Arabidopsis thaliana is the developmental model organism. In plants, there is no special germ line in the embryo and germ cells arise from somatic cells during post-embryonic development, although they all appear to arrive from single cell layer. The plant embryo compromises two organ systems
- Embryonic axis: It gives rise to seedling.
- Cotyledons, either one or two, which provide nourishment as seedling grows. The plants with one cotyledon are monocot plants and one with two cotyledons is dicot plants.
Organization of root apical meristem
- Root apical meristem is the community of undifferentiated cells at the root tips, which give rise to tissues of the root via division.
- Within root there are zones where cells are in various stages of differentiation, all cells in the meristmatic region do not divide at the same rate or at the same frequency.
- The root apical meristem is the zone of cell division; root cap is a protective covering for the root tip.
- Quiescent center The central cells divide slowly in comparison to surrounding cells. The part of the root meristem where the central cells divide slowly is known as the quiescent centre.
- In the zone of maturation, mature cells are differentiated and epidermal cells have root hair.
- In the zone of elongation, cells become longer as they specialize.
- Closed organization The roots having a clear division between the root cap and the root apical meristem are said to show closed organization e.g., Arabidopsis. In mature Arabidopsis embryo, root apical meristem consists of cells derived from embryo and apical suspensor cells.
- Open organization The roots showing no clear division between the root cap and the root apical meristem are said to show open organization e.g., Pea.
Three fundamental cell layers established in embryo are
- L1 It gives rise to epidermis.
- L2 It gives rise predominantly to cortical parenchyma cells and other tissues underlying the epidermis.
- L3 It generates vascular tissues and pith.
Anticlinal divisions These occur in the plane of cell sheet and therefore expand the sheet without increasing its thickness.
Periclinal divisions Occur at right angles to plane of cell sheet and result in its expansion into multiple layers.
Embryonic axis is organized into regions representing:
- Shoot apical meristem (SAM)
- Future seedling shoot (Epicotyl and Hypocotyl)
- Embryonic root (Radicle)
- Root apical meristem (RAM)
Types of germination
- Epigeal germination: If germination is by elongation of hypocotyl it is known as epigeal germination
- Hypogeal germination: If germination is by elongation of epicotyl it is known as hypogeal germination.
- Development of seedling: Seed development comprises two major phases embryo development and seed maturation. Embryogenesis, which is a morphogenesis phase, starts with the formation of a single-cell zygote and ends in the heart stage when all embryo structures have been formed. It is followed by a growth phase during which the embryo fills the seed sac. At the end of the embryo growth phase, cell division in the embryo arrests. Hereafter, the seed, containing a full sized embryo, undergoes maturation during which food reserves accumulate and dormancy and desiccation tolerance develops. During final phase of development embryo‘s of orthodox‘ seeds became tolerant to desiccation, dehydrate losing upto 90% of water.
- Precocious germination: Germination of seeds without passing through the normal quiescent and/or dormant tage of development. Abscisic acid is known to inhibit precocious germination.
- Environmental influences to which seedling respond
- Gravity ensures shoot grows upward and root grows downwards.
- Light 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 gibberlic acid promotes growth. Low ABA and high bioactive GA can break seed dormancy. GA induces synthesis of hydrolytic enzymes in cereal grains. 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 e.g., 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.
Mutants undergo skotomorphogenesis in light and photomorphogenesis in dark.
Seedlings are de-etiolated due to det protein or constitutive photomorphogenic due to cop protein.
Det/cop/fus mutant proteins are responsible for short hypocotyls, open cotyledons in dark and not green seedlings.
Double mutants are det/cop/fus mutant combined with hy5. DET/COP/FUS genes function downstream of HY5.
Product of COP1 (FUS1) gene is localized in cytoplasm of hypocotyl cell in light and in nucleus in dark.
Leaves are lateral outgrowth of a plant shoot which are initiated by the shoot meristem. They have finite growth, are vascularized and usually photosynthetic.
After zygote formation, the embryo is formed inside the seed which leads to embryonic leaves and shoot apical meristem after germination. The embryonic leaves are known as cotyledons. In post-embryonic development, these embryonic leaves give rise to true leaves. The apical cell divides transversely to form upper tier and lower tier. Further transverse division in lower tier gives rise to upper lower tier and lower tier. The upper tier forms the apical tissues- the shoot meristem and cotyledons. The upper lower tier forms the base of cotyledons. The lower tier forms the hypocotyl and most of the root. The root tip including the meristem is derived from hypophysis.
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.