Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Lab 05: Leaf development in Pisum sativum (pea), Stems and Roots Pea leaf mutants demonstrate the plasticity of leaf development. A change in one gene product results in the conversion of compound leaves to simple leaves (unifoliata-uni), the conversion of leaflets to tendrils (afila-af), tendrils to leaflets (tendril-less-tl), partial loss of stipules (reduced stipules-st), change in leaflet and stipule shape (sinuate leaf-sil), and crinkling of leaves due to loss of adaxial identity (crispa-cri). To prepare for this lab, you need to read sections 4.16-4.18 in your book (Cronk, 2010). For the leaf development section of this lab you will: -describe wild-type pea leaves -identify which genes are defective in a series of mutant plants -make predictions of the phenotypes of additional mutant combinations -make predictions of the effects of mutations in other plants -contrast pea mutants with other plants with compound leaves Characterizing mutants Obtain a pot with a wild-type pea plant to share with your table. In the space below, sketch one leaf, including its attachment to the stem. Include the petiole, rachis, stipules, the correct number of leaflets and tendrils. Label each part of the leaf. 55 We will typically have four to five single mutants (may vary year-to-year). Describe and illustrate the plant’s appearance (leaf phenotype) and attempt to determine which of the genes described in the introduction is mutated (genotype) in each case. When you are done, you may check your answers with your TA. 1. Phenotype: Genotype: 2. Phenotype: Genotype: 3. Phenotype: Genotype: 4. Phenotype: Genotype: 5. Phenotype: Genotype: 56 The pots labeled with letters are mutant combinations. These may be a bit trickier. Refer to the descriptions in the introduction and the mutants you just observed. Attempt to identify at least 3 of the mutant combinations. A. Phenotype: Genotype: B. Phenotype: Genotype: C. Phenotype: Genotype: When you are done reasoning your own mutant characterization, consult the mutant key available in lab 57 As you examine the pea plants, answer the following questions: Do mutations that affect the compound leaf also affect the stipules? If so, which ones? ___________________________________________________ Do mutations that affect the stipules also affect the compound leaf? If so, which ones? ___________________________________________________ Do any mutations have similar effects in the leaflets and stipules? If so, which ones? ____________________________________________________ Do regions of stipules and the compound leaf share a developmental program? Compound leaves in other lineages Compound leaves are characteristic of the legume family (Fabaceae). Bean plants (Phaseolus vulgaris) have compound trifoliate leaves, but do not have stipules or tendrils. Given what you know about each of the pea leaf mutants observed, predict the effect of similar mutations on bean leaves. afila tendril-less unifoliata crispa 58 Simple leaves are most certainly ancestral in Angiosperms, and compound leaves have evolved many times. Compound leaf development has been well studied in the tomato family (Solanaceae). Observe available tobacco (Nicotiana tabacum) and two species of Solanum, one with simple leaves (Solanum cheesmanii) and compound-leaf tomato plants (S. lycopersicon). Use your lecture notes and readings to help answer the following questions: How is the development of compound leaves different in pea and tomato plants at a morphological level? How is the development of compound leaves different in pea and tomato plants at a molecular level? What are the underlying molecular mechanisms for compound leaf evolution? Is there evidence that small changes in gene expression can lead to large changes in leaf morphology? 59 Stems Goals: 1. Identify short shoots and long shoots in gymnosperms 2. Become familiar with shoot morphology in angiosperms 3. Observe histological sections of dicot and monocot stems. 1-Branching Patterns in seed plants: Gymnosperms Pine and related conifers – Some conifers such as Douglas fir only have one kind leaf and shoot system. However, two kinds of branches may be found in many conifer species. Long shoots, as the name implies, are shoots in which the internodes are relatively long. Short shoots produce leaves in a tight cluster so that internodes are scarcely detectable. Short shoots are very conspicuous in the pines (Pinus), larches (Larix) and "true" cedars (Cedrus). With the exception of Pinus, branches that have been growing for a number of years as a long shoot may convert to short shoot growth and vice versa. Cedrus deodara. Cedrus is the genus of "true" cedars. They have long shoot/short shoot morphology (LS/SS). The morphology of Larix (larch) is similar to that of Cedrus, but the former is deciduous and may already lack leaves at this time of year. Glance over an entire branch system and notice that some of the new growth (lighter green at the tips of main and side branches) is LS growth (relatively long internodes between needles) and some of it is SS growth (needles in apparent whorls). A terminal bud on the major branch axis usually opens to produce further LS growth, but you may find a major branch axis in which the terminal bud has reverted to SS growth (you may have to look at branches on other benches). Terminal buds on the short shoots usually open to produce further SS growth, but not always. Find a short shoot that has shifted from SS to LS growth. Does this seem to happen frequently? (Look at a number of different branch systems.) __________________________________________________________________ On long shoots, does every needle have an axillary bud associated with it? _________________ Ginkgo biloba Obtain a branch showing LS/SS morphology. Find a region of LS growth for the current year. Study the SS, noting the almost nonexistent internodes. Do SS ever shift to LS growth? _____________ Do LS ever shift to SS growth? ______________ Look at a number of branches before attempting to answer these questions. 