28.4 VARIATIONS ON A THEME: PLANTS VS. ANIMALS

Plants and animals are the best-known examples of complex multicellular organisms. The study of phylogenetic relationships makes it clear that complex multicellularity evolved independently in the two groups. In other words, they do not share a common ancestor that was multicellular. We can see the results of these separate evolutionary events by examining cell adhesion, communication, and development in both plants and animals.

Both plants and animals have evolved sophisticated systems that cause adjacent cells to adhere to each other and that promote the targeted movement of signaling molecules between cells. Plant and animal mechanisms, however, must differ, because plant cells have cell walls and animal cells do not. Likewise, plants and animals have evolved similar genetic logic to govern development, but use mostly distinct sets of genes. In both plants and animals, many proteins switch other genes on or off, so that the spatial organization of multicellular organisms arises from networks of interacting genes and their protein products. Ancestral plants and animals simply recruited distinct families of genes to populate regulatory networks.

28.4.1 Cell walls shape patterns of growth and development in plants.

The plant cell wall (Chapter 5), made of cellulose, imparts structural support to cells and, in fact, provides the mechanical support that allows plants to stand erect (Chapter 29). The presence of cell walls has largely determined the evolutionary fate of plants. For example, because their cells cannot engulf particles or absorb organic molecules, most plants gain carbon and energy only through photosynthesis. Moreover, because all plant cells except eggs and sperm are completely surrounded by cell walls, they have no pseudopodia and no flagella (in conifers and flowering plants, even sperm have lost flagella; see Chapter 30). This being the case, plant cells cannot move, either during development or to obtain nutrients, evade predators, or escape stressful conditions.

Figure 28.10: A meristem. Because of their walls, plant cells can’t move. For this reason, roots grow by cell division of the undifferentiated cells in the meristem region at the root tip.

The inability of plant cells to move has major consequences for development. At the level of the cell, the entire program of growth and development involves cell division, cell expansion (commonly by developing large vacuoles in cell interiors), and cell differentiation. The mechanical consequence is that plant growth is confined to meristems (Fig. 28.10), populations of actively dividing cells at the tips of stems and roots (Chapter 31). More or less permanently undifferentiated cells in meristem regions repeatedly undergo mitosis. A few millimeters from the region of active cell division in a stem or root meristem, cells stop dividing and begin to expand. Within another millimeter or so, this activity is curtailed as well. There, individual cells respond to signaling molecules that induce differentiation, forming the mature cells that will function in photosynthesis, storage, bulk transport, or mechanical support. In general, then, plant development involves cell division, adhesion among the cells that form from meristems, adsorption of water and nutrients supplied by adjacent cells, and differentiation into distinct cell types that govern the function of the plant as a whole.

Because cell walls render plants immobile, plants have evolved mechanisms to transport water and nutrients from the soil to leaves that may be tens of meters distant without the use of any moving parts or even the expenditure of ATP. And because plants are anchored in place, they are unable to move in response to unfavorable growth conditions. For this reason, plants have intricate systems that feed information from the environment to their meristems. Heat, drought, floods, and fire can all leave their mark in altered patterns of growth. In addition, plants can’t flee predators, so they have evolved mechanical structures (hairs and spines) and poisons to keep from being eaten. Indeed, it has been suggested that, in terms of responding to the environment, growth plays a role in plants similar to that played by behavior in animals.

The details of plant growth, development, reproduction, and function are presented in Chapters 29–33. Here, the point to bear in mind is that the properties of a pine tree or a rice plant depend fundamentally on the basic features of cell adhesion, intercellular communication, and regulatory network to guide development, all carried out under the constraints imposed by cell walls.

28.4.2 Animal cells can move relative to one another.

Figure 28.11: Gastrulation, the movement of cells during embryogenesis that transforms a blastula to a gastrula. Unlike plant cells, animal cells move during development.

Having outlined the growth and development of plants, we can appreciate anew the remarkable ballet that characterizes development in animal embryos (Fig. 28.11; Chapter 42). Fertilized eggs undergo several rounds of mitosis to form a ball of undifferentiated cells called a blastula. Then something happens that has no parallel in plant development: Unconstrained by cell walls, animal cells can move relative to one another. Blastula cells migrate, becoming reorganized into a hollow ball that folds inward at one location to form a layered structure called the gastrula.

Gastrula formation brings new populations of cells into direct contact with one another, inducing patterns of molecular signaling and gene regulation that begin the long process of growth and tissue specification. As cells proliferate, molecular signals are expressed in some cells and diffuse into others, generating what amounts to a three-dimensional molecular map of the organism that guides cell differentiation. Gradients in signaling molecules define top, bottom, front, back, left, and right. Plants do much the same thing, but because animal cells can move during development, animal embryos are not restricted to growth only from localized regions like meristems. Cell division and tissue differentiation occur throughout the developing animal body.

Because they are not constrained by cell walls, animals can form organs with moving parts—muscles that power active transport of food and fluids and allow movement. Thus, animals have possibilities for function that are far different from those of plants. If the environment offers a challenge, like drought or predators, animals can respond by changing their behavior. For example, they can move to a new location when threatened or stressed, as is discussed more fully in Chapter 37.

Plants and animals, then, display contrasting patterns of development and function that reflect both their independent origins from different groups of protists and the constraints imposed by cell walls in plants.