核心提示：In an attempt to be concise and understandable, introductory level courses and textbooks frequently present concepts th
At least nine groups of plant hormones have been studied extensively. Auxin, cytokinin, brassinosteroid (BR), gibberellin (GA), and strigolactone (SL) play essential roles in normal growth and development.
In an attempt to be concise and understandable, introductory level courses and textbooks frequently present concepts that are technically correct, but lead to misconceptions on the part of the student because they omit too much. In discussions about mitochondria students frequently come away with a superficial understanding of the true nature of Krebs' cycle, electron transport, respiratory control, and oxidative phosphorylation.
example1 :Integration of Light and Hormone Signaling Pathways Regulates Hypocotyl Elongation
暗形态建成表型 skotomorphogenesis / etiolation
- maximum hypocotyl elongation,
- limited root growth
- closed cotyledons with an apical hook
- suppression of chloroplast development
光照后转变为光形态建成 photomorphogenesis / de-etiolation
- inhibition of hypocotyl elongation
- opening/expanding and greening of cotyledonsand leaves
- acceleration of root growth****
The phytochrome interacting factors (PIFs), a class of basic helix-loop-helix (bHLH) factors, are major posi- tive regulators of shoot cell elongation.
Elongated hypocotyl5 (HY5),GATA2/4, and B-box factors including BZS1
are negative reg-ulators of cell elongation, and they are degraded in
through the E3 ubiquitin ligase constitutive photomorphogenic1(COP1), which is inactivated by both phytochromes and crypto-chromes。
GA, similar to auxin, acts through an intracellular receptor to promote ubiquitination and degradation of key repressor proteins, named DELLA proteins for containing a conserved Asp-Glu-Leu-Leu-Ala amino acid sequence。
www.8522.com，DELLAs were initially found to interact with PIFs and inhibit their DNA-binding activity but have since been found to inhibit DNA-binding activities of many transcription factors, including BZR1 and ARF6 of the BR and auxin pathways, respectively. DELLAs also function as transcriptional co-activators through interaction with several classes of DNA-binding proteins, including ARR1, which is a component of the cytokinin pathway that promotes photomorphogenesis.
the BR, auxin, GA, and phyto- chrome pathways converge through direct interactions among their transcription factors/regulators. BZR1, PIF4, and ARF6 interact with each other, and they share a large number of com- mon target genes.These three transcription factors enhance each other’s target binding and transcriptional activation activities, and their functions in acti- vating many shared target genes and promoting hypocotyl elongation are genetically interdependent on each other.
(A) Light and hormonal signals (red text) are perceived by cell-surface or intracellular receptors (blue), which regulate 澳门新萄京8522，transcription factors (green) through signaling/posttranslational mechanisms (red lines), whereas the transcription factors transcriptionally regulate (blue lines) downstream responses and components of other pathways. Orange: kinases; yellow: phosphatases;** purple: inhibitors of transcription factors.**
(B) Transcriptional integration by the BAP/D-HHbH circuit. Red and blue lines show regulation at the protein and RNA (transcriptional) levels, respectively.
These pages were designed to supplement laboratory work with mitochondria by providing background in as much detail as the student might wish. Students at any level are likely to encounter terms with which they are unfamiliar. A glossary of terms is included in this project for your convenience. A brief overview of mitochondria structure and function is presented just to get you started. You can then wind your way through the main path of detailed information.
example2: The Tradeoffs between Growth and Defense
BR and the flagellin-signaling pathways
A peptide (flg22) from bacterial flagellin protein is perceived as a pathogen-asso- ciated molecular pattern (PAMP) by the LRR-RK named FLS2 (flagellin-sensitive2), which has an overall similar structure as the BR receptor BRI1.
Flg22 induces microRNA miR393, which targets the mRNAs of auxin receptor TIR1, AFB2, and AFB3.
Wounding by herbivores induces production of JA, which acts as a mobile signal to induce systemic defense responses and inhibit vegetative growth
A) Mechanisms of crosstalks of FLS2-mediated flagellin signaling with
the BR and auxin pathways.
(B) Growth regulation in response to herbivore attack, mediated by crosstalk between the GA and JA pathways. Red and blue lines show regulation at the protein and RNA (transcriptional) levels, respectively. Dashed lines indicate unknown mechanisms.
More than half century has passed since C. de Duve discovered lysosomes using cell fractionation procedures. Then, intracellular bulk protein degradation was believed to occur mostly within this organelle. Eukaryotic cells must elaborate a strategy to segregate dangerous lytic enzymes from biosynthetic sites, cytosol and restrict the degradative process in a membrane-bound compartment. The process of degradation of cytoplasmic components in lysosomes is called autophagy in contrast to heterophagy, degradation of extracellular materials through endocytosis. Electron microscopic studies on lysosomes revealed macroautophagy (hereafter simply refer as autophagy) as a major route to deliver the cytoplasmic components to the lytic compartment. The first step of autophagy is a sequestration of a portion of cytoplasm or organelle by a membrane sac, the so-called isolation membrane, resulting a double membrane structure called the autophagosome. Then the autophagosome fuses with the lysosome, gains lytic enzymes and turns to be an autophagolysosome. Lysosomal enzymes disintegrate the inner membrane of autophagosome and digest its contents. Digestion products are transported back to the cytosol and reutilized for new round of protein synthesis.
