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Biological Sciences

Plant Hormones Chapter 19chemical messengers produced in one celltissue and modulate cellular processes in another cell y Plant hormonesby interacting w specific protein receptorsSmall organic molecules 1m 1000fold lower than typical metabolite Advances in analytical instrumentation6 major typesSome molecules meet the criteria for being hormones but have still not been classified as ones eg salicylic acid y Early stagesgrowth stimulators eg division elongation differentiationAuxins cytokinins gibberellins brassinosteroids Auxins and cytokinins ratio is important for cell differentiation eg senescence abscission fruit ripening y Later stagesgrowth inhibitors Abscisic acidethylene ethylene is a gas that can impact other plants at a distance y Responses to bioticabiotic stress eg pathogen attack droughtAbscisic acidethylene y Internalexternal signals perceptionchange in hormoneor target cell sensitivityhormone binding to receptor proteinsignal amplificationtransductioncellular response enzyme activitygene expressiongrowthdevelopmentCell sensitivity means that protein receptors could increasedecrease in number y Tropisms different growth responses to different signals phototropisms light gravitropism gravity thigmotropisms touch y Auxin has different effects one of which is tropismsy Fig 181 Summary of early experiments in auxin research Cells on shaded side grow more rapidly than those on the lit sideWhen the tip is cut out theres no growth responseperception of light should be occurring in tipWhen opaque tip put on the tipno growth responseWhen shaded side is blocked the trophic response is blocked meaning chemical moves down that side to get a responseWhen tip is cut out and a gelatin agar placed between the tip and stump a response still happens chemical signal still moves downConclusion perception is at tip blue light controlled by blue light receptor differential growth response further down the stem so perhaps theres a chemical signal from tip that moves down the stem to target cells y Fig 181 Action spectrum for phototropismAction spectra for curvature which occurs is strongly in response to blue light this is the same receptor controlling guard cell openingthe pigment whose absorption spectrum matched the action spectrum was looked for however it didnt match exactly which isnt a surprise because the absorption spectrum looked for isnt one that occurs naturally and so its different than in vivo y the cotyledon will grow in the direction of light it will also respond to a light gradient meaning that it will grow towards the source with the higher light intensity y this tropism response is also a very rapid response a plant thats grown in the dark then given a 30s pulse of unidirectional blue light within 30 min will show very rapid growth in shaded side slowed growth on the irradiated side the control will show equal growth straight linear linemeasurement of effect of biologically active substance on living materialy Bioassay The tip were cut off placed in agar the chemical then moved down onto agar The agar was then placed on to the tip and you get curvature All the chemicals were taken chemical fractionation applied chemicals in agar separated chemicals were then applied to tipcheck to see if you get a growth responsey The chemical was id indole 3 acetic acid an auxin there are several different types of auxins y Experiment to confirm IAAs ability to increase rate of cell growth stem sections were placed in petri dish for 18 hours in presence or absence of IAAIAA promotes growthmore elongation occurs under v lowof IAA dramatic effect
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