Plant Secondary metabolites Dr. Abdul Latif Khan metabolites
Plant Secondary metabolites Dr. Abdul Latif Khan metabolites Primary andSecondary Secondary metabolism The Growth Hormone - efficient communication among cells, tissues, and organs of multicellular organisms - In higher plants, regulation and coordination of metabolism, growth, and morphogenesis depend on chemical signals from one part of the plant to another.
- Chemical messengers hormones are responsible for the formation and growth of different plant organs. - External factors such as gravity could affect the distribution of these chemical messengers within a plant - Hormones are chemical messengers that are produced in one cell or tissue and modulate cellular processes in another cell by interacting with specific protein receptors - Most plant hormones are synthesized in one tissue and act on specific target sites in another tissue at vanishingly low concentrations. - Endocrine hormones: hormones that are
transported to sites of action in tissues distant from their site of synthesis. - Paracrine hormones: hormones that act on cells sites adjacent to the source site of synthesis. - Plant development is regulated by six major types of hormones: auxins, gibberellins, cytokinins, ethylene, abscisic acid, and brassinosteroids. - Other signaling molecules that play roles in resistance to pathogens and defense against herbivores have also been identified in plants (e.g. jasmonic acid, salicylic acid,
polypeptide systemin) Function of Hormones Light - phototropism Touch - thigmotropism Gravity gravitropism Turgor movements Biological clock - circadian rhythms When to open and close stomata
Control of flowering - photoperiodism Response of Hormones AUXINS - Auxin was the first growth hormone to be studied in plants; the early physiological work on the mechanism of plant cell expansion was carried out in relation to auxin action. - Auxin and cytokinin differ from the other plant hormones in one important aspect: they are required for viability.
(so far, no mutants lacking either auxin or cytokinin have been found, suggesting that mutations that eliminate them are lethal) - points to be covered: - history of auxins discovery - description of the chemical structures of auxins - detection methods of auxins in plant tissues - pathways of auxins biosynthesis - developmental processes controlled by auxin: ---- stem elongation ---- apical dominance ---- root initiation
---- fruit development ---- meristem development ---- oriented or tropic growth Auxin research has been continually advanced by new technologies Abel, S., and Theologis, A. (2010). Odyssey of auxin. Cold Spring Harb Perspect Biol. doi: 10.1101/cshperspect.a004572 Darwin (1890s) studied phototropism movement towards light Darwin and others studied coleoptiles tissues that
protect monocot leaves during germination Darwin, C., and Darwin, F. (1881) The power of movement in plants. Appleton and Co., New York.; Photos courtesy of Dr. R.L. Nielsen Cutting off or covering the coleoptile tip interferes with the response These experiments showed that the light signal is perceived at the tip, although the bending occurs at the base Untreated coleoptile
bends Coleoptiles with tips shielded from light or removed do not bend Darwins (Charles and son) experiment Under normal conditions, shoot tips bend towards the light Without light on the tip, no bending
When not at tip, collar doesnt prevent bending Conclusion: Light is sensed at the tip, but response not at tip New hypothesis: A substance or chemical is transported Auxin later isolated from shoot tips and established to be involved in cell elongation Drawings depicting seedlings of Zea (Gramineae family) Darwin concluded that a signal moves from tip to base We must therefore conclude that when seedlings are freely exposed to a lateral light some influence is transmitted from
the upper to the lower part, causing the latter to bend. 1- The Emergence of the Auxin Concept - Charles Darwin and his son Francis and their studies on plant tropisms - Phototropism, the phenomenon of bending of plants toward light due to differential growth.
- Darwins experimental observations on Coleoptiles: --- If coleoptiles illuminated on one side with short pulse of dim blue light, they will bend (grow) toward the source of the light pulse within an hour. --- If the tips of the coleoptiles covered with foil, the coleoptiles would not bend. (i.e. the tip of the coleoptile perceived the light) --- The region of the coleoptile that is responsible for the bending toward the light (growth zone) is several
millimeters below the tip. - Darwins conclusion: - some sort of signal is produced in the tip, travels to the growth zone, and causes the shaded side to grow faster than the illuminated side - Followed research experimentation on the nature of the growth stimulus in coleoptiles culminated in the presence of a growth-promoting chemical in the tip of coleoptiles - If the tip of a cleoptile was removed, coleoptile growth ceased.
