Role of Meristems in the Plant Development

Plants are unique multicellular organisms that possess the capacity for unlimited growth throughout their lives. This potentiality is due to stem cells (Laux 2003, Byrne et al. 2003) located within meristems (Figure 6-14, inserts). Stem cells perpetuate themselves by cell division and give rise to derivatives that differentiate to form tissues. This results in a repeated initiation of new organs throughout the plant life.

Plant meristems are classified on the basis of their location and function. A plant seedling typically starts out with two meristems. They are primary meristems located at the tip of the shoot and close to the tip of the root (Figure 6-14). They are responsible for primary growth of these organs, i.e. the elongation of the plant body.

Lateral meristems are located on the sides of roots or stems, and are involved in the secondary growth, i.e. the increase in diameter of these organs. These meristems or cambiums are the origin of cork and secondary vascular tissues. The vascular cambium form tissues that are involved in the conduction of water and salts (xylem) or sugars and organic compounds (phloem).

The secondary xylem is called wood. It is composed of several types of cells among which fibers and vessels are lignified which gives rise to the properties of resistance of wood to mechanical stresses. Wood is also greatly responsible for the rigidity of the plant body and allows a vertical growth of woody plant the mass of which can be enormous (commonly several tons). Its formation is dependent upon gravity and this tissue can have different properties in the trunk and in lateral branches. In seedlings that do not possess secondary xylem, structural support for the plant body is made possible by the presence of cell walls. Turgor pressure inside cells also contributes to the rigidity of the organs.

Axillary bud

MB^BWHBBHBi^Bi^Bo^Hi^Bi^BiBHB

Apical meristem

S 1

t||U j| w ■

i^iilli m» ¡iiii

■v: fwlil

Figure 6-14. Plant development. The primary axis of a plant is formed by the primary root and shoot. These organs bear secondary roots and lateral branches respectively. Their growth is due to meristems where cells are dividing rapidly (see inserts). The apical meristem is formed of three layers of cells (LI, L2, L3). Its centra/ zone (CZ) is composed of cells whose activity is regulated by the organizing center (QC) locatedjust below. The fastest cell cycle is observed in the lateral zone (LZ) where the leaf primordia are formed. Stem cells are considered to be located in the central zone. The root meristem is covered by the cap (RC). The quiescent center (QC) is regulating the activity of the stem cells (SC) that are around it. CC: cortical cells; VC: vascular cylinder. Adapted from Laux (2003).

4.1.2 The Plant Body

The adult plant is composed of the shoot-root axis, i.e., the stem and the primary root. Both organs harbor lateral appendices, which are leaves (or branches with leaves) and secondary roots, respectively (Figure 6-14). The origin of these organs is completely different.

In the primary root, the apical meristem is sub terminal and is covered by the root cap, which is involved in the perception of gravity, but it also protects the meristem during growth through the soil. Cell division occurs throughout the apical meristem but is regulated by the quiescent center, which is situated close to the root cap. The outer layer of cells, the epidermis, also protects the root and gives rise to root hairs, which are mainly responsible for water and salt uptakes.

Above the meristem, two regions can be distinguished: the cortex and the vascular cylinder. The outermost cell layer of the vascular cylinder is the pericycle and is surrounded by another layer of cells, the endodermis (Figure 6-15). The former is responsible for a selective screening in the uptake of minerals and the latter is involved in the formation of lateral roots. These organs arise by local divisions in this tissue and have therefore an internal origin. The first divisions are anticline (perpendicular to the root surface) and the following divisions are pericline (parallel to the root surface). This activity is linked to a high concentration of the hormone auxin (3-indole acetic acid) at a certain level of the pericycle (Benkova et al. 2003, Blilou et al. 2005). The auxin gradient within the root primordium is due to the cellular efflux of this hormone mediated by specific carriers belonging to the class of the PIN proteins (Muday and DeLong 2001).

Figure 6-15. Lateral root formation, auxin distribution and auxin transport in Arabidopsis thaliana. I: The fist divisions, which occur in the pericycle, are anticline. They are adjacent to the vascular pole (first vessel formed). II: Pericline divisions take place to form the root primordium. Ill: The primordium grows within the cortex, IV: The arrows indicate the auxin transport within the root primordium. Grey areas show the expression of the reporter gene DR5::GUS. which indicates the zones where there is high concentration of the hormone auxin (the darker the area, the higher the concentrations). Adapted from Benkova et al. (2003).

