Plant Development

Meristems

Plant meristematic tissues are cells that divide in order to give rise to various organs of the plant and keep the plant growing.

Meristems

The adult body of vascular plants is the result of meristematic activity. Plant meristems are centers of mitotic cell division, and are composed of a group of undifferentiated self-renewing stem cells from which most plant structures arise. Meristematic cells are also responsible for keeping the plant growing. The Shoot Apical Meristem (SAM) gives rise to organs like the leaves and flowers, while the Root Apical Meristem (RAM) provides the meristematic cells for the future root growth. The cells of the shoot and root apical meristems divide rapidly and are considered to be indeterminate, which means that they do not possess any defined end fate. In that sense, the meristematic cells are frequently compared to the stem cells in animals, which have an analogous behavior and function.

Meristem tissue and plant development

Meristematic tissues are cells or group of cells that have the ability to divide. These tissues in a plant consist of small, densely packed cells that can keep dividing to form new cells. Meristematic tissue is characterized by small cells, thin cell walls, large cell nuclei, absent or small vacuoles, and no intercellular spaces.

Meristematic tissues are found in many locations, including near the tips of roots and stems (apical meristems), in the buds and nodes of stems, in the cambium between the xylem and phloem in dicotyledonous trees and shrubs, under the epidermis of dicotyledonous trees and shrubs (cork cambium), and in the pericycle of roots, producing branch roots. The two types of meristems are primary meristems and secondary meristems.

Meristem Zones

The apical meristem, also known as the "growing tip," is an undifferentiated meristematic tissue found in the buds and growing tips of roots in plants. Its main function is to trigger the growth of new cells in young seedlings at the tips of roots and shoots and forming buds. Apical meristems are organized into four zones: (1) the central zone, (2) the peripheral zone, (3) the medullary meristem and (3) the medullary tissue.

The central zone is located at the meristem summit, where a small group of slowly dividing cells can be found. Cells of this zone have a stem cell function and are essential for meristem maintenance. The proliferation and growth rates at the meristem summit usually differ considerably from those at the periphery. Surrounding the central zone is the peripheral zone. The rate of cell division in the peripheral zone is higher than that of the central zone. Peripheral zone cells give rise to cells which contribute to the organs of the plant, including leaves, inflorescence meristems, and floral meristems.

An active apical meristem lays down a growing root or shoot behind itself, pushing itself forward. They are very small compared to the cylinder-shaped lateral meristems, and are composed of several layers, which varies according to plant type. The outermost layer is called the tunica, while the innermost layers are cumulatively called the corpus.

Genetic Control of Flowers

A variety of genes control flower development, which involves sexual maturation and growth of reproductive organs as shown by the ABC model.

Genetic Control of Flowers

Flower development is the process by which angiosperms produce a pattern of gene expression in meristems that leads to the appearance of a flower. A flower (also referred to as a bloom or blossom) is the reproductive structure found in flowering plants. There are three physiological developments that must occur in order for reproduction to take place:

1. the plant must pass from sexual immaturity into a sexually mature state
2. the apical meristem must transform from a vegetative meristem into a floral meristem or inflorescence
3. the flowers individual organs must grow (modeled using the ABC model)

Flower Development

A flower develops on a modified shoot or axis from a determinate apical meristem (determinate meaning the axis grows to a set size). The transition to flowering is one of the major phase changes that a plant makes during its life cycle. The transition must take place at a time that is favorable for fertilization and the formation of seeds, hence ensuring maximal reproductive success. In order to flower at an appropriate time, a plant can interpret important endogenous and environmental cues such as changes in levels of plant hormones and seasonable temperature and photoperiod changes. Many perennial and most biennial plants require vernalization to flower.

Genetic Control of Flower Development

When plants recognize an opportunity to flower, signals are transmitted through florigen, which involves a variety of genes, including CONSTANS, FLOWERING LOCUS C and FLOWERING LOCUS T. Florigen is produced in the leaves in reproductively favorable conditions and acts in buds and growing tips to induce a number of different physiological and morphological changes.

From a genetic perspective, two phenotypic changes that control vegetative and floral growth are programmed in the plant. The first genetic change involves the switch from the vegetative to the floral state. If this genetic change is not functioning properly, then flowering will not occur. The second genetic event follows the commitment of the plant to form flowers. The sequential development of plant organs suggests that a genetic mechanism exists in which a series of genes are sequentially turned on and off. This switching is necessary for each whorl to obtain its final unique identity.

ABC Model of Flower Development

In the simple ABC model of floral development, three gene activities (termed A, B, and C-functions) interact to determine the developmental identities of the organ primordia (singular: primordium) within the floral meristem. The ABC model of flower development was first developed to describe the collection of genetic mechanisms that establish floral organ identity in the Rosids and the Asterids; both species have four verticils (sepals, petals, stamens and carpels), which are defined by the differential expression of a number of homeotic genes present in each verticil.

In the first floral whorl only A-genes are expressed, leading to the formation of sepals. In the second whorl both A- and B-genes are expressed, leading to the formation of petals. In the third whorl, B and C genes interact to form stamens and in the center of the flower C-genes alone give rise to carpels. For example, when there is a loss of B-gene function, mutant flowers are produced with sepals in the first whorl as usual, but also in the second whorl instead of the normal petal formation. In the third whorl the lack of B function but presence of C-function mimics the fourth whorl, leading to the formation of carpels also in the third whorl.

Most genes central in this model belong to the MADS-box genes and are transcription factors that regulate the expression of the genes specific for each floral organ.