Roots

• The root system is the part of the plant that absorbs water, minerals, and nutrients from the soil.
• Root hairs are small projections on the surface of roots that increase their surface area to absorb more water and nutrients.
• Mycorrhizae are fungi that form symbiotic relationships with plants' roots, increasing their ability to absorb nutrients like phosphorus.
• land plant colinisation 400 million years ago, first land plants appeared, they were small and fragile, they were not able to support themselves on land
• soil represents a complex environment for plants to forage for nutrient and water
• Roots are important for water , nutrient , storage and anchorage.
• Modern terrestrial environment is due to root evolution changing the terrestrial environment
• Rhuie Chert (410MYA)provides a snapshot of early plants. plants are 10-12cm
• Rhynie chert reveal mycorrhizal fungi interactions with plants
• Arbuscle colonisation within plant cell by mycorrhiza's
• Mycorrhizae increase surface area of roots, allowing them to absorb more nutrients from soil
• Endomycorrhizae penetrate inside the cells of the root cortex
• Ectomycorrhizae form around the outside of the root system
• Roots evolved after land colonisation
• Plants are very efficient at light harvesting
• Designs for solar trees that replicate nature
• Branching directed toward the light
• Height and branching dependent on light
• Mouse


  • Body plan is fixed and determined by its genes
• Arabidopsis


  • Body plan variable, frequency of organs can vary
• How did the necessary innovations arise?
• Over time there has been a trend to more complex multicellular forms
• Charophytes


  • Can form simple filaments
  • Specialist cells
  • Simple bodyplans
  • Meiosis immediately follows zygote formation
  • All cells are haploid except the newly divided zygote
  • Mainly reproduce vegetatively
  • Sexual reproduction does occur
  • And rhizoids!
• 3D growth in the moss Physcomitrium patens enabled diversification of land plants 470 MYA
• 3D Growth


  1. Governed by cell division of the apical cell

  2. Apical Cell with one cutting face - allows 1D growth

  3. Apical Cell with two cutting faces - allows 2D

  4. Tetrahedral Apical Cell with three cutting faces - allows 3D
• Acquisition of 3D growth is present in gametophores of moss
• A tetrahedral apical cell can give rise to shoots with lateral organs
• Phyllotaxis


  • Organs arranged around a single axis
  • Often follow mathematical patterns
• Shoot growth occurs from two apical initials in Selaginella
• Meristems


  • Plant stem cells are found in structures termed meristems
  • Meristems are first laid down in the embryo via asymmetric cell divisions
• The plant body plan is elaborated by post-embryonic organogenesis
• Shoot apical meristem


  • Typically small (50 um in Arabidopsis, up to 3mm in cycads)
  • Central Zone contains the stem cells
  • Periphery Zone cells frequently divide to and produce primordia (e.g. leaves)
  • Rib Zone dividing cells that make the stem
• SAM size is controlled by

  Balancing cells lost to differentiation in PZ with the production of more cells
• Positive regulators are produced proximal to the initials in the central zone


  Negative regulators are produced in the initials and suppress the positive regulators
• Larger meristems


  More initial cells - therefore more negative regulators - this limits the size of the meristem
• Smaller meristems


  Less negative regulators - meristem size increases
• WUSCHEL


  Positive regulator that maintains identity of the apical initials
• CLAVATA


  Negative regulator
• Axillary meristems


  • Form where the upper leaf structure meets the stem
  • May lay dormant
  • Can grow out to become a new stem
• Activation of Axillary Branches is regulated by a host of environmental factors