1. Chapter 35: Plant Structure and Growth The Plant Body Both genes and environment affect plant structure Plants have three basic organs: roots, stems and leaves The Root System Fibrous vs. taproots Root hairs Adventitious roots: roots arising from the shoot or leaves and existing above ground.

2. Figure 35.2 Morphology of a flowering plant: an overview

3. Figure 35.3 Radish root hairs

4. The Shoot System: Stems and Leaves Stems Nodes, internodes, axillary buds and terminal buds Apical Dominance: ability of the terminal bud, through hormonal action, to inhibit the growth of the axillary buds. The concept of “pruning” and “pinching back” plants Modified shoots Stolons: above ground “runners” of strawberry plants; this is a form of asexual reproduction when the parent plant fragments. Rhizomes: horizontal stems that are underground Tubers: starch containing, swollen ends of rhizomes (onions, potatoes)

5. Figure 35.4 Modified shoots: Stolons, strawberry (top left); rhizomes, iris (top right); tubers, potato (bottom left); bulb, onion (bottom right)

6. Leaves Blade, petiole Grasses don’t have petioles. At the base of the leaf there is wrapping around of the blade to form a sheath around the stem. Apical Dominance: ability of the terminal bud, through hormonal action, to inhibit the growth of the axillary buds. The concept of “pruning” and “pinching back” plants Venation Monocots have parallel venation in their leaves Dicots have a branching of the veins.

7. Plant organs are composed of three tissue systems: dermal, vascular, and ground Dermal Tissue or epidermis Protection of the young parts of the plant Can be specialized such as in the formation of root hairs or a cuticle. Vascular Tissue Xylem Tracheids and vessel elements: both dead at maturity Wood is mainly tracheids and vessel elements

8. Figure 35.8 Water-conducting cells of xylem

9. Figure 35.9 Food-conducting cells of the phloem

10. Phloem Composed of sieve tube members, sieve plates Companion Cells nonconducting cell connected to a sieve tube by plasmodesmata its nucleus and ribosomes service the sieve tube with which it is associated. also help to transport (load) sugar into the sieve tube members

11. Ground Tissue tissues that are not vascular or dermal. they could be tissues that simply increase the girth of the plant, or storage cells the pith of a plant and its cortex are considered ground tissues.

12. Figure 35.10 Review of general plant cell structure

13. Plant tissues are composed of three basic cell types: parenchyma, collenchyma, and sclerenchyma Parenchyma Cells Thin, flexible primary cell walls Protoplasm (all cell contents except for cell wall) contains a huge vacuole. Least specialized Aid in sugar transport, contain chloroplasts, store starch, make up most of the fruit tissue that you eat.

14. Collenchyma Cells Thicker primary cell walls than parenchyma cells Help support a young plant Strings of the celery stalk are made of collenchyma cells Living cells, flexible and can elongate Sclerenchyma Cells Thick secondary cell walls cell walls contain lignin cannot elongate like the collenchyma cells dead at maturity Support cells are called fibers and sclereids

15. Figure 35.11 The three major categories of plant cells

16. The Process of Plant Growth and Development Meristems generate cells for new organs throughout the lifetime of a plant Types of Meristems Apical: tips of roots and shoots Primary growth: lengthening of the root or shoot Lateral Meristems: widening of roots and shoots a lateral meristem could add bark another lateral meristem could add vascular tissue

17. Figure 35.12 Locations of major meristems: an overview of plant growth

18. Figure 35.13 Morphology of a winter twig

19. Primary growth: Apical meristems extend roots and shoots by giving rise to the primary plant body Primary Growth of Roots Root Cap Three zones of cells at the cap Zone of cell division (meristematic region) Zone of elongation: pushes root tip through soil Zone of maturation: area of root hairs

20. Figure 35.14 Primary growth of a root

21. Roots have 3 types of meristematic tissues Protoderm which gives rise to epidermis of the root Procambium which produces the stele, which is vascular tissue. Ground Meristem which gives rise to all other types of cells called ground tissue system. Example: parenchyma cells for food storage.

22. The Epidermis single cell layer protection specialized epidermal cells produce root hair extensions that absorb water and minerals The Stele central cylinder in roots vascular tissue has a different organization in dicots and monocots Dicots: stele is made of phloem and xylem. The xylem form spokes with the phloem in between the spokes Monocots: stele is parenchyma cells and xylem and phloem

23. Figure 35.15 Organization of primary tissues in young roots

24. The Ground Tissue System full of parenchyma cells fills the cortex where the roots store food Endodermis: innermost layer of the ground tissue; surrounds the stele in monocot and dicot roots

25. Figure 35.16 The formation of lateral roots Forms from outermost layer of the stele, the pericycle. It will become meristematic and push through the cortex.