Boysen-Jensen (1913) showed that the transmitted influence can move through a gelatin block Before After The signal cannot move through a solid block or butter, demonstrating that it is a water- soluble chemical. Repositioning the tip can induce
bending in uniform light Tip removed and replaced to one side Tip removed Control Before Tip removed and replaced Asymmetric tip placement causes bending
Paal (1919) showed that removing the tip and replacing it on one side of the base is sufficient to cause bending. After In the 1930s, auxin was purified and shown to promote growth Angle of curvature is proportional to amount of auxin in
block Frits Went collected auxin from shoot tips into agar blocks... Indole-3-acetic acid, IAA ...and showed that the material collected in the agar blocks was the growthpromoting substance. This bending assay for the growthpromoting effect of auxin was used
as a basis for its purification. Redrawn from Went, F.W. (1935) Auxin, the plant growth-hormone. Bot. Rev. 1: 162-182. - Major breakthrough by Went (1926): --- allowing the material to diffuse out of excised cleoptile tips into gelatin blocks --- placing these gelatin blocks asymmetrically on top of a decapitated coleoptile, then --- testing for the ability of these gelatin blocks to cause bending of the coleoptile in the absence of a unilateral light source. - (see Figures 19.1 and 19.2) Auxins root-promoting properties were
also known by the 1930s Adventitious roots are initiated from grape stems treated with auxin A more recent experiment: auxintreated radish roots initiate lateral roots at a frequency proportional to auxin concentration M IAA Thimann, K.V. (1938). Hormones and the analysis of growth. Plant Physiol. 13: 437-449. Kerk, N.M., Jiang, K., and
Feldman, L.J. (2000). Auxin metabolism in the root apical meristem. Plant Physiol. 122: 925-932. Auxins role in apical dominance was also known in the 1930s Bud Length Decapitate Replace apex with agar block: without or with auxin. Auxin suppresses
bud outgrowth No auxin Auxin Thimann, K.V., and Skoog, F. (1934). On the inhibition of bud development and other functions of growth substance in Vicia faba. Proceedings of the Royal Society of London B. 114: 317-339 with permission; Went, F.W. and Thimann, K.V. (1937) Phytohormones. The Macmillan Company, New York. Evidence for the role of auxin in apical dominance High auxin concentration Low auxin concentration
Drawings depicting Coleus (Lamiaceae family) Different tissues were recognized to have different sensitivities to auxin Auxin concentrations that promote elongation in stems can be inhibitory in roots. Thimann, K.V. (1938). Hormones and the analysis of growth. Plant Physiol. 13: 437-449. Polar transport of auxin was recognized in the 1930s
A segment cut from a coleoptile can move auxin from tip to base. The upper agar block was loaded with auxin, which the segment translocated to the lower block. When the segment is inverted, it is unable to transport auxin from base to tip. This experiment, carried out by
H.G. Van der Weij revealed that auxin in the shoot is translocated from tip to base. Adapted from Went, F.W. (1935) Auxin, the plant growth-hormone. Bot. Rev. 1: 162-182. What classical studies told us The chemical structure of auxin That auxin promotes root formation and inhibits bud outgrowth That auxin moves through the shoot from tip to base That different tissues have different sensitivities That auxin affects growth in a concentration and tissue-specific way What we didnt know was how auxin promoted growth, moved in a polar direction, limited bud outgrowth and stimulated root
formation. This lecture describes how the tools of molecular biology, genetics, and cell biology have continued the auxin story, and revealed more about how auxin regulates growth. Auxin: a 21st century perspective
Auxin homeostasis Tools in auxin research Polar auxin transport Perception and signaling Auxin action in whole-plant processes Interactions with other signals Auxin signaling pathway Catabolism Synthesis
IAA Conjugation Transport Perception (receptor) TF activation/ inactivation Target genes
Biological Functions Auxins effects depend upon its synthesis, transport, perception, signaling, and target gene responses. Most of these functions are controlled by many genes with differing cell specificities. Adapted from Kieffer, M., Neve, J., and Kepinski, S. (2010). Defining auxin response contexts in plant development. Curr. Opin. Plant Biol.13: 12-20. Auxin response pathway feedback regulation Catabolism Synthesis
IAA Conjugation Transport Perception (receptor) TF activation/ inactivation Target genes
