meaning that cell differentiation is largely controlled by its environment. Thus, a multicellular organism develops not merely through a combination of many separate autonomous intracellular processes (which, due to a low capacity for error correction, would easily lead to instability in the developmental program as a whole), but rather as a complex of interacting cells (frequently linked into symplastic systems) that mutually coordinate and control their activity.

¹ This refers to determinate (mosaic) organismal development, where cell fates are strictly prescribed by the genetic developmental program. This type of development is characteristic of nematodes (e.g., Caenorhabditis). Other organisms, including plants, are more prone to regulative development, which allows for the correction of random errors. — Ed. note.

Substance gradients influence not only the course of differentiation of individual cells or cells within a cellular complex, but they are also responsible for pattern formation¹. This refers to the processes that determine the number and position of differentiating cells and subsequent development. The Chemical Nature of the active substances involved in forming developmental gradients is known only in rare cases (see 7.4.2). However, phytohormones (see 7.6)—especially the auxin indole-3-acetic acid (see 7.6.1)—could play a role here.

¹ The term used here is Musterbildung (a loan translation from the English pattern formation), which translates literally as "form generation" or "pattern creation" for

the temporal window of differentiation events. Thus, the distribution of Stomata in the epidermis, The formation of trichomes, root hairs, and lateral roots, as well as the number and position of leaves on the shoot axis, are all controlled by pattern formation processes.

Unlike most animals, highly developed plants are built on a modular principle. These modules are also called phytomers. They are produced by apical meristems, i.e., shoot meristems and root meristems. A shoot organizational unit consists of a node, an internode, an axillary bud, and a leaf; a root organizational unit consists of a section of the root axis and lateral root primordia. Developing lateral roots and axillary buds in turn produce their own phytomers, and so on. Although shoot architecture shows that the term individual (Latin for indivisible) is, strictly speaking, inappropriate for plants, phytomers are nonetheless not completely independent units of plant development; rather, they themselves are under the strict control of the whole organism. This process of systemic developmental control is termed correlation (see 7.5). For example, the apical bud in the shoot's apical region inhibits the outgrowth of axillary buds (apical dominance — see 7.5; 7.6.1.4). Systemic control can also be observed during the induction of flowering, fruit development, and the formation of storage organs.

7.1.1. Growth

Even in a single cell, growth is a complex process; in a multicellular organism, moreover, the growth of each individual cell must be coordinated with the growth of neighboring cells and all other Cells of the organism in time and space, making the process even more intricate.

Cellular growth encompasses, on the one hand, an increase in the quantity of cell constituents (cytoplasmic growth), which can occur without a significant increase in cell volume (e.g., during the growth of meristematic cells between cell divisions — see Fig. 3.5) and, on the other hand, expansion growth, frequently accompanied by a substantial increase in cell volume and already perceived as a differentiation process, whereby the cell either expands more or less uniformly in all directions (isodiametric growth, e.g., in many parenchymatous cells — see Fig. 3.7, A) or elongates in specific preferential directions (prosenchymatous growth). For instance, in grass coleoptiles, sieve elements, or sclerenchyma fibers, prosenchymatous growth leads to the formation of highly elongated cells (see Fig. 3.20). The volume increase during cell expansion is driven primarily by Water uptake. Therefore, expansion growth is always coupled with vacuole enlargement and the formation of a central vacuole; the total protein content does not necessarily increase during cell expansion. During cell expansion, Cell wall material is synthesized at a moderate rate: in the seta of the liverwort Lophocolea, for example, cell wall mass increases only 1.8-fold over 3 — 4 days against a 48-fold cell elongation.

Expansion growth may encompass the entire cell surface more or less uniformly, or it may be restricted to a specific region of The cell wall. Pronounced tip growth is exhibited, for example, by the apical cells of certain Algae, as well as fungal hyphae, root hairs, pollen tubes, and certain elongated prosenchymatous cells in procambial strands. Non-uniform, vigorous growth at multiple localized surface points forms the basis for the development of more complex cell shapes (e.g., in spongy and stellate parenchyma cells, certain idioblasts and hairs, and individual cells of the alga Micrasterias — see Fig. 3.7, B, C).

