Lecture 14 1 Define the term phytohormone and explain the basic similarities between hormones in plants and animals Phytohormones regulate cellular activities division elongation and differentiation pattern formation organogenesis reproduction sex determination and responses to abiotic and biotic stress Different types of hormones that are produced in a gland where it will be transported to a tissue to induce an effect Similar to animals functions as a signaling molecule trigger for cell to do something different Unique since they are sessile compared to animals In both cases they are integrators and regulators in case of processes coordinate organism with growth conditions Important component how plants respond to cues in environment present in small quantities Integration The hormones in blue are often referred to as the classical plant hormones auxin cytokinins gibberellins ethylene and abscisic acid Those in green are more recent additions to the family of plant hormones You can have more than one cytokinin Some respond specifically to stress 2 Explain that plant hormones act as signaling molecules in plants to regulate growth development reproduction and stress responses Play a role throughout entire life cycle of plant Most are angiosperms understand the most Development of fruit fruit is organ that develops around the seed or seeds the origin of the fruit is the ovule can be very diverse in what they look like houses seed Any point in life cycle there is hormone involvement from the time of germination to flowering is regulated by hormones In addition to regulating normal process also helps plants deal with the environment around them which is not always optimal help plant respond to biotic herbivory viral infections and abiotic heat drought stresses adaptive because it helps plant cope Hormone mediated response pattern 3 Identify the 5 major plant hormones and describe their synthesis accumulation perception transport and how their signals are transduced Synthesis Many tightly regulated biochemical pathways contribute to active hormone accumulation Conjugation can temporarily store a hormone in an inert form lead to catabolic breakdown or be the means for producing the active hormone Jasmonic acid is activated upon conjugation to isoleucine JA Ile Usually though when hormones are conjugated they are inactivated Highly regulated pathways series of enzymatic reaction occurs at one possible point of regulation Can have active and inactive forms Transport and Perception Hormones can move through the xylem or phloem across cellular membranes through regulated transport proteins Several hormone receptors have recently been identified They can be membrane bound or soluble Typically moved through vascular tissue may be able to diffuse through membrane some require specific transport proteins Once they get to where they are going to act there needs to be a receptor where it will bind to so it can be recognized and trigger the next part of the process If receptors are inactive or active can be modified can be present at different places more are bound to membrane Most of the hormones we at least know one receptor Signal Transduction Hormonal signals are transduced in diverse ways Common methods are reversible protein phosphorylation and targeted proteolysis Quite complex in terms of what happens when signal is received Once signal is received it is transduced by modifying or degrading proteins refers to sequential steps signalling cascade Then you can either inactivate or activate the proteins or can lead to degradation of specific proteins Responses Downstream effects can involve changes in gene transcription and changes in other cellular activities like ion transport o Modification to transcription factors is common o If a transcription factor gets activated in previous step and reacts with other genes it could cause diverse effect diversity of function o Extremely common o This can cause very specific and widespread change that can change the profile of the cell o Could have modification of protein that is an ion channel for a movre focused response Auxin Growth Phototropism and gravitropism Branching Embryonic patterning Stem cell maintenance Organ initiation Looks like tryptophan amino acid with similar pathway Phototropism unidirectional light source responds to growing in that direction Gravitropism respond to gravity Apical dominance branching in both roots and shoots Establishes patterns of tissues Stem cell maintenance in meristems Leaf primordia initiation Auxin moves in part by a chemiosmotic mechanism Auxin is a charged anion IAA in the cytoplasm pH 7 In the more acidic cell wall pH 5 5 some is uncharged IAAH The uncharged form crosses the plasma membrane into the cell where it is deprotonated and unable to exit other than through specific transporters Exist in slightly different form depending on the pH it is exposed to At lower pH it will tend to exist mostly in form of IAAH protonated At higher pH IAA Transport molecules tend to be specific for one or the other Polar auxin transport Auxin transport out of cells is controlled by three families of transport proteins that collectively control the directionality of auxin movement Asymmetric distribution of the transporters controls polar auxin transport Polar transport so it predominantly moves in one direction Auxin moves from shoot down to root body to the roots Gradient is established by unidirectional transport mechanism which is dependent upon different forms of auxin Auxin biosynthesis IAA is produced from tryptophan Trp via several semi independent pathways and one Trp independent pathway Environmental and developmental control of the genes controlling auxin biosynthesis conjugation and degradation maintain auxin homeostasis Auxin signaling pathway o o o 1 Auxin binds to SCFTIR1 and Aux IAA Auxin interacts with soluble receptor When auxin is not present the repressor keeps protein from initiating gene expression If IAA enters cells interacts with another receptor and together they move through and 2 Aux IAA ubiquitinated and degraded by 26S proteasome 3 Degradation of repressor permits transcriptional activation by ARF transcription factors Now it is active ARF Mediation is degradation of complex Cytokinins cell division control of leaf senescence control of nutrient allocation root nodule development stem cell maintenance regulates auxin action antagonistic effect with auxin Regulated by CK and nitrogen Tissue
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