PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Chapter 20: Gibberellins: Regulators of plantHeight and seed germinationGibberellin biosynthesis-Gibberellins are diterpenoids formed from four isoprene units-Synthesis begins in the chloroplast, continues in the ER & finishes in the cytosol-136 natural gibberellins have been identified: few of them have activity; many are precursors to active forms or are deactivated products AlkaloidsGibberellin biosynthesis: tissue expressionEffects of Gibberellins on growth and developmentStimulate stem growthStimulate fruit growthRegulate transition from juvenile to adultPromote seed development and germinationPromote pollen development and tube growthDose dependent growth response to GAGrowth effects of Gibberellins:Gibberellin (GA1) stimulates stem growth Ultradwarf is a GA biosynthetic mutant Tall is WT Slender is a GA deactivation mutantGibberellin regulationNotice the cumulative effects of the different GA formsWhy would GA autoinactivate itself?Negative regulatorGA20ox and GA3ox are key regulatory enzymes which catalyze the formation of bioactive GA9 and GA4Negative feedback regulation of bioactive GA: GA2ox synthesizes GA34 which inhibits GA20ox & GA30x activityAuxin stimulates GA3ox and inhibits GA2ox Mendel’s dwarfGibberellin growth stimulation Mechanism that explain GA induced growth are not as well understood as they are for auxin 1) It doesn’t appear that GA increases growth through osmotic potential 2) GA does increase cell wall extensibility A) GA does not stimulate proton pumping B) GA has been shown to increase the expression of some expansins C) upregulates XTH an enzyme that may facilitate the penetration of expansins into the cell wall 3) GA induces stem growth by stimulating cell division Phenotypes of knockout (A)and OE (C) lines of a GA induced expansinNorman Borlaug and the green revolutionPlant height can be genetically engineered GA2ox (catalyzes GA deactivation) overexpression produces dwarf varieties of wheatDwarfing is an important agronomic trait in wheat1) Resources can be partitioned to produce more seed instead of stem tissue2) Tall slender wheat lodges under the weight of heavy seed heads especially with fertilizer application; breeding for dwarf varieties that didn’t lodge contributed significantly to drastic increases in yield during green revolutionWhy engineer dwarf plant, don’t we want them bigger?Nobel Peace PrizePresidential Medal of FreedomCongressional Gold MedalGibberellin growth signaling pathwayDella is a constitutive repressor of GA response genesGID1 is a putative GA receptor; when it binds GA it interacts with GID2 which ubiquinates Della resulting in its degradationGibberellin activation of seed germinationEmbryo produces GA which diffuses to the aleurone cells surrounding the starch rich endospermIn response to GA the aleurone cells produce protein bodies that release hydrolytic enzymes α-amylase into the endosperm which convert the starch to sugars to fuel embryo growthGibberellin activated starch degradation pathway GA stimulates the transcription factor GAMYB which induces the transcription of α-amylase (starch hydrolysis) and other genes involved in seed germinationGA also facilitates the transport of secretory vesicles containing α-amylase Aleurone cellChapter 20 Homework questionsOutline the key enzymatic control points in the GA biosynthetic pathway (slide 6) that determines height growth in plants; include an explanation of the negative feedback pathway in GA synthesis and explain why plants would place limits on their height growth. How did manipulating traits for height related to GA regulation lead to the Green Revolution?Explain at the molecular and cellular levels how GA increases stem growth.Explain at the molecular and cellular levels how GA stimulates seed