In addition to cell growth, multicellular growth also includes cell proliferation (divisional growth). In some organs, such as roots, the division zone (meristematic or embryonic zone) is sharply demarcated from the expansion growth zone, whereas in the shoot apex, they gradually transition into one another. The cessation of cell expansion is typically followed by further differentiation processes.

Table 7.1. Duration and growth rate of certain plant organs via elongation

The growth of plant parts is often explained exclusively by cell elongation without Cell Division. This holds true, for example, for

✵ the growth of cereal coleoptiles,

✵ bud break and flowering in many trees over the course of several days in spring;

✵ the initial phase of radicle growth;

✵ rapid elongation of certain shoots (e.g., bamboo);

✵ elongation of stamens (e.g., in grasses);

✵ elongation of the seta in moss sporogonia;

✵ the stalks of basidiomycete fruiting bodies.

The elongation rate of organs in these cases is often substantial (Table 7.1). In underground roots, the zone of elongation growth is located just behind the tip and is only a few millimeters long (Fig. 7.3). The apical meristem of a maize root produces about 10,000 root cap cells daily, thereby completely renewing the root cap every day, as well as about 170,000 cells for longitudinal root growth. In the root Hair zone, cells have typically already reached their maximum size and begin terminal differentiation. In aerial roots, the zone of elongation growth is longer, and it is significantly longer still in shoots. For instance, in asparagus (Asparagus officinalis), it exceeds 50 cm. In shoot axes, which are divided into nodes and internodes, the Base of the internode retains its growth capacity for the longest time.

In grasses, this intercalary growth persists for a long time; moreover, in the internodal regions above the nodes, cytoplasmic growth and division growth are observed alongside elongation growth. Similar basal intercalary growth zones also form on leaves (particularly clearly in conifers and monocots, but also in dicots). For example, the leaf petiole grows intercalarily between the leaf blade and the leaf base.

When observing the local growth rate along the growth zone—approximately in the root elongation zone (Fig. 7.3, A)—one can notice a gradual increase in the growth rate to a maximum, followed by a decline to a standstill (the "grand period of growth"). Every individual cell that "passes" through the elongation zone exhibits this type of increase and decrease in growth rate. The supply of cells from the meristem and the onset of their elongation are so harmoniously coupled with the decline in growth intensity in older PARTS OF THE organ that roots as a whole continue to grow evenly. In shoots, a circadian¹ increase and decrease in growth rate is frequently observed: the growth rate is slightly higher in the dark than during the day. This process is light-regulated (see 7.7.2). Periodic growth can occur (e.g., in Poaceae) when a young internode is just beginning elongation growth, while an older one is largely finishing it. In barley, a periodic rise in the concentration of the phytohormone gibberellin GA₁, which stimulates internode elongation, has been detected (see 7.6.3): its concentration increases each time before the growth rate of the internode rises (for mechanisms of cytoplasmic growth see 7.3.1, division growth see 7.3.2, and cell elongation see 7.3.3; 7.6.1.4).

¹ With a 24-hour period. — Note of the Editor.

Fig. 7.3. Root growth. A — distribution of growth rates along the primary root of a maize seedling; relative longitudinal growth per 1 h (0.1 = 10%) was determined from short-term measurements at various locations on the root. B — distribution of growth in the root tip of Vicia faba; ticks indicate the positions of ink marks applied to the root at THE START OF the experiment at 1 mm intervals (left), and 22 hours later (right). Due to the uneven growth of individual zones, the ink marks end up at varying distances from one another.

Differentiation and correlative control during development are discussed further below (see 7.3 – 7.5); we now turn to the Genetic foundations of development.