Mitochondria are bacteria-sized organelles, found in the cytoplasm of virtually all eukaryotic cells. They are especially abundant in cells and parts of cells that are associated with active processes. For example, in flagellated protozoa or in mammalian sperm, mitochondria are concentrated around the base of the flagellum or flagella. In cardiac muscle, mitochondria surround the contractile elements. Hummingbird flight muscle is one of the richest sources of mitochondria known. Thus, from their distribution alone one suspects that they are involved in energy production.
We know now that multicellular organisms probably could not exist without mitochondria. Mitochondria make efficient use of nutrient molecules, requiring oxygen in the process. They are, in fact, why we need oxygen at all.
The double-membraned mitochondrion can be loosely described as a large wrinkled bag packed inside of a smaller, unwrinkled bag. The two membranes create distinct compartments within the organelle, and are themselves very different in structure and in function.
The outer membrane is a relatively simple phospholipid bilayer, containing protein structures called porins which render it permeable to molecules of about 10 kilodaltons or less . Ions, nutrient molecules, ATP, ADP, etc. can pass through the outer membrane with ease.
The inner membrane is freely permeable only to oxygen, carbon dioxide, and water. Its structure is highly complex, including all of the complexes of the electron transport system, the ATP synthetase complex, and transport proteins. The wrinkles, or folds, are organized into lamillae , called the cristae . The cristae greatly increase the total surface area of the inner membrane. The larger surface area makes room for many more of the above-named structures than if the inner membrane were shaped like the outer membrane.
The membranes create two compartments. The intermembrane space, as implied, is the region between the inner and outer membranes. It has an important role in the primary function of mitochondria, which is oxidative phosphorylation.
The matrix contains the enzymes that are responsible for the citric acid cycle reactions. The matrix also contains dissolved oxygen, water, carbon dioxide, the recyclable intermediates that serve as energy shuttles, and much more. Because of the folds of the cristae, no part of the matrix is far from the inner membrane. Therefore matrix components can quickly reach inner membrane complexes and transport proteins.
Electron micrographs have revealed the three dimensional structure of mitochondria. However, since micrographs are themselves two dimensional, their interpretation can be misleading.
Texts frequently show a picture of a 'typical' mitochondrion as a bacteria-sized ellipsoid . However, they vary widely in shape and size. Electron micrographs seldom show such variation, because they are two-dimensional images.
Isolated mitochondria, such as from homogenized muscle tissue, show a rounded appearance in electron micrographs, implying that mitochondria are spherical organelles.
Mitochondria in situ can be free in the cytoplasm or packed in among more rigid structures, such as among the myofibrils of cardiac muscle tissue. In cells such as muscle, it is clear that mitochondria are not spherical, and often are not even ellipsoid. In some tissues, the mitochondria are almost filamentous, a characteristic that two dimensional micrographs may fail to reveal.
A planar section cuts through one or several parts of the organelle, making it appear that there is more than one. The image we see of a circular or ellipsoidal organelle may disguise the true nature of the mitochondrion.
example3 :Shade Avoidance Syndrome: A Case of Inter-organ Growth Coordination
In nature, successful competition with neighbors for sunlight is crucial for plant survival, and thus canopy shade induces a vari- ety of morphological changes that are collectively called** shade avoidance syndrome (SAS)**. These include elongation of stem and petiole, leaf hyponasty (upward bending of the leaves caused by growth of the lower side), reduced shoot branching and root growth, and decreased seed and fruit production.
(A) Diagram of light-hormone interactions in growth regulation under
full light (left) and shade (right) conditions. HBL: high blue light,
LBL: low blue light, HRFR: high red:far-red ratio, LRFR: low red:far-red
ratio. Dark text and arrows show active components and their activities,
and dimmed text and arrows indicated inactivated components and
activities. Red arrows show the flow of auxin.
(B and C) Venn diagram shows overlaps between genes induced by 1 hr low R:FR treatment, or by 6 hr low blue light treatment of light-grown seedlings, and the target genes of BZR1, PIF4, and ARF6 identified by ChIP-seq in dark-grown seedlings.
Autophagy is involved in non-selective and bulk degradation of cellular proteins. While the ubiquitin/proteasome system is responsible for highly selective degradation of short-lived proteins. Since more than 90% of cellular proteins have long lifetimes, the turnover of long-lived proteins is important to understand cell physiology.
Communication between cells is a crucial step to coordinate organ formation and tissue patterning. In plants, the intercellular transport of metabolites and signalling molecules occur symplastically through membranous structures (named plasmodesmata) that traverse the cell wall to connect the cytoplasm and endoplasmic reticulum of neighbouring cells.
- Plasmodesmata (PD) play a role in organ development and patterning.
- Proteins that localize and/or interact to regulate PD have been identified.
- Novel mobile transcription factors and RNAs support PD role in organ formation.
