Plant Physiological Responses

Course Organization

• Understanding the course is organized into three modules is important
• Module I focuses on Energy/Nutrient Harvesting, Acquisition & Storage.
• Module II covers Material Exchange with the Environment & Transport within Organisms.
• Module III addresses Sensing and Responding to Environmental Variability.

Learning Goals

• Identification of key environmental variables is important as they drive plant physiological responses
• Describing how blue light regulates stomatal opening is a learning goal
• Discussing how phytochromes regulate the timing of seed germination is important
• Understanding how phytochromes enable plants to detect photo-period length is a key element

Introduction to Plant Responses

• Plants are immobile, which influences their response to stress
• Plants and animals respond to environmental cues differently
• Animals typically respond to stress with "Fight or Flight"
• Plants respond with "Fight or Die" due to their lack of mobility

Environmental Variability and Plant Stress

• Most plant stress responses are triggered by environmental variability
• Key factors include light intensity and spectral quality, water potential, nutrients, temperature, day length (photo-period), herbivores, pathogens, and gravity
• Plants sense chemical, physical, biological, and environmental cues for survival
• Plants respond differently to daily and seasonal changes.

Daily vs Seasonal Variability

• Plants sense Photosynthetic Active Radiation (PAR) daily cycles (day/night) and day-to-day variability
• Plants regulate simple physiological processes like stomatal opening in response to daily cycles
• Plants sense differences in the average PAR between seasons.
• They manage intricate developmental programs like seed germination, flowering, fruit development, and senescence in response to seasonal changes
• Plant responses to environmental variability rely on time scale, and factors such as light, temperature and rainfall

Sensing Mechanisms in Plants

• Plants utilize sensing mechanisms to detect environmental cues
• Plants and animals share similar cellular-level sensing mechanisms
• The physiological response is triggered by different stimuli plants respond to
• The plant sensory response system has three basic components: reception, transduction, and response
• Cellular receptors sense external signals as part of the reception phase
• Phosphorylation cascades and second messengers translate signals into a chemical language during transduction
• The response generally involves DNA transcription, translation, or protein activation/inactivation
• Plants can sense the wavelengths of the sunlight spectrum, focusing on blue, red, and far-red wavelengths

Plant Sensory Responses to Light

• Plants can sense and differentiate between various types of light.
• The phototropic response in plants specifically requires blue light and a receptor for blue light
• Multiple light sensors identify light intensity and wavelength,triggering physiological responses
• Blue light stimuli are linked to phototropins, resulting in :phototropism, stomatal opening, and high light avoidance.
• Red light stimuli linked to phytochrome system results in: germination, stomatal opening, and flowering
• Far-red light stimuli also use the phytochrome system, resulting in: germination and flowering

Blue Light and Stomatal Regulation

• In C3 plants, stomatal opening and transpiration are partially controlled by light
• Stomatal conductance (how open stomata are) and plant transpiration increase with light intensity
• Stomata aperture and chlorophyll absorption are linked processes,utilizing dual blue/red beams in commercial greenhouses
• The purple lighting used stimulates photosynthesis, forces stomatal opening, and induces flowering
• Blue light is sensed by phototropins PHOT1 and PHOT2 receptors functioning as protein kinases
• PHOT1 and PHOT2 are activated by interactions with the Flavin mononucleotide (FMN) chromophore
• The blue light signaling pathway triggers stomata opening

Stomata Function

• Stomata opening and closing in C3 and C4 plants are regulated by light
• Light-Oxygen-Voltage (LOV) domains of PHOT1 bind non-covalently (weakly) to the Flavin mononucleotide (FMN) chromophore in the dark
• LOV domains bind covalently (strongly) to the FMN chromophore in blue light, inducing autophosphorylation (activation)
• Blue light absorption causes 'auto-phosphorylation' of the PHOT1/PHOT2 photoreceptors.
• Phosphorylated PHOT1/PHOT2 activates H+-ATPases leading to protons leaving guard cells
• A proton gradient is then generated
• Potassium (K+) and chloride (Cl-) enter the guard cells via inward-directed K+ channels and H+/Cl- cotransporters
• Water then flows inside the guard cells leading to increased turgor pressure, causing them to separate and the pore to open as a result
• K+ is an essential osmotic regulator in stomata and is commonly added to fertilizers
• PHOT1 and PHOT2 also regulate chloroplast distribution in the cell

Phytochromes and Seed Germination

• Seeds prefer sunny locations for germination
• Sunlight is rich in red light but poor in far-red light
• Plants receive more red (R) photons than far-red (FR) photons in sunny places
• Far-red light is poorly absorbed by chlorophyll a; red light is strongly absorbed.
• Seeds sense the FR: R ratio to determine the sun or shade levels
• Germination is determined by the FR: R light ratio
• A Single phytochrome receptor senses the FR: R ratio, and it contains two conformations
• A High Far Red to Red (FR: R) light ratio refers to shade
• A Low Far Red to Red (FR: R) light ratio refers to sun
• In the sun, the plant experiences metabolism activation and high germination

Phytochrome Conformations and Light

• The phytochrome receptor changes to the predominant light wavelength
• Pr absorbs red light and changes to Pfr, while Pfr absorbs far-red light and changes to Pr
• Changes in phytochrome conformation when exposed to light are very fast (within minutes)
• Pfr is the active form of the phytochrome that will trigger germination.
• The Pr moves to the Pfr resulting in a high Pfr:Pr ratio, in full sun,
• The Pfr moves to the Pr conformation giving low Pfr:Pr ratio, which refers to shade

Night Length and Phytochrome System

• Shade is the presence of Red and Far Red photons, while night is the absence
• Plants need to adjust their physiology to the seasons
• Plants measure the time of year to schedule complex seasonal changes such as flowering, programming senescence and when to begin dormancy
• Plants use the phytochrome system to measure the night length, a reliable indicator of the season
• Night length is measured using the phytochrome system.
• During the night, Pfr slowly reverts (hours) to Pr, so the Pfr: Pr ratio decreases without light
• The lower the Pfr: Pr ratio detected at sunrise, the longer the period of darkness
• Plants use two ways to change the Pfr: Pr ratio: conformation changes in response to light and night reversion
• Conformation changes are a fast process for seed germination and night reversion is a slow process for flowering