Plant Response to Environmental Stressors

• Plants respond to their environment to facilitate survival
• Plants recognize changes in their environment through various stimuli
• Plants can react to the following: damage, light, heat, and chemicals

Response to Wounding

• The wounding response involves rapid MAPK module activation

• Hormones and a slower MAPK module are also involved

• Perception of abiotic stress leads to a cascade of events

  • Second messengers and calcium ions ($Ca^{2+}$) are released
  • Signal transduction occurs, e.g., MAP Kinase cascade
  • Transcription factors are activated, leading to responsive gene expression
  • Physiological response is induced

• When cells injured, they break open and release fluids

• Enzymes are created to repair the damage

• Browning starts as a result of oxidative processes

• The process of cell division is started by other chemical reactions

• Plants express genes and produce chemical products when exposed biological or environmental stressors

• Responses can happen quickly and it takes minutes

• Jasmonic acid is an example of a chemical signal, which notifies other areas of the plant when something occurs

Response to Light

• Plants undergo growth and greening when exposed to light
• Phototropism describes how plant bends towards light

Phototropism

• Light is received by phytochrome in the plasma membrane in the cytoplasm.
• cGMP is a second messenger
• Protein kinases are activated
• Transcription factors 1 and 2 are activated in the nucleus
• Response is de-etiolation (greening) response proteins
• Auxins move to the shaded part of the shoot causes cell elongation
• A positive phototropic response in shoots results from auxins

Sun Tracking

• Sun tracker plants face east in the morning and west in the evening
• At night, the plants switch back to face east again
• Stem growth occurs on the west side at night, causing flowers to turn east, waiting for the sun to rise
• Stem growth occurs on the west side in the daytime, causing flowers to bend east, waiting for the sunset
• Sunflowers seeds produce oil

Gravitropism

• Gravitropism describes how plants response to gravity
• Positive gravitropism describes when the roots grow down
• Negative gravitropism describes when the stems grow against gravity
• No matter how a corn seed is oriented in the soil, the seedling's main root will sprout downward, and its primary shoot will sprout upward
• Plant reorientation involves amyloplast sedimentation
• Signal transduction occurs via second messengers
• Auxin transporter relocates
• Differential auxin transport happens
• And then organ curvature

Thigmotropism

• Thigma means touch in Greek
• If touched, plants can change their behavior
• Pea plants wrap themselves around something when their stems/leaves are touched
• Stem tendrils help the plant to climb up something

Photoperiod

• Photoperiod describes how plants responses to the different lengths of time of exposure to light
• Short-day plants require a period of darkness exceeds a critical night length to flower
• Long-day plants flower when a period of darkness is less than a critical night length flower

Phytochrome

• The phytochrome protein senses light, containing two identical subunits
• The protein contains a Chromophore, a Photoreceptor activity and a Kinase activity
• Red light activates phytochrome
• Responses to red light is seed germination and controlling flowering
• Far-red light inhibits phytochrome
• In darkness, there is slow conversions(some plants) or Enzymatic destruction

Plant Responses Summary

• Plants respond of light by growing toward it, so they get maximum exposure(phototropism)
• Plants respond to touch and gravity in different ways
• Auxin production (a hormone that increase cell elongation) is involved in these movements
• Plants respond to the photoperiod, day length, by being long-day, short-day or neutral. Phytochrome is involved

Other Responses

• Plants use temperature and day length to regulate dormancy and regrowth
• Plant growth rate is related to temperature as well
• Seed germination is related to temperature

Hormone Environmental Change Responsiveness

• Auxin stimulates cell elongation and regulates organ bending and branching
• Cytokinins stimulate plant cell division, promote later bud growth, and slow organ death
• Gibberellins promote stem elongation and help seeds break dormancy and use stored reserves
• Brassinosteroids (chemically similar to sex hormones of animals) accelerate cell division and elongation
• Abscisic acid promotes stomatal closure during drought and promotes seed dormancy
• Strigolactones regulate apical dominance, seed germination, and mycorrhizal associations
• Ethylene mediates fruit ripening
• Ripening of fruit is induced by ethylene
• Ripening includes changes in cell wall structure, pigment accumulation, flavor and aromatic volatile production, and conversions of starches to sugars
• Ethylene has a commerical use to ripen fruits like, bananas, tomatoes, avocados, mangos, papayas, persimmons and honeydew melons