Shoot Apical Meristem (SAM)
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The shoot apical meristem (SAM) is a population of cells located at the tip of the shoot axis. It produces lateral organs, stem tissues and regenerates itself. In most plants little to no shoot tissue results from embryogenesis: essentially the entire shoot system of plants derives from postembryonic development in the SAM.
Shoot growth consists of primary and secondary growth. Primary growth is the addition of tissue to the shoot by the apical meristem. Secondary growth is tissue derived from zones other than the meristem, such as cambial zones.
Control of organ initiation patterns
Three major hypotheses exist to explain how organ initiation patterns are generated. These are not mutually exclusive hypotheses, although proponents argue about them as if they were.
Inhibitory field hypothesis:
One way to explain the pattern of organ initiation is that organ primordia produce an inhibitory field that prevents organ initiation within a certain proximity. Surgical manipulations in which the position of organ initiation was assessed following removal of a primordium are generally consistent with this theory. Subsequent organs that would normally have contacted the removed primordium initiated at positions closer than normal to the missing primordium. A corollary is that the apical dome should also produce an inhibitory field.
Auxin appears important for leaf initiation. Application of auxin transport inhibitor to an apex results in the simultaneous initiation of primordia in a continuous collar surrounding the apex. If the auxin transport inhibitor is spotted on primordia, they occupy a larger position on the meristem. An inhibitor of auxin action spotted onto primordia caused them to occupy a smaller area. Both these treatments resulted in altered phyllotaxy. The results suggest that a localized high auxin concentration promotes organ initiation and determines the size of the field occupied by the primordium. The inability to transport auxin away from the apex results in a uniformly high concentration causing the continuous collar. The inability to transport auxin away from a primordium results in a higher localized concentration and larger field.
According to biophysical explanations of phyllotaxy, patterns of stress and reinforcement in the surface layer of the apex determine the pattern of organ initiation. The inner tissues are hypothesized to provide the driving pressure, much like the air in a balloon. Polarized light microscopy shows a correlation between phyllotaxy and cellulose microfibril orientation in the apex surface.Results of the surgical and auxin experiments are not incompatible with this model because the surgery might have altered the pattern of physical stress in the apex and the auxin may have affected reinforcement by cell wall loosening.
The protein expansin is also able to induce cell wall loosening. Expansin coated beads were placed on meristems in a position other than the site of the next predicted primordium initial. This treatment inhibited formation of the primordium that normally would have formed and allowed the formation of a primordium in the ectopic location. Ie. the pattern of phyllotaxy was altered. The new phyllotaxy was propagated in subsequent primordia. This supports the role of biophysical properties of the surface in organ initiation. Expansin proteins are expressed at sites of organ initiation in the SAM, supporting the possible role of expansin in normal organ initiation but how the pattern of protein accumulation is regulated is not yet known.
There are several genes known to be important for meristem function. The knotted gene of maize and STM gene of Arabidopsis encode homeodomain proteins that are expressed in the apical meristem. Expression patterns are consistent with the proposed function of promoting meristem function/inhibiting differentiation. Both are expressed throughout the meristem except in organ primordia. Loss of expression precedes visible histological appearance of the primordium. clavata mutants of arabidopsis have abnormally large meristems with altered phyllotaxy. Specifically, the central zone is enlarged and more than the normal number of organs are formed (this is most apparent in flowers). This is consistent with inhibitory fields controlling primordia initiation. Another gene of maize called abphyl1 also has an enlarged meristem with altered phyllotaxy. Instead of forming leaves alternately, mutants form leaves oppositely. The expression pattern of knotted is altered in accordance with the pattern of leaf initiation but the identity of abphyl1 is not yet known.
Determination of leaf primordia was studied in ferns by explanting primordia to sterile culture free of hormones. Plastochron 2 primordia usually developed shoots (ie. they still had a meristem identity. By plastochron 8, primordia are 1 mm and developed into normal looking leaves (although smaller than usual). The primordia acquired leaf identity and contained the information to develop autonomously.
Leaves are determinate organs. That is they grow to a specified size and no further. Thus along with the acquisition of leaf identity is the loss of indeterminate growth.