Autophagy in mammals had been studied mostly using electron microscopy by detecting autophagosomes and autophagolysosomes. Since the lysosomal system consists of very dynamic and complicated membrane structures, it was not easy to analyze lysosomes and its related membrane structures biochemically. Many efforts to detect specific proteins on the autophagosome failed and genes required for autophagy had not been identified.
Plasmodesmata regulation during organ formation and vascular patterning
plant cells communicate to each other and form higher order structures that characterize organisms – plasmodesmata (PD) 胞间连丝
In simple terms, PD are channels made of plasma mem- brane (PM) that provide cytoplasmic and membranous continuity between neighbouring cells forming the symplasm
Transport pathways and PD regulation by callose.
(a) Intercellular transport occur through PD cytoplasmic sleeve (black arrows), by diffusion in the lumen of the endoplasmic reticulum (ER) and the desmotubule (DT) (orange arrows) and, potentially, by lateral segregation in the membranes (green discontinuous arrows). Plasma membrane (PM), cell wall (CW) and PD-cytoplasmic aperture (in discontinuous blue) are indicated.
(b) PD transport is regulated by the deposition of callose in the surrounding cell wall. Callose 胼胝质 is produced at PD sites from UDP-glucose by callose synthases (CALS) and degraded to glucose subunits by PD-located beta-1,3 glucanases (PdBG). Callose turnover depends on the activity of these enzymes. High levels of callose restricts PD-cytoplasmic aperture blocking molecular transport thus cell-to-cell symplastic connectivity.
In this talk, I will focus on the recent progress in the molecular dissection of autophagy in the yeast, Saccharomyces cerevisiae, and its relevance to understand autophagic protein degradation in higher eukaryotes.
Receptor proteins act at plasmodesmata to regulate organ development
Analysis of the PD proteome in Arabi- dopsis identified three receptor-like kinases (RLKs) and a number of receptor-like proteins including the PD-Lo- cated Proteins (PDLPs).
Although most of the research in PD-located receptor proteins focuses on their function in plant–pathogen interactions, new cumulative data support their role in development.
Interaction of receptor proteins is also proposed as a mechanism to regulate the PD transport of factors main- taining stem cell fate in the apical meristems
Identification and developmental function of novel mobile proteins and RNAs
List of mobile proteins (blue shaded cells) and small RNAs (in green) studied in the last five years with a function in organ development and patterning.
The importance of mobile RNA molecules in tissue patterning and organ development emerged from recent publications.
Thousands of transcripts moving in the phloem of Arabidopsis and other plant species were identified using different strategies (such as grafting, translocation of RNA between host and parasitic plants and phloem sap analysis). These phloem RNA molecules move long-distances and between CC and SE, by diffusion or in complex with RNA-binding proteins, to regulate development in target tissues. However questions remain regarding the selectivity for phloem translocation or the final destination of these transcripts.
The transport of miRNA and siRNA molecules can also be phloem independent. Research on miR394 suggests that it moves from the L1 layer of the SAM to the inner stem cell layers to repress LEAF CURLING RESPONSIVENESS (LCR, a gene involved in leaf and shoot meristem development) acting as a positional cue to maintain shoot stem cell activity. Also important for patterning, miR165/166 move between cell layers in embryos and root meristem to regulate CLASS III HOMEODOMAIN LEUCINE ZIPPER (HD-ZIP III) proteins.
In turn, tasiR-ARF, a trans-acting siRNA that targets AUXIN RESPONSE FACTOR 3 (ARF3) and ARF4, diffuses from the adaxial to the abaxial side to establish leaf polarity. siRNA can also move long-distances in a phloem-independent pathway from root to shoot . When produced in roots, siRNA moves to the shoot by a combination of short-range cell-to-cell communication events and amplification of the signal in all cells en route. Interestingly, the spreading of the silencing signal is affected in mutants in a hydrogen peroxide (H2O2)-producing type III peroxidase (named RCI3) concordant with previous research indicating the role of H2O2 on regulating PD permeability.
Together, the findings support the involvement of PD in the transport of siRNA and their role in developmental signalling.
- M. Notaguchi Identification of phloem-mobile mRNA J Plant Res, 128 (2015), pp. 27–35
- C. Zhang, L. Han, T.L. Slewinski, J. Sun, J. Zhang, Z.Y. Wang, R. Turgeon Symplastic phloem loading in poplar Plant Physiol, 166 (2014), pp. 306–313
- Z. Spiegelman, G. Golan, S. Wolf Don’t kill the messenger: long-distance trafficking of mRNA molecules Plant Sci, 213 (2013), pp. 1–8
- D.J. Hannapel, P. Sharma, T. Lin Phloem-mobile messenger RNAs and root development Front Plant Sci, 4 (2013), p. 257
- B.K. Ham, G. Li, W. Jia, J.A. Leary, W.J. Lucas Systemic delivery of siRNA in pumpkin by a plant PHLOEM SMALL RNA-BINDING PROTEIN 1-ribonucleoprotein complex Plant J, 80 (2014), pp. 683–694