Plant Responses to the Environment

Plant Responses to the Environment

• Plant hormones help coordinate growth, development, and responses to environmental stimuli
• Plants have cellular receptors that detect important changes in their environment

Signal Transduction Pathways

• Link internal and environmental signals to cellular responses
• Signals are detected by receptors, which change shape in response to a specific stimulus
• Receptors initiate a specific series of biochemical steps, leading to the regulation of cellular activities

Light and Plant Responses

• Light is important for photosynthesis and key events in plant growth and development
• Plants detect direction, intensity, and wavelengths of light using phytochrome
• Phytochrome is a photoreceptor that consists of a protein covalently bonded to a non-protein part that functions as a chromophore
• Phytochrome occurs in two interchangeable forms: PR and PFR, which respond to red and far-red light respectively

Photomorphogenesis

• Light reception is important for photomorphogenesis, which affects plant morphology
• Phytochrome is involved in resetting the internal biological clock of many plants
• The biological clock has a period of approximately 24 hours and controls plant circadian rhythms (e.g., leaf movement)

The Flowering Process

• Long-day plants develop flowers when the day length is longer than a critical period (e.g., 14 hours)
• Short-day plants develop flowers when the day is shorter than a critical period (e.g., 15.5 hours)
• Day-neutral plants develop flowers regardless of day length
• Phytochrome can reset the biological clock, influencing flowering

Hormones and Plant Growth

• Auxins regulate phototropism and gravitropism
• Auxins promote stem growth, cell elongation, and vascular differentiation
• Auxins delay leaf abscission and promote lateral root growth
• Indole-3-acetic acid (IAA) is the natural auxin, derived from tryptophan

Cytokinins

• Produced mainly in roots and travels through the xylem
• Promote cell division and stimulate lateral bud growth
• Delay leaf senescence or abscission
• Regulate protein synthesis

Gibberellins

• Produced in embryonic tissues, developing seeds and fruits, and root apices
• Promote stem elongation and enhance the effects of auxin
• Stimulate germination in buds and seeds
• Used commercially to break dormancy

Ethylene

• Found in all plant tissues and is a gaseous hormone
• Formed from the amino acid methionine
• Promotes leaf abscission and fruit ripening
• Used commercially for fruit ripening

Abscisic Acid

• Produced in leaves, stems, and green fruits
• Causes stomatal closure during water stress
• Promotes dormancy in seeds and buds
• Plays an important role in embryogenesis by stimulating starch and protein accumulation
• Regulates seed maturation and germination through gene expression in direct competition with gibberellins