| Plastids | Perform metabolic functions. Descended from cyanobacteria symbiont. Semi-autonomous genome; encodes rRNA/tRNA + proteins for photosynthesis. Double membrane + thylakoids |
| Structure | Grana, stuck of thylakoids, CURVATURE THYLAKOID1 (CURT1) maintains grana structure; it bends the end of thylakoids by its oligomers. |
| Plastid differentiation | Proplastids in undifferentiated cells (where plastids derive). Etioplasts (ready to develop into chloroplasts upon light). Chloroplasts (chlorophyll and carotenoids). Chromoplasts (accumulate carotenoids). Amyloplasts (accumulate starch for sotrage). |
| Thylakoid biogenesis | Lamellae develop first, which is followed by grana stacks. In mature clp, there is no continuity between thylakoids and inner envelope. Thylakoid membranes have a unique lipid composition (Rich in galactolipids and Asymmetrically distributed). Thylakoid membrane proteins are also distributed asymmetrically. Thylakoid membranes are not made de novo but derive from the inner envelope (no lipid biosynthesis, lipids are made at the CIE). Vipp1 mutants have no thylakoids and no vesicles |
| Development of chloroplasts from etioplasts is light dependent. | Genes encoding photosynthetic proteins and enzymes are expressed, the proteins translated and imported into the organelle. The colourless chlorophyll precursor protochlorophyllide is converted to chlorophyll. The prolamellar body (PR) contains very high amounts of lipids that assemble with the newly synthesised membrane proteins to form the thylakoids. |
| Genome reduction | Clp contain about 3000 different proteins. But genomes have only 50-200 protein coding genes
What about the rest? Evidence for transfer of large segments of clp DNA to the nucleus. A small number of membrane proteins must be retained to avoid mistargeting |
| Plastid biogenesis | Requires transport of proteins from the nucleus to the organelle; Translocases. It also requires communication between the clp and the nucleus to coordinate expression levels. Plastids do not make their own proteins but acquire their proteins from the nucleus. |
| Chloroplasts divide | Independently of cell division. Complex process; 3 membranes to sort out. Division requires: ex-bacterial components (FtsZ) and dynamin-related proteins (DRP). Interference with any component results in defective division |
| Plastids ‘extensions’: stromules | Stromule function is still unclear: increase of surface area, initiate synthesis of starch grains in cereal seeds, potential connections with other organelles, Seem to be induced by pathogen attack. |
| Transplastomic plants | You can insert genes into the chloroplast genome; By particle bombardment. Every chloroplast will eventually express them (homoplasmy). Massive expression levels. Maternal inheritance |
| Vacuoles | Provacuoles are vesicles from the ER/Golgi. Large, vegetative (Lytic) vacuoles contain a dilute solution of: ions and organic acids, secondary metabolites, hydrolytic enzymes and metabolic waste products. Specialised protein storage vacuoles (PSV) store proteins in developing seeds (all dicots). |
| Protein Storage Vacuoles in mature seed | Cotyledon PSV (in seeds) and Radicle PSV (mature cells) |
| Cell wall: functions | A network of polysaccharides and proteins. Determines cell shape and volume. Confers mechanical strength. Determines relationship between volume and turgor pressure. Forms vascular tissue. Acts as a diffusion barrier/defence. CW fragments can act as signalling molecules |
| Cell walls: uses | Paper.
Textiles.
Fibres.
Charcoal.
Timber.
Source of polysaccharides.
Roughage in diet.