Biological Functions The pathway is extensively self-regulated through positive and negative feedback. Adapted from Kieffer, M., Neve, J., and Kepinski, S. (2010). Defining auxin response contexts in plant development. Curr. Opin. Plant Biol.13: 12-20. Regulation by environmental factors and other hormones Catabolism Synthesis Conjugation
IAA Transport Perception (receptor) TF activation/ inactivation Target genes Biological
Functions Gravity, Nutrient nutrient status, ionic Ionic environment, pathogens, Pathogens, Light: directionality, intensity, wavelength Ethylene, brassinosteroids, cytokinins, gibberellins, jasmonates, strigolactones.... Adapted from Kieffer, M., Neve, J., and Kepinski, S. (2010). Defining auxin response contexts in plant development. Curr. Opin. Plant Biol.13: 12-20. 2- Identification, Biosynthesis, and Metabolism of Auxin The principal auxin in higher plants is indole-3acetic acid (IAA): - (see figures 19.3 and 19.4 for structures of natural and synthetic auxins, respectively)
- Early definition of auxins: all natural and synthetic chemical substances that stimulate elongation in coleoptiles and stem sections. - Auxins can be defined as compounds with developmental biological activities similar to those of (or associated with) IAA. Biosynthesis and homeostasis IAA is produced from tryptophan (Trp) via several pathways and one Trp-independent pathway (black arrow). The IAOx pathway
may be restricted to Arabidopsis and its close relatives. Indole Tryptophan Indole-3pyruvic acid (IPA) Tryptamine Indole-3acetamide (IAM) Indole-3acetaldoximine
(IAOx) Indole-3acetaldehyde IAA Adapted from Quittenden, L.J., Davies, N.W., Smith, J.A., Molesworth, P.P., Tivendale, N.D., and Ross, J.J. (2009). Auxin biosynthesis in pea: Characterization of the tryptamine pathway. Plant Physiol. 151: 1130-1138.. IAA is synthesized in meristems and young dividing tissues: Biosynthesis of IAA is associated with rapidly dividing and rapidly growing tissues. Sites of auxin synthesis: --- Shoot apical meristems --- young leaves
--- Root apical meristems --- Young fruits and seeds ? Basipetal shift in auxin production in very young leaf primordia correlates closely with the basipetal maturation sequence of leaf development Auxin synthesis is developmentally and environmentally controlled Indole Methyl Jasmonate Tryptophan
Ethylene Indole-3pyruvic acid Tryptamine Red / Far-red light ratio Temperature Auxin biosynthesis is influenced by other hormones and environmental conditions Indole-3acetamide
Indole-3acetaldoximine Indole-3acetaldehyde IAA Adapted from Quittenden, L.J., Davies, N.W., Smith, J.A., Molesworth, P.P., Tivendale, N.D., and Ross, J.J. (2009). Auxin biosynthesis in pea: Characterization of the tryptamine pathway. Plant Physiol. 151: 1130-1138.. Auxin synthesis promotes shade-avoidance hypocotyl-elongation response Auxin synthesis White light Shade Wild-type
Wild-type taa1 An environment enriched with far-red light (which simulates shading by other plants) promotes hypocotyl elongation, but not in a mutant blocked in an auxin synthesis pathway. taa1 TAA1 Tryptophan
Indole-3pyruvic acid In wild-type plants but not loss-offunction taa1 mutants, auxin synthesis is increased in shade conditions. Reprinted from Tao, Y., et al. (2008) Rapid synthesis of auxin via a new tryptophan-dependent pathway is required for shade avoidance in plants. Cell 133: 164176, with permission from Elsevier. Auxin homeostasis is also regulated by conjugation and degradation
Overexpression of an auxin conjugating enzyme encoded by a GH3 gene reduces auxin levels in the plant and causes a dwarfed phenotype. IAA GH3 genes are auxininduced Rice Arabidopsis (GH3 genes) Wild-type GH3.13 overexpression
Wild-type GH3.13 overexpression Zhang, S.-W., et al., (2009) Altered architecture and enhanced drought tolerance in rice via the cown-regulation of indole-3-acetic acid by TLD1/OsGH3.13 activation. Plant Physiol. 151:1889-1901. Staswick, P.E., et al., (2005) Characterization of an Arabidopsis enzyme family that conjugates amino acids to indole-3-acetic acid. Plant Cell 17: 616-627. 3- Auxin Transport - The main axes of shoots and roots show apexbase structural polarity. - The apex-base structural polarity of shoots and roots is dependent on the polarity of auxin transport. - In excised oat coleoptile sections, IAA moves mainly from the apical to the basal end
(basipetally), i.e. unidirectional transport (polar transport). - Polar auxin transport was reported in 375 million year old fossil wood. - If polar auxin transport is disrupted in some regions (by the presence of buds or branches), the tracheary elements that differentiate in these regions form circular patterns. - Since the shoot apex is the primary source of auxin in the plant, polar transport is the principal cause of an auxin gradient extending from the shoot tip to the root tip. - The longitudinal gradient of auxin from the shoot to the root affects various developmental
