Responses to Light

Pigments are molecules that are capable of absorbing light energy

• Some are used in photosynthesis

• Others detect light and mediate the plant’s response to it.

Photomorphogenesis

• Nondirectional, light-triggered development as in flowering

Phototropism

• Directional growth in response to light.

Both compensate for inability to move

Other Responses to Abiotic Factors

Gravitropism- root growth towards gravity)

Thigmotropism (substrate-dependent growth as with vines)

Shade Avoidance (leaf growth not occurring in the beneath the canopy in a forest)

Resistance to Short-term Drought

Seasonal Changes (as in leaves falling in autumn and leaves coming back in spring

Hormones and Sensory Systems

Hormones are chemicals produced in one part of an organism and transported to another part where they exert a response

In plants, hormones are not produced by specialized tissues

Eight major kinds of plant hormones

• Auxin, cytokinins, gibberellins, strigolactones, brassinosteroids, oligosaccharins, ethylene, and abscisic acid.

Auxin

Effects of auxin discovered in 1881 by Charles and Francis Darwin

• They reported experiments on the response of growing plants to light.

  • Grass seedlings do not bend if the tip is covered with a lightproof cap.

  • They do bend when a collar is placed below the tip.

Thirty years later, Peter Boysen-Jensen and Arpad Paal demonstrated that the “influence” was actually a chemical

Cytokinins

Plant hormone that, in combination with auxin, stimulates cell division and differentiation.

Other Effects of Cytokinins

Promote the synthesis or activation of cytokinesis proteins

Agrobacterium inserts genes that increase rate of cytokinin and auxin production

• Causes massive cell division.

• Formation of crown gall tumor.

Gibberellin as a Signal

GA is used as a signal from the embryo that turns on transcription of genes encoding hydrolytic enzymes (like amylase) to break seed dormancy.

Ethylene

• Gaseous hydrocarbon (H2 C − CH2)

• Auxin stimulates ethylene production in the tissues around the lateral bud and thus slows their growth

• Ethylene also suppresses stem and root elongation

• Major role in fruit development – promotes ripening

• Target in agriculture to delay fruit ripening so that fruits do not over-ripen prior to purchase (example on next slide is a transgenic method, but sprays are also used)

Abscisic Acid

• Synthesized mainly in mature green leaves, fruits, and root caps

• Little evidence that this hormone plays a role in abscission, but that is where its name come from.

• Induces formation of dormant winter buds

• Counteracts gibberellins by suppressing bud growth and elongation

• Counteracts auxin by promoting senescence

• Necessary for dormancy in seeds