60 2-Branching Patterns in Angiosperms Herbaceous shoot– Begin your observations by looking at an herbaceous plant such as Coleus, locate the following, a) apical meristem (shoot tip with leaf primordia), b) axillary bud, c) node and internode, d) lateral branches, and f) leaves What is the leaf arrangement? (alternate, opposite, whorled)_______________________ Woody Twig of Horse Chestnut – On the benches in the lab there will be a number of woody twigs of Horse Chestnut, obtain one. Using the illustration below, label the following: a) terminal bud, b) terminal bud scale scars, c) axillary (lateral) bud, d) leaf scar, e) leaf vein scars, and lenticels. What was the leaf arrangement on this plant?____________________________________ What happened to the leaves? Are they deciduous?_______________________________ Home many years of growth can you see on your twig?___________________________ (Clue: look at terminal bud scale scars!) 61 3-Cross sections of stems Observe a cross section of a corn stem (monocot) and a Coleus stem (dicot herb, primary growth only). Illustrate each below, identifying: epidermis, cortex, vascular bundles, primary xylem and phloem, cambium and pith. Stems vocabulary: Terminal bud- dormant bud found at the tip of the shoot (includes apical meristem). Lateral/ axillary buds- usually dormant meristems surrounded by leaf primordia/bud scales found in the axil of a leaf Adventitious bud- a bud initiated from a location other than an axil. Apical dominance- inhibition of axillary bud growth by the terminal bud. Indeterminate growth- continued growth from a long-lived meristem (most vegetative growth). Determinate growth- defined amount of growth before termination of meristem (most reproductive). Dichotomous branching- apical meristem splits to form two branches. Monopodial growth- growth associated with a persistent apical meristem. Sympodial growth- cessation of growth by apical meristem allows axillary branches to grow. Phyllotaxy- geometric arrangement of leaves on stem. Alternate, spiral- single leaf at each node, each occurring at about 135° from last leaf. Opposite, decussate- two leaves at each node, with each pair occurring at 90° from last pair. 62 Roots Goal: Identify and label anatomical and morphological structures of angiosperm roots. Branch (lateral) root initiation. Obtain a slide of Lupinus branch roots l.s. There will be branch roots of various ages, some of which are tiny primordia at the periphery of the vascular cylinder and others that have pushed through the cortex and emerged at the surface of the parental root (Fig. 8.6). a b Cross section of roots showing lateral root initiation: a, two lateral roots mid- through the cortex of the parent root; b, two lateral roots, growing through the cortex of the parent root. Eudicot Mature root x-section. Obtain a slide of Ranunculus. Use the diagrams below to help you find all of the following tissues on the slide: a. epidermis b. cortex. In many roots, the cortical cells are filled with starch. c. endodermis. In a stained cross-section of young root tissue, the casparian strip is seen as red dots in the radial walls of endodermal cells. But, in most of your slides, an old region of the root has been sectioned and the endodermal wall is more uniformly thickened. d. stele. The stele refers to the primary vascular tissue. e. xylem. Xylem is a tissue easy to pick out because it contains the large water-conducting cells in the stele. Note the four xylem "arms". f. protoxylem. Notice that the smallest water-conducting cells are toward the outside (at the tips of the xylem arms.) g. metaxylem. These xylem elements are the larger, more centrally located vessels and tracheids. f. phloem occurs in discrete regions between the xylem arms. 63 a, root cross section; b, detail of root stele. Monocot mature root cross section In the space below draw and label a root cross section of corn (Zea), include the following: a) root hairs, b) epidermis, c) cortex, d) endodermis, f) vascular bundles, and g) pith. Compare to the dicot root above. Root Morphology Fibrous and Tap roots To get an idea of how the morphology of the root system differs in monocots and dicots, examine available examples of each. Tap root systems are common in dicots. Fibrous root systems are characteristic of monocots; note the absence of a dominant root. Also note where the fibrous roots originate. Roots originating from stem tissue are adventitious roots (a.k.a. nodal roots, crown roots, prop roots). For both examples identify: root cap, root hairs, primary root, lateral roots. You may also detect the slimy mucigel secreted by the roots. 64 Each root hair is an outward extension of a single specialized epidermal cell. Are the root hairs randomly distributed over the surface, or do they have a defined arrangement? Why are the root hairs longer farther from the root tip? _________________________________ Hand-section of dicot root. Obtain a carrot longitudinal and cross section. Compare to the dicot cross section viewed previously. What is the function of this part of the carrot? Are root hairs evident on your carrot? Why or why not? Is the root cap evident on your carrot? Why or why not? Can you see lateral roots on the outside of the carrot? On the inside of the carrot? Is the stele evident? Root vocabulary Rhizoid- root-like structure with no vascular tissue (seedless plant gametophytes) Primary root- root arising from the embryonic RAM Lateral root- root arising from the pericycle (endogenous origin, in contrast to branch stems) Pericycle- meristem just exterior to stele, which produces lateral roots Adventitious roots- root arising from the shoot Root hairs- single-cell extensions of root epidermal cells, which greatly increase surface area (occur just behind zone of elongation and are ephemeral) Fibrous root- highly branched root system with many lateral roots of equal size Taproot- large, dominant primary root with reduced branching and strong gravitropism Endodermis- sheath of cells surrounding stele which function as barrier between root and soil environment Casparian strip- waterproof, waxy (suberin) layer found in the radial walls of epidermal cells Mycorrhizae- fungal symbionts that aid in plant absorption of nutrients Root nodules- modified lateral roots that allow for symbiotic association with nitrogen- fixing bacteria Prop roots- roots which function in support of the stem 65