Concepts of Biology (BIOL116) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/

Plant Growth and Development

I.  Definitions & Life Cycle Revisited

• growth, development, differentiation
• plant life cycle

II. Meristems

A.  General

• growth in plant occurs in meristems/meristematic regions
• meristems are areas of active cell division
• meristems are only the region of plant where cell division/growth occur
• growth is restricted in plants because of the cell wall & architectural design
• typical pattern of growth:  meristem �  new cells � cells enlarge � cells differentiate 

A.  Apical (or Primary) Meristems

• responsible for growth in length
• located at tip of roots & shoot
• derived from meristem originally present in the embryo (at tip of radicle and epicotyl)
• remember plants grow in length from the tips (Check out Gink & Go Visit the Playground for more information)
• root meristem - growth in length of the root; protected by root cap; recall the regions of roots (cell division = meristem, cell elongation, differentiation)
• shoot meristem - produces growth in length of stem; source of new leaves; produces buds at base of leaves

B.  Buds

• produced at base of leaves (or a corollary, every leaf has a bud at its base)
• embryonic leaves & stem - essentially a miniature shoot that is "waiting" to develop.  
• buds give rise to branches

C.  Pericycle

• meristem in root
• beneath endodermis
• gives rise to lateral roots
• type of secondary meristem - ultimately derived from the primary (or apical) meristem

D.  Vascular Cambium

• produces xylem (to inside) and phloem (to the outside)
• growth in width (girth)
• location in woody plants - stem anatomy (bark, wood, heartwood, sapwood, phloem, xylem, rays, spring wood, summer wood, annual rings, dendrochronology)
• location in herbaceous plants - vascular bundles 
• another secondary meristem

E.  Cork cambium

• produces cork or periderm - protects phloem and wood
• secondary meristem

III.  Seed Structure. (a baby in a suitcase carrying its lunch)

• Coat (testa)
• Embryo (cotyledon, hypocotyl, radicle, epicotyl)
• Nutrients (endosperm, and/or cotyledons)
• Types (monocot/grass; dicot with endosperm; dicot with minimal endosperm)
• seeds vs. spores

IV.  Germination

• Requirements (oxygen, temperature, water)
• Imbibition
• Radicle protrusion
• Quiescent vs. dormant vs. dead
• Breaking Dormancy (scarification, stratification, light, after ripening, leaching)
• Early Metabolism - glycolysis, respiration, photosynthesis

V.  Growth is controlled by hormones

1. Definition:  small, organic molecules, synthesized by the plant, active in low concentration (<10^-6 M), typically there is a separation of site of synthesis and site of action; promote or inhibit growth and developmental responses.  Based on these criteria, would the following substances be considered hormones? Ca²⁺, sucrose, 2 4 - D, glycine, or K⁺
   
       
2. Evidence for existence
   
3. A comparison of plant vs. animal hormones
   

   	Plants	Animals
   number of hormones	few	many
   specificity of action	non-specific	specific
   work together	yes	no
   site action/production separation	no	yes

       Thus, there are comparatively few plant hormones, each elicits a variety of responses and often works together with other hormones. In contrast, animals have numerous different kinds of hormones, each with a specific function, and it works alone to induce a response.

       Why the differences? Good question. It may be partly a function of our ignorance; in other words, there are likely to be many more plant hormones that simply haven�t yet been identified. Nevertheless, I suspect  that at least part of the reason for the differences is related to body design. Recall that plants have an architectural with a limited number of parts that are repeated. It follows that only a limited number of hormones would be necessary to induce growth/developmental responses. With a mechanical design and numerous separate parts, animals required unique chemical messengers to interact with each one.

   
   

4. Types

1. Auxin (indole acetic acid, IAA) - cell elongation, adventitious root formation, tropisms, fruit development, apical dominance, leaf abscission
   
   
2. Gibberellin - stem elongation, bolting, flowering, germination (beer making)
   
   
3. Cytokinin - cell division & differentiation, retard senescence (aging)
   
   
4. Ethylene - gas, fruit ripening, senescence, abscission
   
   
5. Abscisic acid (ABA) - stress (i.e., water), dormancy
   
   
6. Others (polyamines - gene expression: systemin - defense: oligosaccharins - defense: salicylic acid - defense; jasmonates & systemin - defense; brassinolides - stem elongation, pollen tube growth, vascular tissue development; "florigen" - flowering)

VI.  Tropisms & other Growth Responses

1. General Response System:  environmental signal/stimulus � receptor � transducing mechanism � response
   
   
2. Gravitropism:  signal - gravity; receptor - root cap; statoliths (starch grains); Mechanism - auxin transport/synthesis; differential sensitivity; Responses - cells on top of root or bottom of shoot grow more
   
   
3. Phototropism:  signal - light (blue); receptor - pigment (flavin); Mechanism - auxin transport, shaded side elongates
   
   
4. Apical Dominance - apical meristem inhibits development of nearby lateral buds.   Gardeners have long known that you pinch off the tip of the plant and it stimulates it to branch.  Also responsible for the xmas-tree shape of many plants, especially conifers.  Functions:  (a) to prevent plant from getting too top-heavy; (b) minimizes shading of lower leaves; (c) allow another shoot to replace one that is damaged.  Response due to hormones, especially IAA, ethylene and cytokinin.  In the simplest hypothesis - IAA produced at the tip moves toward the base of the plant.  The concentration decreases from shoot tip toward root.  Lateral buds are inhibited by high concentrations of IAA and therefore don't develop until the tip has grown sufficiently far away that the IAA concentration is lower.  Cytokinin and ethylene are also important.
   
   
5. Etiolation - response of growth to dark.  Dark-grown (or light-limited) plants:  (a) lack chlorophyll; (b) have elongated internodes; or in other words grow faster as they attempt to "find" light; (c) apical hook; (d) smaller leaves or unopened cotyledons.  These are all mechanisms to allow the plant to conserve as much energy as possible until it once again "finds" light.

VII.  Flowering

A.  General Sequence of Events:  juvenile vegetative � adult vegetative � adult reproductive � flowering

• juvenile to adult transition - influenced by factors such as age, size, leaf number, nutritional status, growth condition
• vegetative to reproductive transition - influenced by a "signal" such as photoperiod or temperature

B.  Photoperiod

1. Stimulus -daylength (short day = long night; long day = short night)
2. Receptor - leaves, phytochrome (exists in two forms - Pfr, Pr; converted by light)
3. Transducing Mechanism - hormone involved - florigen? GA?  Simplistic view - Pfr stimulates flowering in LDP, inhibits in SDP.

C.  Vernalization - cold treatment required to flower; stimulus - cold; receptor - imbibed seeds, seedlings; transducing mechanism - hormone? GA? vernalin?