Meristematic Plant Development
Phyllotaxis is known as the study of plant patterns. Diversity is present, still similar patterns can be observed in many different types of plants. A common pattern consists of two sets of spiral that form a lattice i.e., observed in the stamens and carpels of flowers, the florets of compound flowers, the scales of pine cones, cycads and seed ferns. This type of pattern is known as Spiral Phyllotaxis. The apex is known as the tip of a plant shoot. The region that contains undifferentiated stem cells is apex. This region is called as apical meristem. The production of plant organs such as leaves, thorns, tendrils, sepals, petals etc. is carried by apical meristem. Apical ring is present near the boundary of apical meristem. The process is observed under the microscope. When extensive cell division start it leads to the formation of plant organ in the spot along the apical ring. Primordia exhibit the phyllotactic pattern which is preserved and developed in various plant organs.
Fibonacci numbers appear naturally as a consequence of the dynamical systems approach. Many research work shows that Fibonacci numbers promoted the survival of plants. The prevalence of the Fibonacci numbers was then seen as a consequence of Darwinian Theory of Evolution; survival of the fittest is the consequence of the Fibonacci numbers. The adult plants preserve the pattern of the early development. This does not contradict the theory of evolution of course. Natural selection is acting to promote various but some typical types of developmental processes.
Two important processes in plant growth at the cellular level are:
- Cell division
- Cell expansion
These two processes are responsible for the change in the shape and size of a plant and its various organs. These two processes share no casual relationship. Growth of plant cells depend mainly on water consumption although they consume many other compounds also the cell division of plant cells consist of the duplication of nucleus and the building of cell wall to separate the two new nuclei. The parent cell gives rise to complete two daughter cells. A plant shoot is approximately a cylinder so it is convenient to use a cylindrical coordinate system to indicate position on a shoot. Cell division is classified into three types according to the orientation of the new cell wall with respect to the three mutually orthogonal directions of the cylindrical coordinate system. In transverse cell division, the new cell wall is roughly orthogonal to the axis of the shoot, in periclinal cell division the new cell wall is parallel to the surface of the plant and in anticlinal cell division the new wall is contained in a plane passing through the axis of the shoot.
There are two basic types of development in plants:
The embryonic development results in seed or spore. After dormancy period is over seed begins to absorb water and cells undergo expansion, this results in the bursting of new plant, this is known as germination. After germination, if all goes well, the seedling has a root growing down into the ground and a shoot growing up from the ground. The line formed by the root and shoot form the main axis of the plant. In meristematic development, some species of plants have meristems that are present before germination while in other species the meristems form after germination. In the plant, there may be present a clear boundary between embryonic and meristematic development. The entire plant organ develops from the primodium. The type of cells that develop is not completely determined until after the formation of the primordium. A variety of factors decide which type of plant organ develops.
Development of seedling Plant embryogenesis is followed by germination. Germination marks a switch from an anabolic phase of nutrient accumulation to a catabolic phase of nutrient consumption and growth. There are two stages separated by period of quiescence, where embryo become desiccated and is stored in seed.
Environmental influences to which seedling respond:
Gravity: It ensures shoot grows upward and root grows downwards.
Light: It enables seedling to extend through soil in dark and then switch to vegetative growth once it has emerged at surface.
Transition to germination
It is controlled by abscisic acid and gibberlic acid. Abscisic acid inhibits growth and gibberellic acid promotes growth. There are number of genes which affect transition when mutated but are not involved in hormone synthesis. Instead these genes encode regulatory proteins that control downstream embryo specific and seedling specific gene expression. Mutations in these genes cause appearance of trichomes on cotyledons. Trichomes are epidermal hair usually found on leaves. For example LEC1 and FUSCA 3
Dark development or Skotomorphogenesis
It is the first stage of the germination. It is characterized by rapid extension of shoot upwards through soil. Shoot is pale and spindly, condition described as etiolated. Light development or Photomorphogenesis It is the type of development where seedling emerges from soil and is exposed to light. The seedling turns green as it begins to accumulate chlorophyll. Photosynthesis begins and shoot apical meristem is activated leading to the formation of true leaves.
Shoot and root development
The vascular plant begins its existence as a single cell, the zygote. The zygote grows into embryo; the early embryo is globular whereas a mature embryo has a defined apical basal growth axis. This polar structure has two distinct zones during longitudinal axis formation. The two zones have the capacity of continuous growth and are set apart at opposite poles. These regions are the apical meristems, one producing the shoot system, the other producing the root system. These are open ended indeterminate growth systems from which the same kinds of organs and/or tissues are produced continuously and which result in the primary plant body.
In response to environmental conditions, such as photoperiod and low temperature, the shoot apical meristem may undergo transition to a floral state. In some cases, the meristem become determinate and ceases to produce new organs and most root meristems remain indeterminate, although lateral roots which branch off a primary axis can become determinate. Shoot buds containing meristematic cells give rise both to terminal apices and to lateral. Roots also branch profusely, but from meristematic tissue deep within the root axis, so generating extensive root systems typical of most land plants. In monocotyledons, an intercalary meristem located at each node of the stem provides the facility for continued longitudinal growth if the shoot tip is destroyed. Plants with extended lifespans have additional meristem layers called cambium which develop within roots and stems, and lead to an increase in girth along the plant‘s longitudinal axis. Vascular cambium generates extra conducting tissue; cork cambium produces protective tissue, replacing the functions of epidermis in stems, and cortex and epidermis in roots. Cambial meristems and their derivative tissues are referred to as the secondary plant body. Although no new organs are produced by these lateral meristems, the secondary plant body may constitute the bulk of the plant, for example a tree‘s trunk, branches and roots. The group of cells which generates the vascular tissues including the pericycle in the roots of higher plants is called procambium. It is a meristematic tissue that is concerned with providing the primary tissues of the vascular system.