Figure 6-15. Lateral root formation, auxin distribution and auxin transport in Arabidopsis thaliana. I: The fist divisions, which occur in the pericycle, are anticline. They are adjacent to the vascular pole (first vessel formed). II: Pericline divisions take place to form the root primordium. Ill: The primordium grows within the cortex, IV: The arrows indicate the auxin transport within the root primordium. Grey areas show the expression of the reporter gene DR5::GUS. which indicates the zones where there is high concentration of the hormone auxin (the darker the area, the higher the concentrations). Adapted from Benkova et al. (2003).

The shoot apex is covered by a series of leaf primordia with the youngest being closest to the top (Figure 6-16A). The shoot apical meristem is restricted to the part of the shoot apex with is above the youngest primordia. As in the root apex, the central zone is a region of low mitotic activity. The cells of this zone divide slowly and produce cells below and on the sides, which give rise to the rib meristem and the peripheral zone, respectively. Cell division is intense in localized areas in the peripheral zone where lateral organs such as leaves are initiated and in the rib meristem that contributes to internodal elongation. The surface and sub-epidermal layers are designated LI, L2, and L3 (Gross-Hardt and Laux 2003, Castellano and Sablowski 2005). LI and L2 divide anticlinally, whereas L3 divides both anticlinally and periclinally (Figure 6-16B).

Leaves are highly plastic organs and show a great variety of form and size. Typical leaves are flat, dorso-ventral organs adapted for photosynthesis. It is now well established that a dorsiventrally flattened lamina requires adjacent abaxial and adaxial domains (Byrne 2005, Bowman et al. 2002). A loss of either the adaxial or abaxial domain results in a complete or partial radialisation of the leaf.

Figure 6-16. Leaf'formation and auxin transport. A. Transverse section in the shoot meristem showing the formation of the primordio (PI to P5) in Arabidopsis thaliana. P0 indicates the zone where the next primordium will be formed. The primordio present very early an abaxial and an adaxial domains (represented by different gray levels) which are characterized by a differential expression of some genes (PHB/PHV/REV). It is hypothesized that primordio represent a sink for auxin (they need auxin to develop). The youngest they are the more they need this hormone. Depletion of auxin in some areas should inhibit primordium formation, so that new leaf can be only initiated in a given area of the shoot meristem. Adapted from Castellano and Sablowski (2005), and Byrne (2005). B. Auxin transport in the young primordium. This hormone is transported in the peripheral layers toward the top of the primordium and then toward its center. Adapted from Benkova et al. (2003).

Figure 6-16. Leaf'formation and auxin transport. A. Transverse section in the shoot meristem showing the formation of the primordio (PI to P5) in Arabidopsis thaliana. P0 indicates the zone where the next primordium will be formed. The primordio present very early an abaxial and an adaxial domains (represented by different gray levels) which are characterized by a differential expression of some genes (PHB/PHV/REV). It is hypothesized that primordio represent a sink for auxin (they need auxin to develop). The youngest they are the more they need this hormone. Depletion of auxin in some areas should inhibit primordium formation, so that new leaf can be only initiated in a given area of the shoot meristem. Adapted from Castellano and Sablowski (2005), and Byrne (2005). B. Auxin transport in the young primordium. This hormone is transported in the peripheral layers toward the top of the primordium and then toward its center. Adapted from Benkova et al. (2003).

Recent work has revealed that one of the signals for the initiation of leaf primordia might be auxin, which is continuously transported to the shoot meristem and controls the position at which cells are recruited into organogenesis (Castellano and Sablowski 2005). Primordia emerge in regions of the meristem that have high auxin concentration. Once a primordium is established, it functions as a sink for auxin (this hormone is concentrated at this level), depleting auxin in the surrounding cells (Figure 6-16A). The next primordium can only emerge at a certain distance, in a position at which auxin can accumulate. Auxin enters the primordia via the peripheral layers whereas it flows out of them via the internal tissues (Figure 6-16B).

Axillary buds are often initiated at the flank of the shoot meristem by cell division in the axil of leaves (see Figure 6-14). The axillary bud is formed by the shoot apex and leaf primordia.

0 0

Responses

  • Frances
    What is the role of meristem in plant?
    1 year ago
  • amanuel
    What is meristemDiscuss it's role in plants?
    1 year ago
  • Mirrin Mitchell
    What is the importance of meristems on development of plant body?
    8 months ago
  • amanuel
    What are function of meristem to development of the plant body?
    7 months ago

Post a comment