26. Primary Growth of Shoots 3 Primary Meristems (just like in the roots) Protoderm: gives rise to the epidermis (just like the root) Here the epidermis covers the stems and leaves Procambium: gives rise to the vascular tissue Ground meristem: gives rise to the ground tissue such as the pith of a stem and the cortex of the stem.

27. Primary Tissues of Stems Vascular Bundles run up and down the stems in strands whereas in roots the vascular bundles are in the vascular cylinder. Xylem faces the inside of the stem (the pith) and the phloem is to the cortex. Monocots: the vascular bundles of xylem and phloem are scattered throughout the ground tissue Dicots: the vascular bundles of xylem and phloem are arranged in rings.

28. Figure 35.18 Organization of primary tissues in young stems

29. Figure 35.19 Leaf anatomy

30. Secondary growth: lateral meristems add girth by producing secondary vascular tissue and periderm. Introduction There are two meristematic regions concerned with lateral growth: 1.Vascular cambium: produces secondary xylem and phloem 2. Cork cambium: tough outer covering of stems and roots and in the process it replaces the epidermis.

31. Secondary Growth of Stems Vascular Cambium and the Production of Secondary V. Tissue accounts for the increase in girth of gymnosperms and dicots produces secondary xylem to the interior and secondary phloem to the exterior. It’s the secondary growth of xylem that we call wood.

32. Figure 35.20 Production of secondary xylem and phloem by the vascular cambium This occurs NOT at the apical meristem (that’s primary growth where things are lengthening) but farther down the stem.

33. Wood Consists mainly of tracheids and vessel elements and sclereid fibers. All are dead These cells have hard, lignified cell walls. In temperate regions, where there is a winter and spring/summer, the vascular cambium stops differentiating in winter, forming rings. Spring tracheids are large because here is lots of water present which expands the cells before they die.

34. Figure 35.21 Secondary growth of a stem (Layer 3)

35. Figure 35.22 Anatomy of a three-year-old stem

36. Figure 35.22x Secondary growth of a stem

37. Cork Cambium and the Production of Periderm As the girth increases, the epidermis will split and fall off. Another meristematic tissue, the cork cambium, produces cork cells to the exterior. Mature cork cells secrete suberin which is the wax coating the cell walls. Cork functions as a pathogen barrier. Periderm: cork plus cork cambium Lenticels: small openings through the periderm for gas exchange. Bark: all tissues external to the vascular cambium which includes the secondary phloem, cork cambium and cork but this phloem does not transport sugar

38. Figure 35.23 Anatomy of a tree trunk Consists of secondary xylem No water transport here

39. Mechanisms of Plant Growth and Development Introduction The same questions pertain to plants as they do to animals: How does the fertilized egg, the zygote become so many different types of tissues and organs? If they all have the same genome what is regulating these processes; the branching, the formation of leaves, flowering, the dropping of leaves. There are three fundamental processes that will transform the zygote into a plant: 1.Growth 2.Morphogenesis 3.Differentiation

40. Molecular biology is revolutionizing the study of plants The plant most used for research is Arabidopsis thaliana or Arabidopsis Arabidopsis has the expected characteristics of an organism you could work with in the lab: small and easily growth in the lab or greenhouse short generation time of about 6 weeks so you get new seeds (offspring) quickly. It has a small genome (Arabidopsis has had its genome sequenced.) Knowing its genes and determining their function allows the scientists to know how plants develop. One way to know what a gene does is to mutate the gene and see what structure or function has been goofed up. Mechanisms of Plant Growth and Development

41. Figure 35.25x Arabidopsis thaliana

42. Figure 35.25 The proportion of Arabidopsis genes in different functional categories

43. Growth, morphogenesis, and differentiation produce the plant body Growth: simply the increase in size and mass of the plant This results from cell division and cell enlargement Mechanisms of Plant Growth and Development Morphogenesis: the formation of the form of the organism, in this case the plant, with its leaves, branching pattern, roots, stems and also the organization of these structures So, same question, what genes are being turned on, off, at what time in development, what genes are being turned on together. Differentiation: as far as plants are concerned it would be the formation of the diversity of cell types, such as guard cells, apical meristems, roots and root hairs, xylem and phloem. How are the genetic instructions regulated to get a healthy plant? It’s the same as asking that about a human.. How do the genes get regulated to produce a healthy baby?

