Growth and Development
THE VEGETATIVE PHASE OF DEVELOPMENT
begins with embryogenesis, but development continues throughout the life of a plant. Plant
developmental biologists are concerned with questions such as, How
does a zygote give rise to an embryo, an embryo to a seedling? How do
new plant structures arise from preexisting structures? Organs are generated by cell division and expansion, but they are also composed of tissues in which groups of cells have acquired specialized functions, and
these tissues are arranged in specific patterns. How do these tissues form
in a particular pattern, and how do cells differentiate? What are the basic
principles that govern the size increase (growth) that occurs throughout
plant development?
Understanding how growth, cell differentiation, and pattern formation are regulated at the cellular, biochemical, and molecular levels is the
ultimate goal of developmental biologists. Such an understanding also
must include the genetic basis of development. Ultimately, development
is the unfolding of genetically encoded programs. Which genes are
involved, what is their hierarchical order, and how do they bring about
developmental change?
In this chapter we will explore what is known about these questions,
beginning with embryogenesis. Embryogenesis initiates plant development, but unlike animal development, plant development is an ongoing
process. Embryogenesis establishes the basic plant body plan and forms
the meristems that generate additional organs in the adult.
After discussing the formation of the embryo, we will examine root
and shoot meristems. Most plant development is postembryonic, and it
occurs from meristems. Meristems can be considered to be cell factories
in which the ongoing processes of cell division, expansion, and differentiation generate the plant body. Cells derived from meristems become
the tissues and organs that determine the overall size, shape, and structure of the plant.
Vegetative meristems are highly repetitive—they produce the same
or similar structures over and over again—and their activity can con
tinue indefinitely, a phenomenon known as indeterminate
growth. Some long-lived trees, such as bristlecone pines and
the California redwoods, continue to grow for thousands
of years. Others, particularly annual plants, may cease vegetative development with the initiation of flowering after
only a few weeks or months of growth. Eventually the
adult plant undergoes a transition from vegetative to reproductive development, culminating in the production of a
zygote, and the process begins again. Reproductive development will be discussed in Chapter 24.
Cells derived from the apical meristems exhibit specific
patterns of cell expansion, and these expansion patterns
determine the overall shape and size of the plant. We will examine how plant growth is analyzed after discussing meristems, with an emphasis on growth patterns in space (relationship of plant structures) and time (when events occur).
Finally, despite their indeterminate growth habit, plants,
like all other multicellular organisms, senesce and die. At
the end of the chapter we will consider death as a developmental phenomenon, at both the cellular and organismal
levels. Foe an historical overviw of the study of plant
development, see Web Essay 16.1.
EMBRYOGENESIS
The developmental process known as embryogenesis initiates plant development. Although embryogenesis usually
begins with the union of a sperm with an egg, forming a
single-celled zygote, somatic cells also may undergo
embryogenesis under special circumstances. Fertilization
also initiates three other developmental programs: endosperm, seed, and fruit development. Here we will focus on
embryogenesis because it provides the key to understanding plant development.
Embryogenesis transforms a single-celled zygote into a
multicellular, microscopic, embryonic plant. A completedembryo has the basic body plan of the mature plant and
many of the tissue types of the adult, although these are
present in a rudimentary form.
Double fertilization is unique to the flowering plants
(see Web Topics 1.1 and 1.2). In plants, as in all other
eukaryotes, the union of one sperm with the egg forms a
single-celled zygote. In angiosperms, however, this event
is accompanied by a second fertilization event, in which
another sperm unites with two polar nuclei to form the
triploid endosperm nucleus, from which the endosperm(the tissue that supplies food for the growing embryo) will
develop.
Embryogenesis occurs within the embryo sac of the
ovule while the ovule and associated structures develop
into the seed. Embryogenesis and endosperm development
typically occur in parallel with seed development, and the
embryo is part of the seed. Endosperm may also be part of
the mature seed, but in some species the endosperm disappears before seed development is completed. Embryogenesis and seed development are highly ordered, integrated processes, both of which are initiated by double fertilization. When completed, both the seed and the embryo
within it become dormant and are able to survive long
periods unfavorable for growth. The ability to form seeds
is one of the keys to the evolutionary success of
angiosperms as well as gymnosperms.
The fact that a zygote gives rise to an organized embryo
with a predictable and species-specific structure tells us
that the zygote is genetically programmed to develop in a
particular way, and that cell division, cell expansion, and
cell differentiation are tightly controlled during embryogenesis. If these processes were to occur at random in the
embryo, the result would be a clump of disorganized cells
with no definable form or function.
In this section we will examine these changes in greater
detail. We will focus on molecular genetic studies that have
been conducted with the model plant Arabidopsis that have
provided insights into plant development. It is most likely
that most angiosperms probably use similar developmental mechanisms that appeared early in the evolution of the
flowering plants and that the diversity of plant form is
brought about by relatively subtle changes in the time and
place where the molecular regulators of development are
expressed, rather than by different mechanisms altogether
(Doebley and Lukens 1998).
Arabidopsis thaliana is a member of the Brassicaceae, or
mustard family (Figure 16.1). It is a small plant, well suited
for laboratory culture and experimentation. It has been
called the Drosophila of plant biology because of its widespread use in the study of plant genetics and molecular
genetic mechanisms, particularly in an effort to understand
plant developmental change. It was the first higher plant
to have its genome completely sequenced. Furthermore,
there is a concerted international effort to understand the
function of every gene in the Arabidopsis genome by the
year 2010. As a result, we are much closer to an understanding of the molecular mechanisms governing Arabidopsis embryogenesis than of those for any other plant
species.Embryogenesis Establishes the Essential Features
of the Mature PlantPlants differ from most animals in that embryogenesis does
THE VEGETATIVE PHASE OF DEVELOPMENT
begins with embryogenesis, but development continues throughout the life of a plant. Plant
developmental biologists are concerned with questions such as, How
does a zygote give rise to an embryo, an embryo to a seedling? How do
new plant structures arise from preexisting structures? Organs are generated by cell division and expansion, but they are also composed of tissues in which groups of cells have acquired specialized functions, and
these tissues are arranged in specific patterns. How do these tissues form
in a particular pattern, and how do cells differentiate? What are the basic
principles that govern the size increase (growth) that occurs throughout
plant development?
