AGR2451 Lecture 8 Notes -- Raizada

Review of Last lecture•Plants are immobile•The plant is simply a nutrient gathering machine. The nutrients are gathered to allow the organism's DNA to propagate.•Vegetative body is entirely designed to get nutrients•Plant architecture - phyllotaxy, branching, reiteration•Organ shape - cell elongation and orientation of cell division•Immobile, so needs to disperse DNA by other means•Plant body plan is determined post-embryonically from meristems - flexibility•Many types of meristems •Explain how a vegetative meristem gives rise to a leaf primordia (picture)•Flexibility in means of reproduction -- where and how - sexual and asexual

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Slide 8.2

A Overview of Plant Reproduction

Biochemistry and Molecular Biology of PlantsW.Gruissem, B. Buchanan and R.Jones p.989ASPP, Rockville MD, 2000

Lecture 8 -- Plant Development II - "The Mechanics of Plant Reproduction"

Reproduction is the key process for plant agriculture and the key target for breeding.

1. Flowers The flower is made up of 4 concentric rings, each ring containing a

different organ type.

QuickTime™ and aPNG decompressor

are needed to see this picture.

Biochemistry and Molecular Biology of PlantsW.Gruissem, B. Buchanan and R.Jones p.997ASPP, Rockville MD, 2000

Describe the functions of the following floral structures:sepals-petals -stamens-carpels-

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How is diversity created amongst the flowers of the 235,000 species of Angiosperms?

From Biology of Plants p.382P. Raven, R. Evert and S.EichhornWorth Publishers, New York, 1992

Martin, C. and Gerats, T. (1993) Control of pigment biosynthesis genes duringPetal development. Plant Cell 5, 1254. ASPP Publishing, Rockville, MD, USA

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-***The key decision made by a plant is when to flower = age + environmentWhat are these environmental cues and why?

How does a plant sense daylength?-pigment phytochrome = receptor, 2 forms, Pr and Pfr -- Pfr builds up in leaves and is the active signal to vegetative meristem, converts it to an inflorescence (flower-producing) meristem-This is not chlorophyll

Define:Short-day plantLong-day plant

Effect on HumanCivilization?

Slide 8.5

From Biology of Plants p.579 and p.948P. Raven, R. Evert and S.EichhornWorth Publishers, New York, 1992

2. Gametes -The male gamete (sperm) is stored in a multicellular (micro)gametophyte, the pollen grain. -Each pollen grain actually possesses 3 haploid cells formed by two

rounds of mitosis following meiosis: -a vegetative cell -its nucleus regulates the enzymes that regulate the gametophyte including pollen tube growth after fertilization-two sperm cells -- function?

From Biology of Plants p.266P. Raven, R. Evert and S.EichhornWorth Publishers, New York, 1992

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Gametes - Female•Meiosis produces 4 haploid spores. •Three die. •The last one (the "megaspore") undergoes 3 mitotic divisions to form 8 nuclei (contained in 7 cells). •The central cell contains 2 nuclei, and both of these nuclei will be fertilized by one sperm to form a triploid endosperm. •Another cell is the egg cell which is the female gamete. •All of the 8 nuclei are located inside a multicellular

(mega)gametophyte, the embryo sac. •The embryo sac is located inside the carpel (ovary) of the flower

embryo sac

Reiser, L. and Fischer, R.L. (1993) The ovule and the embryo sac.Plant Cell 5, 1294. ASPP Publishing, Rockville, MD, USA

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Gametes (Continued)

Why do plant gametophytes consist of multiple haploid cells and multiple nuclei? Do the different haploid cells in an embryo sac send signals to each other and if so why?

Why do plants have double fertilization?

A single corn plant produces 25 million pollen grains but only 300-600 female eggs (in embryo sacs). What use is there for producing so much pollen?

