BIOLOGY 205/SECTION 7 DEVELOPMENT- LILJEGREN Lecture 11 Arabidopsis flower development 1) Arabidopsis has become the ideal model plant to study development. a. other plants were used as models in the past. Mendel studied peas, maize (corn) was one of the two major model organisms (the other being Drosophila) studied by geneticists in the early 20th century. b. rice is now a really popular model crop plant to study, since like Arabidopsis, its genome has been completely sequenced, and its a monocot (Arabidopsis is a dicot). 2) Advantages and history of this model weed. a. Much faster generation time than other model plants like maize, rice, and tomato, or compared to other more familiar Brassicaceae family members (cauliflower, cabbage etc) b. compact genome, not polyploid like many crop plants (maize)! Makes genetic analysis much easier if don’t have duplicated genomes (ie only 2 copies of a gene instead of 4). c. Elliot Meyerowitz was one of the first scientists who switched from studying development in flies to developing Arabidopsis as a model genetic system more than twenty years ago. 3) Genetic analysis in Arabidopsis: Finding mutants through forward genetics a. Finding mutants the same as in flies or worms. Treat seeds with mutagen (EMS), screen next generation (M2) for mutant phenotype you’re interested in. b. EMS introduces point mutations (changes at random places in the genome G>A, C>T). If point mutation changes codon, can get different amino acid. Many mutations are silent: ie. change in 3rd nucleotide (wobble position) of a codon may not change the amino acid, and some amino acid changes wouldn’t affect a protein’s function significantly. c. Since Arabidopsis self-fertilizes have same advantage as worms (and opposed to flies)—can skip a generation to get homozygous mutant. d. Many classic flower mutants we’ll be talking about were first identified in mutant screens a long time ago. 4) Genetic analysis in Arabidopsis: Making transgenic plants a. Since early 1980’s can make transgenic plants using Agrobacterium. Like ability to make transgenic mice, this was a major breakthrough for plant research because of all the additional genetic experiments it made possible. b. Agrobacterium in nature (unmodified by scientists) transfers a linear piece of DNA (T-DNA=transferred DNA) into a plant cell where it becomes integrated into the plant genome. The T-DNA contains genes encoding for hormones that in some cases cause uncontrolled growth of the infected plant cells (crown gall tumor specific for Agrobacterium tumefaciens). c. Scientists use modifed Agrobacterium that no longer contains these hormone-producing genes (so no tumors) on its T-DNA. Instead a gene providing kanamycin resistance is present and you can clone in your favorite gene. When this modified Agrobacterium is used to transform Arabidopsis, both genes are integrated together at random spots throughout the genome. Can then select for positive transformants (transgenic seedlings) by screening for kanamycin resistance. 5) Plants are different than animals!a. Plant cells have rigid cell walls and don’t migrate during development. Ie. Gastrulation is NOT used to set up the multilayered body plan present in plants (epidermis, endoderm, and vascular). b. Unlike animals, plants continue to generate new structures after embryogenesis from meristems (clusters of actively dividing cells that are like stem cells) present in the shoot and the root. Shoot meristems, which are indeterminate, are continuously replenished as cells at their flanks become part of new organs (ie. leaves, flowers, secondary shoot meristems). Flower meristems are determinate, they generate all the parts of the flower and in the process are used up. 6) Flower development. a. Arabidopsis flower has 4 types of floral organs which are arranged in whorls. From outside to inside, whorl of 4 sepals, whorl of 4 petals, whorl of 6 stamens and center whorl of 2 fused carpels that make up the fruit. b. Over past 15 years much of the molecular pathways underlying flower development has been revealed—we’ll cover just two stages of flower development: flower meristem identity and floral organ identity. 7) Flower meristem identity: LEAFY, APETALA1, CAULIFLOWER a. At this stage in development, the shoot apical meristem is producing leaves with associated secondary shoot meristems, so floral meristem identity genes control when the apical meristem starts producing flower meristems on its flanks instead of shoot meristems/leaves. b. apetala1 mutant: sepals converted into bracts (leaf-like structure), no petals, additional secondary flower meristems produced so these mutant flowers have some of the characteristics of shoots. c. cauliflower mutant: no phenotype d. apetala1 cauliflower double mutant: flowers converted into inflorescence (shoot) meristems. Both the APETALA1 and CAULIFLOWER genes encode closely related MADS-box transcription factors. They show partial functional redundancy, ie. cal mutant has no phenotype on its own, and the ap1 mutant phenotype is dramatically enhanced by additional mutations in CAL. e. leafy mutant: several additional shoot meristems/leaves produced instead of flowers, some of later flowers produced have shoot-like characteristics (associated leaves). Wildtype flowers never have an associated leaf in Arabidopsis. LEAFY encodes a different type of transcription factor. f. leafy apetala1 double mutant: all flowers transformed into shoot meristems/leaves. This is evidence of functional redundancy between LEAFY and APETALA1 in specifying flower meristems. 8) Floral organ identity: the ABC model a. How does an undifferentiated ball of cells (the flower meristem) become a flower? What controls which floral organs are produced where? b. A activity controls whorls 1&2 (sepals and petals), B activity controls whorls 2&3 (petals and stamens), C activity controls whorls 3&4 (stamens and carpels). c. A activity in whorls 1&2 prevents expansion of C activity (normally only in whorls 3&4) into whorls 1&2, and vice versa. B activity unaffected by changes in A and C. d. This is the famous ABC model (“war of the whorls”) proposed by John Bowman and Elliot Meyerowitz 15 years ago: A alone produces sepals, A+B produces petals, B+C produces stamens, C alone produces carpels. e. Expression patterns of
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