The mode of development in animals and plants is different Outcome of animal embryogenesis is a mini edition of the adult Outcome of plant embryogenesis is a simple structure with -root apical meristem -embryonic root -hypocotyl -1 or 2 cotyledons -shoot apical meristem All the other organs of the plant are formed postembryogenically in the meristems
In contrast with animal development, plant development is an ongoing process WHY? General Sherman sequoia -2,200 years old and still growing
Plants cannot move out of their environment so they react to environmental changes (e.g. changes in light, temperature, soil conditions, water availability, day length) by changing their growth and development Plants grown under different light conditions
This week Introduction to meristems 1. What are meristems? 2. Where are the meristems in a plant? 3. When do meristems develop? (4. How do we know which cells give rise to each tissue?)
What are Meristems? - growing points in the plant from which develop new tissues and organs develop e.g the shoot apical meristem gives rise to the stems, leaves, and reproductive structures
Meristems are tiny (rarely more than 250 micrometres) 1000 shoot apical meristems in an eppendorf test tube
Meristem sizes vary but are all very small
Meristems are tiny (rarely more than 250 micrometres) 1000 shoot apical meristems in an eppendorf test tube The shoot apical meristem stays the same size as cells continually leave it and begin to differentiate.
Meristems can be indeterminate (show no predetermined limit to growth) -in some species the meristems can grow for thousands of years Methuselah (a Bristlecone Pine, Pinus longaeva) is about 4,841 years old (it was more than 200years old when the pyramids were built). California
As long as conditions are favorable, the vegetative apical meristems will form phytomers phytomer modular units consisting of a leaf, bud and internode (which contains a meristem).
How can meristems keep making new tissues and at the same time regenerate themselves (self-perpetuate)? Some meristematic cells remain undifferentiated and retain the capacity for cell division indefinitely. These are called stem cells (or initial cells )
What are stem-cells? Relatively undifferentiated cells that can divide to give rise to one daughter cell that continues to be a stem cell and another that will differentiate into plant organs and tissues Stem cell division and differentiation
How is the fate of the daughter cells decided after stem cell division? The fate of each individual daughter cell is determined by its relative position. Those that stay at the most apical position of the shoot meristem, that is in a stem cell niche, renew the stem cell population, whereas those that are displaced from this position differentiate.
Characteristics of plant stem-cells -are found in meristems -divide slowly -are the ultimate source of the tissues that make up the plant body -identity of stem cells is not an inherent property of the cell-lineage, but conferred by positional cues
Animal and plant stem-cells have different tasks Animal stem-cells - replenish highly specialized body cells with limited life spans e.g. blood and skin cells Plant stem-cells - provide material for formation of entire new organs e.g. leaves, flowers and roots
Stem cells in plants have different tasks from those in animals but may be regulated in a similar way. -are found in stem-cell niches -are maintained in an undifferentiated state by signals from surrounding cells
2. Where are the meristems in a plant?
There are two categories of meristems found in plants Apical Responsible for increase in length Lateral Responsible for increase in girth "the meristem makes the plant."
Apical and lateral meristems
Shoot apical meristems -produces stems, leaves, and reproductive structures
Shoot apical meristems -located at the extreme tip of the shoot -flat or mounded region with small thin-walled cells containing dense cytoplasm but no large central vacuoles Cross section of a shoot apical meristem
Cross section of a cabbage showing the shoot apical meristem
Shoot apical meristems have different zones CZ Rib PZ CZ central zone contains the stem cells Rib zone generates the central tissues of the stem PZ peripheral zone generates organ primordia
Primordia -localised regions of the apical meristem that have high levels of cell division and give rise to cells with an identifiable function e.g. leaves Leaves
Organ primordia form at the edge of the meristem Cross section of a shoot apical meristem
Scanning electron micrograph of a shoot apical meristem from above Primordia forming at the edge of the meristem
Shoot apical meristems are composed of up to three distinct layers of cells: L1, L2, and L3/corpus L1 (outermost layer, 1 cell thick) L2 (lies beneath L1, 1 cell thick) L3 (inner most layer) Cross section of a shoot apical meristem
Cell division in the apical meristem L1 and L2 cells divide anticlinaly to generate the epidermis and ground tissue L3 cells divide in any plane and generate the internal tissues of the shoot L1 L2 L3
Where are the stem cells in the shoot apical meristem? Each of the meristem layers (L1-L3) probably contains 1-3 stem cells (shown darkly shaded)
Root apical meristems -produces the primary root only (no lateral organs) Secondary root primary root
In the root the meristem lies behind the root cap (in Arabidopsis it is about 0.25mm long) Cell differentiation Cell elongation Vascular tissue cortex Epidermis Cell division Root Apical meristem Root cap (protects meristem)
In the root of a young seedling stem cells in the root divide rapidly to produce long files (rows) of cells Cross section of a root apical meristem
There are two categories of meristems found in plants Apical Responsible for increase in length Lateral Responsible for increase in girth
Lateral Meristems - cylindrical meristems found in shoots and roots that result in secondary growth e.g. the vascular cambium located between the xylem and phloem in woody plants - found in all woody and some herbaceous plants
Lateral Meristems located between the phloem and xylem. On its inside surface, vascular cambium produces new xylem cells, on its outside new phloem cells.
Tree rings are generated by the vascular cambium Cross section of a portion of tree trunk
Intercalary meristem - found in monocot stems (particularly grass stems) - meristem at the base of the internode
Rows of cells being produced by the intercalary meristem Cross section of a monocot stem
Intercalary meristem -only the apical meristem (node) is active. -If the tip of the stem is removed, the uppermost intact intercalary meristem becomes the apical meristem Cross section of monocot stem
When do the meristems develop? Primary meristems Shoot and root apical meristems arise during embryogenesis. Secondary meristems Other meristems e.g.intercalary meristems arise during post-embryonic development. This shows that in plants differentiated cells can reverse their state and go back to being undifferentiated meristem cells.
Not all meristems are indeterminate As plants change from a vegetative to a reproductive state so does the meristem. vegetative reproductive
The transition to flowering involves major changes in the pattern of morphogenesis and cell differentiation at the shoot apical meristem
During vegetative growth the Arabidopsis shoot apical meristem produces phytomeres with very short internodes. As reproductive development is initiated, the vegetative meristem is transformed into an determinate inflorescence meristem. Floral meristems are produced on the sides of the inflorescence meristem.
Vegetative meristem to Inflorescence meristem to Floral meristem
Arabidopsis inflorescence meristem with floral meristems produced at the sides
Scanning electron micrograph of an Arabidopsis infloresense meristem floral bud sepals Organ primordia forming (in this case floral buds with floral meristem ) Stem cells
The anatomy of an Arabidopsis flower stamen petal anther filament stigma carpels style ovary 1 mm sepal
4. How do we know which cells give rise to each tissue?
Label a single cell and follow its fate To label a single cell -make transgenic plants with the 35S::Ac::GUS construct -when the mobile Ac element is excised the 35S promoter will switch on the GUS gene
Label a single cell and follow its fate A single cell in an inflorescence meristem is labeled, this results in one floral meristem being labeled.
Contributions of the meristem layers to Arabidopsis flowers GUS staining -of cross sections of floral meristem L1 L2 L3 -whole flowers
Diagram of the contributions of the meristem layers to Arabidopsis flowers L1 L2 L3 filament stigma anther petal style ovary 1 mm sepal
Next lecture: How does the plant maintain a stem cell population?