44. Growth involves both cell division and cell expansion The Plane and Symmetry of Cell Division It is important to have cells simply divide in one plane so a plant gets taller. Asymmetrical division is important in the formation of specialized cells such as guard cells. A key factor in controlling this is the distribution of the cytoplasm into the daughter cells Mechanisms of Plant Growth and Development A Preprophase Band of microtubules can orient itself one of two ways around the nucleus and control the plane of division.

45. Figure 35.26 The plane and symmetry of cell division influence development of form A totally different plane of division occurred to produce these two guard cells.

46. Figure 35.27 The preprophase band and the plane of cell division A Preprophase Band of microtubules can orient itself one of two ways around the nucleus and control the plane of division. Occurs during late interphase The microtubules hold the nucleus in place during spindle formation so the spindles are directed to their respective positions to produce the determined division pattern.

47. The Orientation of Cell Expansion The uptake of water is very important and this produces the cell’s expansion. The water goes into the central vacuole which grows due to the coalescing of many small vacuoles. They don’t expand equally in all directions; usually it is along the main axis which is controlled by the cellulose in the cell walls. Mechanisms of Plant Growth and Development

48. Figure 35.28 The orientation of plant cell expansion

49. Figure 35.29 A hypothetical mechanism for how microtubules orient cellulose microfibrils

50. The Importance of Cortical Microtubules in Plant Growth Mutants have been made of Arabidopsis, called Fass mutants. The microtubular arrangement in these mutants is abnormal The preprophase bands do not form in an organized fashion which affects the arrangement of the cellulose microfibrils in the cell wall. The poor arrangement of the cellulose microfibrils affects the elongation of the cell causing it to elongate in all directions equally and to divide every which way. Mechanisms of Plant Growth and Development

51. The fass mutant of Arabidopsis confirms the importance of cortical microtubules to plant growth Wild-Type Seedling Fass Seedling Mature Fass Mutant

52. Morphogenesis depends on pattern formation Pattern Formation: the development of specific structures, like a branch or a sepal, in a specific location. Pattern Formation depends on signals that each cell detects when it is part of the embryo. Even when a cell is within an organ, such as a developing flower, it continues to receive cues about its role in flower development. Mechanisms of Plant Growth and Development

53. What produces this positional information? What are the cues? Polarity Remember that a plant has a root and a shoot end and these two poles of the plant must form very different structures. The first cell division of the plant zygote is asymmetrical and this polarizes the plant into a shoot and a root. Mechanisms of Plant Growth and Development

54. Cellular differentiation depends on the control of gene expression Cellular differentiation is accompanied by differential protein production with a cell. Cells with the same genome follow different sets of instructions. Do you see why the control of transcription and translation is so important? It’s the proteins that are produced by the expression of these genes that control the plant development. Mechanisms of Plant Growth and Development

55. Cellular differentiation depends on the control of gene expression These cells are root cap cells which will be lost as the root hairs differentiate from the root cells. A single cortical cell is in contact with two epidermal cells. This causes a specific gene to be expressed and these cells will not form root hairs. This epidermal cell touches two cortical cells and a gene is not expressed and root hairs will develop

56. Clonal analysis of the shoot apex emphasizes the importance of a cell’s location in its developmental fate Phase changes mark major shifts in development Definition: the changing from one developmental phase to another such as changing of a leaf’s change when it is young to when it is older. (Figure 35.34) Phase changes can occur throughout a plant’s life. An older branch can produce young or juvenile leaves and make that part of the plant look young. In animals, this does not happen; the entire organism ages. You don’t find a young liver in an older person because the liver developed later in life. Mechanisms of Plant Growth and Development

57. Figure 35.34 Phase change in the shoot system of Eucalyptus Young leave structure Older leave structure

58. Genes controlling transcription play key roles in a meristem’s change from a vegetative to a floral phase. What this is referring to is simply, what changes occur when a plant begins to flower? What goes on cellularly to make the plant produce a flower? Length of night, hormones, and intracellular signals do all this The genes associated with these changes are called meristem identity genes. And like all genes, they make proteins, and in this case they are transcription factors that take the nonflowering part of the apical meristem and make it flower. Some of the genes responsible for forming stamens and carpels have been identified by mutating certain genes and seeing what malfunctioned. Mechanisms of Plant Growth and Development