biofuel. |
| Cell wall: structure | Middle lamella: gel, mainly pectin and derives from the cell plate (which is formed by the phragmoplast), the cell wall is formed during cytokinesis. Primary wall: formed by growing cells. Secondary wall: formed after growth stops. |
| Primary wall | Extensible. Cellulose microfibrils: extruded from plasma membrane, cellulose synthase complex at the plasma membrane and oriented by microtubules. Matrix of pectin, hemicellulose and structural proteins exported in vesicles from Golgi. Cross-linking is affected by highly glycosylated proteins: Hydroxyproline-rich glycoproteins (extensions), Proline-rich proteins and Arabinogalactan proteins. Generally, cellulose has high tensile strength, orientation of the microfibrils determines cell growth and shape and cells without cell wall are spherical. |
| Cell wall growth | The primary walls of growing cells are under tremendous tension (10-100 atm), this force drives wall growth. Cell wall can stretch without breaking by polymer creep: Matrix yields allowing stretch and by selective loosening and shifting of linkages between microfibrils, expansins and glycolytic enzymes. |
| Secondary wall | Variable structure, inextensible, thickening, extensive cross-linking and it may contain lignin. |
| Lignin occludes the other CW polysaccharides | →lignin buried during the Carboniferous is our current ‘biofuel’
→major obstacle to the development of cellulosic biofuels
→remodelling lignin (‘cleavable’ lignin) is a major research challenge |
| Plasmodesmata | Cytoplasmic and ER connections: 40-50nm diameter, cytoplasmic sleeve with protein links, central tubule of ER, link many but not all cells and they identify a cytoplasmic continuum: symplast. Free diffusion under 1kD. Specific RNAs, proteins and viruses can also pass through. The size exclusion limit varies with the tissue type |
| Reticulons | Ubiquitous proteins in eukaryotes, located (mostly) in the ER membrane, wide array of biological functions and possible role in shaping tubular ER. Constrict the desmotubule. |
| Animal vs plant development | Similarities: Cell division, patterning and organogenesis, differentiation and growth |
| Plant development | Post-embryonic organ formation occurs through the action of meristems. Shoot apical: stem, leaves, seeds and fruits. Root apical: primary and lateral roots. |
| Plant development is repetitive | The shoot is made up of repeating units called phytomers. A phytomer consists of a leaf, a node,
an internode and an axillary meristem. The same processes that learn to pattern formation in the embryo are repeated again and again throughout the lifespan of the plant |
| During development | 1. Cell division 2. Cell differentiation; introduce variation either locally or gradual. 3. Pattern formation; allow the cells to divide even more. |
| Apical-basal polarity | Development of three primary layers in the basal region: Epidermis, Ground tissue and Vascular primodium. At the late heart stage, the ground tissue stem cells divide laterally to form the cortex and the endodermis. The patterns of cell division are fixed in the early embryo. The fate of any cell’s progeny can be predicted. |
| Mechanisms of development can be investigated through mutants defective in the process | Tonneau (ton) mutation causes: disorganised cell division. gnom mutation: lack apical-basal polarity. It encodes a protein with a role in the secretory pathway. There is abnormal localisation of PIN1, a transporter for the hormone auxin. Shortroot (shr) and scarecrow (scr) mutants: both lack one cell layer, the endodermis. |
| Shortroot and Scarecrow | Shortroot (SHR) mRNA is expressed in the vascular tissue of the root. Scarecrow mRNA is expressed in the ground tissue. SCR and SHR are in different tissues, yet they regulate the same process. Scarecrow is reduced in the shortroot mutant. |
| how does it work? (SHR and SCR) | The SHR mRNA is expressed in the vascular tissue. The SHR protein is expressed in the vascular tissue. It moves into the adjacent ground tissue where it becomes localised to the nucleus. The SHT protein can then activate SCR transcription in the ground tissue. In the ground tissue, the SHR protein functions as a transcription factor. It induces expression of SCR and other endodermis-specific genes. SCR is also required for periclinal cell divisions which lead to the generation of two layers of ground tissue. |
| How can we explain the mutant phenotypes? | shortroot mutant: NO SCR or SHR. The ground tissue adopts the default fate.
scarecrow mutant: In the absence of SCR expression, SHR causes the ground tissue to takes characteristics of both cortex and endodermis. |
| Formation of root hair from the root epidermis | Werewolf (WER) encodes a transcription factor expressed in non-hair cell. Initially all epidermal cells express WER. WER turns on GL2, which maintains cells in a non-hair (NH) state. Root hair differentiation is triggered by a signal from a protein called JACKDAW (JKD) in cortical cell. This results in inhibition of Werewolf (WER) expression. GL2 expression is switched off, leading to
differentiation into a hair cell (H). Non-hair cells express CAPRICE (CPC), which migrates to hair cell to inhibit expression of WER. |
| The Shoot apical meristem | Cells in the central zone are a pool of undifferentiated stem cells. Cells in the peripheral zone proliferate and differentiate into lateral organs. Cells in the rib meristem proliferate and differentiate into the stem |
| Meristems and leaf formation | Shoot tip: primary growth 9in stems, primary xylem and phloem.