Excess pollen permits competition between pollen grains for females, by testing the fitness of the parental genes in single copy form -- 60-90% of the genes expressed in the vegetative part of the plant are expressed in haploid pollen. Excess airborne pollen has implications for breeding and GMOs. What are these?

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3. Fertilization•The embryo sac(eggs) is buried deep in the flower where it isprotected.•Pollen, however, lands far away from the egg on the surface of the flower (of the same species) on the stigma.

•The stigma has a sticky surface of protruding papillae to catch the pollen. •The stigma is at the protruding tip of a long hollow cylinder (the "style") elevated to catch pollen. •At the base of the style is the embryo sac containing the egg. How do the two sperm get to the embryo sac?

Biochemistry and Molecular Biology of PlantsW.Gruissem, B. Buchanan and R.Jones p.1021ASPP, Rockville MD, 2000

From Biology of Plants p.392P. Raven, R. Evert and S.EichhornWorth Publishers, New York, 1992

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Biochemistry and Molecular Biology of PlantsW.Gruissem, B. Buchanan and R.Jones p.1019ASPP, Rockville MD, 2000

Fertilization (Continued)A corn silk is a long style terminating in an embryo sac on an ear of corn. (demo)

Obviously, pollen from one species can land on the floral stigma of a different species. Why doesn't cross-species fertilization happen?

Some crops are self-fertilizing whereas others are outcrossers. What are the advantages and disadvantages of each strategy?

Because pollen and eggs are often produced in the same flower, how do outcrossers manage to have their eggs fertilized by sperm (pollen) originating from other plants?

*Self-incompability is found in many plants, including Solanaceous species and Brassica. *Corn and the major crop grasses do not reject their own pollen.

Biochemistry and Molecular Biology of PlantsW.Gruissem, B. Buchanan and R.Jones p.1021ASPP, Rockville MD, 2000

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Fertilization (continued)The embryo and the endosperm are formed from separate fertilization events with two sperm.

Why do higher plants have double fertilization?

QuickTime™ and aPNG decompressor

are needed to see this picture.

Biochemistry and Molecular Biology of PlantsW.Gruissem, B. Buchanan and R.Jones p.1023ASPP, Rockville MD, 2000

Some higher plants can produce embryos without meiosis or fertilization. Instead, cells from the parent embryo sac divide and directly produce an embryo inside the sac. This process is called apomixis.

Hybrid vigor occurs in the F1 (first generation) progeny of twodivergent parents, but diminishes in the next generation. For corn, how is hybrid seed generated?•Apomixis may help farmers in the Developing World.

•Dandelions reproduce via apomixis. They are clones of one another.Slide 8.11

4. Seeds •Fertilization -embryo, an endosperm surrounded by a seed coat. •As the seed is being formed, the plant continues to senesce (a slow-down in carbon fixation +photosynthesis) at end of the growing season. •However, carbon and nutrient reserves are needed by the seed. •This occurs via a breakdown of starch, protein bodies and oils in the leaves, then translocated to the growing seeds (= "grain fill" period).

***Much of the success in increasing crop yields (as in corn) has come from delaying the onset of senescence, allowing the plants to continue to fix carbon during grain fill. •After the seed has matured, it becomes dormant - low water content•Annual plants make a huge investment in reproduction as a percentage of their dry matter. Perennial plants do not. Why not?

•The seed has to be full of pesticides, and humans have selected for mutations in the genes that encode the enzymes that produce these toxic biochemicals, a process that is ongoing,incomplete and poorly researched.

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Lopes, M.A. and Larkins, B.A. (1993) Endosperm origin, development and function.Plant Cell 5, 1389. ASPP Publishing, Rockville, MD, USA

5. Seed germination

What environmental signals does a seed use to determine when to germinate or not germinate and why??

In general, how would a seed sense these signals at the molecular level?

These signals ultimately cause the turning on of genes via transcription, that cause cell division and lead to seedling germination and the orderly determination of cell types and organs. The storage reserves are broken down and used by the developing seedling.

Please hand in questionnaire!!

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