BOT Exam 2: Hormones and Growth

Determinate Growth

Maximum size genetically determined. Growth stops after a time. Usually cannot heal/regrow. Maximum size rarely achieved in nature. Leaves, flowers and fruits

Indeterminate Growth

No maximum size… genetically. Able to keep growing through life. Usually can heal/regrow. Limited by resources and environmental factors. Roots and stems

Meristematic Tissue

The tissue where all tissues originate from

Totipotent

Stem cells with the potential to differentiate into any type of cell.

Meristems

Regions of continuous cell division and growth located in different points of the plant. The three types are Apical meristems, Lateral meristems and Intercalary meristems (only in grasses)

Axillary Buds

Can give rise to branching - new stems.

Monopodial

Buds do NOT degrade, and all shoots continue to grow

Sympodial

Terminal buds degrade, and lateral buds closest to the apex become the new terminal shoot

Internal Conditions

Conditions that control the development of growth: heredity, age and position. For example, the shape of leaves tends to be an inherited trait common across members of a species

External Environment

Environment that involves light, wind, temperature, etc. For example, wind-blown trees in sub-alpine areas form what is known as krummholz

Development: Growth

Cell division and enlargement

Development: Differentiation

Specialization of cells (via expression of genes)

Induced Changes

Modifications to cells through the life of the plant (e.g. preparation for winter)

Signals

Hormones, electric pulses, hydraulic impulses

Totipotency

Also known as cell potency. The ability of a cell to differentiate into other cell types. Animal cells are only totipotent early in the development of the zygote (then become determined)

Reversible Cells

Adventitious shoots and roots. Protoderm might divide and become part of the ground tissue. Tissue culturing

Plant Hormones

Substances that provide long-range signaling to coordinate growth, development and response to environmental stimuli.

Target Cells

Those affected by the hormone; have receptor proteins with binding sites. Higher calcium activates enzymes

Target Cells - Enzymes

Can play several roles in target cells including the activation of transcription factors that regulate gene expression

Auxins

A class of hormones that play an essential role in regulating growth (e.g. lateral root growth) and responses to the environment (e.g. gravitropism). Produced in shoot apical meristems and young leaves. Initiates lateral roots; promotes xylem differentiation

Apical Dominance - Auxins

Inhibits growth of axillary buds. High levels of auxin near the apical meristem prevent branching. Levels decrease further down - releasing axillary buds. Pruning also releases apical dominance.

Phototropism

Movement with respect to light sources

Gravitropism

Movement with respect to direction of gravitational force

Gibberellin

Hormone that plays a key role in the division and elongation of cells in the internode areas. Promotes germination of dormant seeds; promotes phloem differentiation

Cytokinin

A class of plant hormones that often play a complementary role to auxins (e.g. growth of axillary buds depends on the ratio of auxin to cytokinin). Produced in root apical meristem and developing fruits. Main function is to promote cell division (cytokinesis); diverts nutrients towards areas with high concentrations

Abscisic Acid

Plays a key role in preventing seed germination and promoting dormancy. Higher ABA levels in seeds allow them to survive low water conditions and prevent premature germination. Produced in leaves and ovules

Ethylene

An unusual hormone which exists as a gas in normal conditions. Produced in damaged (wounded or aged) tissue! Induces fruit ripening, signals leaf and flower senescence, and is involved in root hair formation

Homeostasis

Plant hormone balance determines processes: Auxins (stimulate root growth) vs. Cytokinins (stimulate axillary/bud growth) & Gibberellins (promotes growth of stems and fruit) vs. Abscisic Acid (inhibits growth until conditions improve)

Tissue Culturing

Allows seedlings to be produced from individual plant cells grown in the laboratory

Rosettes

Plants with internodes so short, leaves seem to grow from the same point.

Bolting

When flower-bearing stems (scapes) elongate quickly. Can be induced by spraying a plant with Gibberellin

Abscisic Acid - Seed Development

Plays a key role in viable seed formation: induces production of proteins/nutrients for growing embryo and accumulates in seed coat to maintain dormancy

Gibberellin - Seed Development

Promotes germination by stimulating cell division and elongation in the embryo; promotes breakdown of storage materials

Ethylene - Senescence (Aging)

Upregulates genes leading to synthesis of chlorophyllase and protease - enzymes that breakdown chlorophyll and proteins respectively