Clonal analysis 1. fate of meristem cells in the shoot.
chimera—a plant composed of genetically different cells (ie. a genetic mosaic)
periclinal chimera—a chimera in which entire cell layers of the apical meristem are genetically different from one another.Albino mutations are commonly used to mark cell lineages. These chimeras are described with G and W according to the cell layer. Thus chimeras with a white L1 and green L2 and L3 would be denoted W-G-G, white L2 would be G-W-G, white L2 and L3, G-W-W etc.
mericlinal or sectorial chimera—chimeras that do not occupy the whole circumference of the meristem
Genetically marking cells within meristems allows the cells to be followed and their fates revealed. Such lineage studies have revealed several things about meristems. First, periclinal chimeras are fairly stable indicating that the tunica corpus arrangement of cells in the meristem represent stable cell lineages. This allows propagation of several variegated horticultural plants.
Rarely, cells from one apical layer will enter another layer. In ivy a clonal analysis was done by examining rare changes in the variegation pattern. The few permanent changes observed affected a quarter of the shoot indicating that each apical layer contained 4 apical initial cells. Most changes were ephemeral and the longest ephemeral changes occupied 8 nodes indicating that one daughter of the apical initials contributed to 8 nodes worth of growth.
Fate maps have been generated for the apical meristem in the embryo of several plants by using ionizing radiation on seeds to induce albino sectors. The general pattern observed is that a few cells at the center of the meristem contribute to many nodes at the top of the plant, while many cells on the sides of the meristem contribute to few nodes at the bottom of the plant.
Clonal analysis 2. fate of meristem cells in lateral organs.
Examination of the pattern of green tissues on leaves of periclinal chimeras reveals the fate of each cell layer in the organ. This analysis shows that L1 cells contribute the epidermis and in some plants, tissues around the leaf margin. L2 cells contribute most of the mesophyll tissue and L3 cells contribute some of the mesophyll in the central region of the leaf near the midvein. In G-W-G chimeras, the occasional invasion of epidermally derived cells into the mesophyll is easy to detect. Single green cells have been observed in otherwise albino areas of mesophyll. These cells differentiate as mesophyll cells indicating that cell fate is not determined until after the last cell division and emphasizing the independence of cell lineage and cell differentiation.
A study was done in maize where sectors were induced at the time of leaf initiation and the relative contribution of the sectors were analyzed in the resultant leaf. By analyzing the fractional contribution of the sectors to the leaf, the number of apical meristem cells that form the primordium could be estimated. Epidermal sectors either occupied 2/3 of the top side, 2/3 of the bottom side or 1/3 the bottom and top sides at the tip. This indicated that 3 cells in the longitudinal dimension contributed to leaf formation. Sectors occupied either the epidermis or mesophyll indicating that at least 2 cells in depth contributed to the leaf. Most sectors occupied 1/40th the width of the leaf indicating that the primordium was 40 cells wide. Thus roughly 240 cells contributed to the formation of a maize leaf.
Development in the meristem is highly regulative
The late stage at which cell fate is determined (ie. after the last cell division) and the ability of cells to invade different cell layers and differentiate according to their final position indicate a highly regulative development. This can be further demonstrated by inducing periclinal chimeras in which clones have different growth characteristics. The relative contributions of different cell layers to leaves can be altered in various chimeras but the overall size and shape of the meristem or leaves does not change.
There are two types of branching patterns in plants: terminal and lateral.
Terminal branching involves the separation of the apical meristem into 2, which then both continue growing as a shoot branch. This mechanism is common in lower plants but rare in angiosperms. How the single integrated growing center is divided is not understood. Experimental manipulations such as bisecting the meristem can artificially induce terminal branching, but these do not appear to mimic any naturally occurring events. They do speak to the capacity of isolated meristem segments to reorganize into complete meristems.
Lateral branching in seed plants occurs mainly by growth of axillary buds. When leaf primordia are initiated, a small group of cells in their axils remain small and unvacuolated. These are referred to as “detached meristems” because they retain meristematic characteristics and do not differentiate as the cells around them do, but they become spatially separated from the apical meristem. As the shoot grows and the detached meristems attain a distance from the apex, they organize into a shoot meristem with several leaf primordia (a bud). At this point they arrest because of apical dominance (the shoot apical meristem inhibits growth of the axillary meristems by producing the hormone auxin). In bushy plants the arrest is temporary until the main shoot has grown beyond the inhibitory range of the apex. In unbranched plants the arrest is permanent unless the apical meristem is removed. Upon release from apical inhibition, the lateral bud begins growth as a branch shoot.