Plants are sessile www.mccullagh.org/db9/ 10d-17/giraffe-grazing.jpg
Plants have distinct requirements because of their sessile nature Organism-level requirements Must adjust to environment at given location Pollen and seed dispersal Cell-level requirements All materials move by mass flow, hydraulic pressure, and active or passive transport across cell membranes No morphogenic cell movement or apoptosis to drive developmental events
Plant cells are cemented in place Plant form depends on the orientation of new cell walls Cell walls are rigid and glued together, preventing cell movement Cell death rarely used to generate form
Plants cells have flexible fates Most plant cells remain totipotent: Totipotent = the ability of a single cell to produce every kind of cell in an organism. Individual plant cells can regenerate, give rise to an entire plant. Differentiated plant cells thus have the capacity to: 1. De-differentiate - lose specialized characteristics 2. Re-enter cell cycle - reinitiate cell division 3. Establish new meristems
Plants display extensive post-embryonic development Plants continue to initiate and develop organs throughout their life. This allows them to adapt to changing conditions as they grow. Maintaining eternally embryonic tissues = meristems. Meristem: a Group of dividing cells capable of making all different cell types, structures Embryogenesis sets polarity, symmetry and establishes meristems. http://www.thenakedscientists.com
In embryogenesis: Embryonic vs. Post-embryonic Development Apical-basal axis established. Radial pattern established. Apical meristems are established RAM SAM But the development of leaves, flowers, the stem, secondary roots, etc. all occurs post-embryonically. Bio 4028 Lecture 5 6
Post-Embryonic Development Relies on Meristems Plants continue to initiate and develop organs throughout their life. This allows them to adapt to changing conditions as they grow. Relies on meristems that were made during embryogenesis. MERISTEM = small collection of undifferentiated, self-renewing cells that are the ultimate source of all post-embryonic structures of the plant.
Flexible cell fates and post-embryonic development are critical for coping with the environment www.mccullagh.org/db9/ 10d-17/giraffe-grazing.jpg
Plant responses to the environment Plants constantly adjusting to changes in their environment Involves changes in: gene expression, biochemistry, cell biology, morphology & growth Plants alter their shape, size, and direction of growth to continually adjust to their changing environment Responses to changes often governed by plant growth regulators Developmental plasticity governed by differential production & distribution of hormones
What constitutes a plant s environment? Light: wave length, intensity, direction Gravity Temperature: heat, cold Water: drought, flooding Nutrients Salt, toxic compounds Other organisms: plants, animals, insects, pathogens
Light as an environmental cue Day vs night Day length: seasonal information (flowering) Light quality: Neighboring plants & shade avoidance Directional information Quality & amount perceived by photoreceptors
Phytochrome Absorbs in the red (Pr) and far-red (Pfr) wavelengths Pr & Pfr are photo-reversible Each form has different absorption optimum Relative proportions of Pr and Pfr determined by degree of vegetative shading in the canopy Light at top of canopy Light filtered through green leaf
Shade avoidance example R:FR ratios allow plant to sense, respond to shading by other plants (Pfr/Ptotal) Full sunlight: more R light than far-red light Shade: more FR (R absorbed by green leaves). Different quality of light: eg full sun vs sun filtered by leaves (shade) As shading increases: R:FR decreases, Pfr:Ptotal decreases, stimulates stem elongation, increase chances of growing up, beyond shading leaves
Red FarRed As shading increases: R:FR decreases, Pfr:Ptotal decreases, stimulates stem elongation, increase chances of growing up, beyond shading leaves
Tropism Change in direction of growth, towards or away from stimulus Occurs in shoots and in roots
Tropisms Directional growth with respect to an exogenous signal towards or away from an environmental stimulus Phototropism - growth in response to light Gravitropism - growth in response to gravity Thigmotropism - growth in response to touch Why have tropisms? Responding to changes in the environment is essential for sessile organisms.
Phototropism
Sunflowers track the sun and they get ready for the next day
Lots of plants can tell the difference between night and day
Tropisms Directional growth with respect to an exogenous signal towards or away from an environmental stimulus Phototropism - growth in response to light Gravitropism - growth in response to gravity Thigmotropism - growth in response to touch
Gravitropism is a movement in response to gravity
Tropisms Directional growth with respect to an exogenous signal towards or away from an environmental stimulus Phototropism - growth in response to light Gravitropism - growth in response to gravity Thigmotropism - growth in response to touch
Thigmotropism is the response to touch
Some plants can count
Phototropism
Charles and Francis Darwin predict a mobile signal regulates phototropism Charles and Francis Darwin (1880s) studied movement of seedlings in response to unidirectional light PLANT PHYSIOLOGY, 3 rd Ed.
Additional important observations led to the identification of auxin PLANT PHYSIOLOGY, 3 rd Ed.
Auxin Auxein = to grow (Greek) First plant hormone discovered Indole acetic acid (IAA) is the predominant natural occurring form Indole 3-acetic acid (IAA)
Phototropism and auxin Cholodney Went model (1920s): In response to unilateral light, auxin produced at tip of coleoptile is transported laterally to shaded side of seedling. Cells elongate Auxin then transported from tip to elongation zone: stimulates cell elongation on dark side.
Auxin levels are carefully regulated Synthesis, conjugation & degradation Transport Sequestration within cellular compartments Transported cell-cell throughout plant AUX/LAX transporters PIN PGP/MDR
Transport proteins are required for setting up auxin gradients for controlling plant growth Fig. 2 Vanneste & Friml, 2009 Cell
Gravity Perception Note: In roots: high concentration of auxin INHIBITS cell elongation. Gravistimulation results in assymetric axuin accumulation in lateral root cells on non-elongating side. (C) Auxin accumulation indicated by DR5-GFP reporter T&Z Figs. 19.32, 19.33.
TAKE-HOME MESSAGES 1- The sessile nature of plants (and plant cells) require unique mechanisms to regulate growth and development. 2- Plant cells have flexible fates and display post-embryonic development. 3- Plants use developmental processes to cope with their environment (amongst other processes). 4- Plant hormones are critical for response to stimuli (including mechanical stimuli).