Understanding how growth, cell differentiation, and pattern formation are regulated at the cellular, biochemical, and molecular levels is the
ultimate goal of developmental biologists. Such an understanding also
must include the genetic basis of development. Ultimately, development
is the unfolding of genetically encoded programs. Which genes are
involved, what is their hierarchical order, and how do they bring about
developmental change?
In this chapter we will explore what is known about these questions,
beginning with embryogenesis. Embryogenesis initiates plant development, but unlike animal development, plant development is an ongoing
process. Embryogenesis establishes the basic plant body plan and forms
the meristems that generate additional organs in the adult.
After discussing the formation of the embryo, we will examine root
and shoot meristems. Most plant development is postembryonic, and it
occurs from meristems. Meristems can be considered to be cell factories
in which the ongoing processes of cell division, expansion, and differentiation generate the plant body. Cells derived from meristems become
the tissues and organs that determine the overall size, shape, and structure of the plant.
Vegetative meristems are highly repetitive—they produce the same
or similar structures over and over again—and their activity can con
tinue indefinitely, a phenomenon known as indeterminate
growth. Some long-lived trees, such as bristlecone pines and
the California redwoods, continue to grow for thousands
of years. Others, particularly annual plants, may cease vegetative development with the initiation of flowering after
only a few weeks or months of growth. Eventually the
adult plant undergoes a transition from vegetative to reproductive development, culminating in the production of a
zygote, and the process begins again. Reproductive development will be discussed in Chapter 24.
Cells derived from the apical meristems exhibit specific
patterns of cell expansion, and these expansion patterns
determine the overall shape and size of the plant. We will examine how plant growth is analyzed after discussing meristems, with an emphasis on growth patterns in space (relationship of plant structures) and time (when events occur).
Finally, despite their indeterminate growth habit, plants,
like all other multicellular organisms, senesce and die. At
the end of the chapter we will consider death as a developmental phenomenon, at both the cellular and organismal
levels. Foe an historical overviw of the study of plant
development, see Web Essay 16.1.
EMBRYOGENESIS
The developmental process known as embryogenesis initiates plant development. Although embryogenesis usually
begins with the union of a sperm with an egg, forming a
single-celled zygote, somatic cells also may undergo
embryogenesis under special circumstances. Fertilization
also initiates three other developmental programs: endosperm, seed, and fruit development. Here we will focus on
embryogenesis because it provides the key to understanding plant development.
Embryogenesis transforms a single-celled zygote into a
multicellular, microscopic, embryonic plant. A completedembryo has the basic body plan of the mature plant and
many of the tissue types of the adult, although these are
present in a rudimentary form.
Double fertilization is unique to the flowering plants
(see Web Topics 1.1 and 1.2). In plants, as in all other
eukaryotes, the union of one sperm with the egg forms a
single-celled zygote. In angiosperms, however, this event
is accompanied by a second fertilization event, in which
another sperm unites with two polar nuclei to form the
triploid endosperm nucleus, from which the endosperm(the tissue that supplies food for the growing embryo) will
develop.
Embryogenesis occurs within the embryo sac of the
ovule while the ovule and associated structures develop
into the seed. Embryogenesis and endosperm development
typically occur in parallel with seed development, and the
embryo is part of the seed. Endosperm may also be part of
the mature seed, but in some species the endosperm disappears before seed development is completed. Embryogenesis and seed development are highly ordered, integrated processes, both of which are initiated by double fertilization. When completed, both the seed and the embryo
within it become dormant and are able to survive long
periods unfavorable for growth. The ability to form seeds
is one of the keys to the evolutionary success of
angiosperms as well as gymnosperms.
The fact that a zygote gives rise to an organized embryo
with a predictable and species-specific structure tells us
that the zygote is genetically programmed to develop in a
particular way, and that cell division, cell expansion, and
cell differentiation are tightly controlled during embryogenesis. If these processes were to occur at random in the
embryo, the result would be a clump of disorganized cells
with no definable form or function.
In this section we will examine these changes in greater
detail. We will focus on molecular genetic studies that have
been conducted with the model plant Arabidopsis that have
provided insights into plant development. It is most likely
that most angiosperms probably use similar developmental mechanisms that appeared early in the evolution of the
flowering plants and that the diversity of plant form is
brought about by relatively subtle changes in the time and
place where the molecular regulators of development are
expressed, rather than by different mechanisms altogether
(Doebley and Lukens 1998).
Arabidopsis thaliana is a member of the Brassicaceae, or
mustard family (Figure 16.1). It is a small plant, well suited
for laboratory culture and experimentation. It has been
called the Drosophila of plant biology because of its widespread use in the study of plant genetics and molecular
genetic mechanisms, particularly in an effort to understand
plant developmental change. It was the first higher plant
to have its genome completely sequenced. Furthermore,
there is a concerted international effort to understand the
function of every gene in the Arabidopsis genome by the
year 2010. As a result, we are much closer to an understanding of the molecular mechanisms governing Arabidopsis embryogenesis than of those for any other plant
species.Embryogenesis Establishes the Essential Features
of the Mature PlantPlants differ from most animals in that embryogenesis does
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