Lateral meristems: secondary growth, primary + secondary xylem and phloem, vascular cambium. |
| Meristem and maintenance | Meristem need to continuously regenerate themselves while producing new organs. Mutants that use up their meristem after formation of a few leaves (wox mutants). Mutants that have enlarged meristems (clavata mutants). The wushel (wus) mutant carries a mutation in a WOX gene. |
| Wox and Clavata 1, 2, 3 | WOX genes function to maintain the population of stem cells in the shoot apical meristem.
The CLAVATA 1,2,3 genes act to prevent the meristem from expanding too much. CLV3 regulates the expression of WUS mRNA. |
| In situ hybridization with WUS digoxigenin-labelled antisense probe. | WUS is expressed in a deep region of the central zone, often described as the organising centre. In the enlarged meristem of a clv3 mutant, the WUS expression domain is expanded. Arrested meristem of a CaMV35S::CLV3 plant. WUS RNA is not detectable. |
| WUS regulates transcription of the CLV3 gene. | WUSHEL (WOX) and CLAVATA function as part of a transcriptional feedback loop. Organising centre cells express WUS. They signal to neighbouring stem cells to express CLV. CLV-expressing cells signal to the organising centre to reduce WUS expression – resulting reduced CLV expression in stem cells. This acts to limit the pool of both organising centre and stem cells. Cell that express CLV are unable to differentiate and retain stem cell identity. As cells divide, they are gradually pushed away from WUS-expressing cells and receive less signal from WUS. As a result, they express less CLV and become able to differentiate |
| Leaf formation | 1. cells have to acquire a new identity as “leaf” rather than meristem. 2. Growth involves
regulated patterns of cell division and cell expansion. 3. Next the leaf has to acquire polarity: outer vs inner surfaces. |
| Cells in the meristem are indeterminate (they are not programmed to stop growing) | Cells in the meristem and in the the leaf primordia look similar but are functionally distinct. Cells in the meristem are indeterminate and serve as a stem-cell population. |
| KNOX1 genes maintain the meristem in an indeterminate state | Class I KNOX genes (KNOX1): Encode transcription factors, Expressed in meristem, Not expressed in forming primordia, Help maintain indeterminate growth |
| Genetic control | Leaves are lateral organs that derive from founder cells. 1. Cells at the periphery of
the SAM are specified to adopt a different fate to surrounding cells 2. This process gives rise to leaf founder cells that are morphologically indistinguishable from the surrounding cells 3. Activation of founder cells involves subsequent cell divisions to create a primordium that will develop into a leaf. |
| Cells in leaf primordia become determinate | Cells in the primordia are functionally distinct from those in the meristem. They become determinate. ARP genes :“ARP” is derived from three genes, ASYMMETRIC LEAF1, ROUGH SHEATH2, and PHANTASTICA. ARP genes encode MYB transcription factors. Expressed in cells of leaf primordia. Promote determinate growth and differentiation. Promotes leaf primordium development and represses meristem identity. |
| ARP in KNOX1 mutant | In the stm (knox) mutant, AS1 is expressed in the meristem. The two classes of transcription
factors are mutually repressive, and help establish a separate identity for the emerging leaf primordium. |
| Genetic control of leaf polarity | Primordia have polarity because one side is closer to the meristem. Adaxial surface (meristem side) –light harvesting. Abaxial surface (away side) - transpirational water loss, respiratory gas exchange. |
| Establishment of leaf polarity. | Default state: abaxial surface. A signal from the meristem moves through the epidermis into the incipient primordium. The signal conveys adaxial positional information. |
| PHANTASTICA (PHAN) is required for adaxial cell fate. | The phantastica mutant has radially symmetrical leaves. PHAN encodes a transcription factor.
Mutant phan leaves are abaxialized, indicating that PHAN is necessary for adaxial cell fate. |
| KANADI (KAN) genes are required for abaxial differentation. | PHAN and KAN antagonise each other’s expression to specific both domains of the leaf. |
| Control of leaf size and shape | A distinct “middle domain” is necessary for blade outgrowth. No WOX = narrow leaves. KNOX1 factor correlates with leaf complexit, which contributes to indeterminacy. 1. KNOX-1 genes are
repressed at the site of leaf primordium initiation. 2. Simple leaves KNOX-1, genes stay off or 2. Compound leaves KNOX-1 genes turn on again in developing leaf primordia, conferring prolonged organogenic activity on the leaf edges. |