processes (e.g. embryonic development, stem elongation, apical dominance, wound healing, and leaf senescence). - In roots, acropetal transport of auxin occurs in phloem, and phloem-based movement, driven by source-sink translocation of sugars. Mechanisms responsible for the distribution of auxin in the plant (Cellular mechanisms underlying and regulating auxin transport and their roles in plant adaptation to various environmental signals) Polar transport requires energy and is gravity independent - Donor- receiver agar block method (Fig. 19.10):
-- a donor block contains radioisotope- labeled auxin -- a tissue segment -- a receiver block -- measuring the radioactivity in the receiver block General properties of polar IAA transport: 1- Tissues differ in the degree of polarity of IAA transport. -- Basipetal transport of IAA predominates in cleoptiles, vegetative stems, and leaf petioles. -- Acropetal transport of IAA occurs in the stelar tissues of the root. -- Polar transport is not affected by the
orientation of the tissue, so it is independent of gravity. -- See Fig. 19.11 for the lack of gravity effects on basipetal auxin transport in the inverted or upright orientation of stem cuttings. -- Roots form at the base end because root differentiation is stimulated by auxin accumulation due to basipetal transport. -- Shoots form at the apical end where the auxin concentration is lowest. 2- Polar transport proceeds in a cell-to-cell fashion -- Auxin exits the cell through the plasma membrane, diffuses across the compound middle lamella, and enter the next cell through its plasma
membrane. -- Auxin efflux is the loss of auxin from cells. -- Auxin influx or uptake is the entry of auxin into cells. -- The overall process requires metabolic energy. 3- The velocity of polar auxin transport ranges from 2 to 20 cm/h. -- Higher rates of polar transport are observed in tissue immediately adjacent to the shoot and root apical meristems. -- Polar transport is specific for active auxin; auxin is recognized by protein carriers on the plasma membrane. 4- The major sites of polar auxin transport in
stems, leaves, and roots is the vascular parenchyma tissue (the xylem). -- In vascular parenchyma, the overall direction of auxin transport is downward; basipetally in the shoot and acropetally in the root. -- In the coleoptile of grasses, basipetal polar transport of auxin occurs mainly in the nonvascular parenchyma tissues. -- Auxin translocated in the phloem sieve tubes contributes to transport from shoot tissues to the growing root tips. -- Basipetal auxin transport from the apex occurs in root as well. -- Basipetal auxin transport in the root
occurs in the epidermal and cortical tissues, and is important in gravitropism. Evidence for the role of auxin in adventitious root formation With synthetic auxin Without synthetic auxin Adventitious roots growing from stem tissue Saintpaulia (Gesneriaceae family) Another example of misleading common name
The African violet is not in the violet family Evidence for the role of auxin in formation of fruit and structures of similar function (e.g. receptacle in strawberry) Normal All achenes Band of achenes conditions removed removed Without seed formation, fruits do not develop. Developing seeds are a source of auxin. What do you expect?
Not shown: Auxin replacement restores normal fruit formation and can be used commercially to produce seedless fruits However, too much auxin can kill the plant and thus synthetic auxins used commercially as herbicides Fragaria (Rosaceae family) Summary In the past 30 years we have identified many of the molecular characters in the auxin story, and have a pretty good idea of its major themes, but the story is far from complete. Catabolism Synthesis
IAA Conjugation Transport Perception (receptor) TF activation/ inactivation Target genes
Biological Functions Adapted from Kieffer, M., Neve, J., and Kepinski, S. (2010). Defining auxin response contexts in plant development. Current Opinion in Plant Biology 13: 12-20. Ongoing investigations Where does auxin elimination fit in? Catabolism Synthesis What do all those transport proteins do? What controls their
activity and distribution?? Conjugation IAA What regulates auxin synthesis? Transport Perception (receptor) What are the other
TIR1 like proteins doing, and what does ABP1 do? TF activation/ inactivation Why so many ARFs and Aux/IAAs? What are the target genes, and what do they do?
Target genes Biological Functions How do all these pieces fit together to make a functioning plant???? Adapted from Kieffer, M., Neve, J., and Kepinski, S. (2010). Defining auxin response contexts in plant development. Current Opinion in Plant Biology 13: 12-20.
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