Root Development

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Root Development Overview

  • Review of previous lecture on shoot development:
    • Shoot apical meristem, maintenance.
    • Shoot branching.
    • Long distance transport.
  • Upcoming lectures on root development:
    • General root development.
    • Lateral root formation.

Today's Lecture

  • Properties and functions of roots.
  • Evolution of roots: from primitive plants to dicots.
  • Dicot root development:
    • Embryonic root formation.
    • Differentiation of cell types in the root.
    • Cell fate specification: lineage vs. position.
    • Root apical meristem (if time permits).

Root System

  • Dicot root system:
    • Tap root: originates from the embryonic root.
    • Primary lateral roots: come off the main root.
    • Secondary lateral roots: come off the primary lateral roots.
  • Main root (tap root) anchors the plant.
  • Vascular tissue (xylem and phloem) extends into lateral roots for nutrient and water exchange.
  • Epidermis differentiates into root hairs for nutrient and water exchange.
    • Nutrient and water exchange mostly happens at the root tip.
    • Older, woody roots lose epidermis and stop exchanging nutrients.

Root Cap

  • Protects the root apical meristem as the root grows through the soil.
  • Gets shed; cells live only a few days.
  • Exudes carbon (maybe 20%20\% of fixed carbon) to feed microbes and fuel mutualistic interactions.

Main Functions of the Root

  • Water and nutrient uptake from the soil.
  • Cortical cell layers (one in Arabidopsis).
  • Epidermal cells differentiate into root hairs.
  • Root hair specialization in nutrient uptake, responding to nutrient limitations.
  • Root hair proliferation in response to phosphorus deficiency in poor soils.
  • Anchorage.
  • Aerial roots in mangroves for oxygen uptake in flooded conditions.
  • Root system architecture is plastic, influenced by water availability, nutrients, and soil hardness, and is species-dependent.
  • Root system size: usually at least as big as the above-ground part of the plant, often much bigger.

Specialist Functions of Roots

  • Storage of starch.
  • Nodules for nitrogen fixation.
  • Mycorrhizal networks of roots:
    • Hyphae from mycorrhizal fungi infect roots and expand the root system.
    • Help plants take up sparse nutrients, especially phosphorus.
    • Extremely common (85-90% of land plants), particularly under low phosphorus conditions.

Evolution of Roots

  • Green algae (karyophytes) didn't have roots.
  • Bryophytes (liverworts, mosses, hornworts) have single-celled rhizoids, not true roots.
  • Roots evolved in lycophytes and then became more complex in gymnosperms and angiosperms.

Timeline of Plant Development

  • Plants in water.
  • Cryptogamic plants (bryophytes) on land without true roots, relying on mycorrhizal associations.
  • Evolution of proper roots allowing plants to grow bigger and have better anchoring functions.
    • Mycorrhizal fungi are ancient and widespread.
    • Fossil evidence of mycorrhizal-like structures in primitive plants from 400 million years ago.
    • Mycorrhizal fungi form hyphae, invade the root, and form arbuscules inside plant cells.
    • Arbuscules: tree-like structures for trading phosphorus for carbon.

Types of Mycorrhizae

  • Arbuscular mycorrhizae: fungal spores on the outside.
  • Ectomycorrhizal fungi: on the outside of the cells.
  • Ectomycorrhizae changes the root development and cause roots to split dichotomously.
  • Roots evolved around 300 million years ago.
  • The plants might have relied heavily on mycorrhizal associations for 100 million years.

Liverworts and Rhizoids

  • Small, creeping plants in wet places.
  • Rhizoids: single-celled extensions from the epidermis, not analogous to root structures.
  • Genetic evidence:
    • Marcanthia rhizoids.
    • Rice roots (angiosperms).
    • RSL1 and RSL2 genes are essential for rhizoid formation in Fyscometrella; double mutants can't make rhizoids.
  • Arabidopsis root hair mutants (RHD) genes are related to rhizoid formation genes.

Genetic Evidence and Experiment

  • Homologous Genes for Rhizoid and Root Tear Formation.
  • Experiment performed on Arabidopsis.
    • Arabidopsis wild type plant makes root tears.
    • Arabidopsis mutants that have the genes knocked out, will no longer make root tears.
    • However, performing a genetic rescue using the wild type gene will allow for root tears to be made again.
    • ATRHD63RHD11
    • AT is and indication of Arabidopsis thaliana.
    • Mutant Arabidopsis plant receives a Marcancia RSL gene and now develops mutate development.

Summary of Function and Evolution of Roots

  • Early land plants lacked roots
  • Mycorrhizal fungi may have substituted for the role of roots in nutrient uptake.
  • Bryophytes: origin of root-like structures (rhizoids), similar to epidermis root hairs, regulated by conserved genes involved in root hair formation.

Origin of the Root System

  • Embryonic development in Arabidopsis: root specified in the late heart stage on the basal side.
  • Auxin maximum specifies the root (opposite of shoot apical meristem, which is specified by an auxin minimum).
  • Cytokinin interacts with auxin in specification of the root apex.
  • Cytokinin and auxin responses are switched on in the cells that form the base of the embryo.

Construct Development

  • Find a promoter element the responds to certain hormones.
  • The promoter is usually found within the 2000 base pair part located upstream of the coding region of a gene.
  • The scientist can delete parts of the promoter until the hormone is no longer inducible.
  • This allows one to identify what part of the promoter is the hormone responsive element.

Gene Expression and Hormone Production

  • Initially one cell splits into lens cell and basal cell.
  • Lineage dependent specification of different cell types in the heart stage embryo.
  • Complex interactions between auxin and cytokinin.
  • Auxin switches on auxin response factors, which switch on genes that transport auxin into the basal cell.
  • Auxin switches on cytokinin response genes (AIA's) which subsequently inhibit cytokinin responses.

Columella Stem Cells

  • The columella stem cells are the tissue that gives rise to the root cap.

Monopterous

  • Monopterous mutant lacks a root system due to the inability to form an auxin maximum at the basal side of the embryo.
  • Monopterous is a transcription factor that regulates the expression of other genes, including pin encoding proteins.
  • Wherever monopterous expression is, the auxing influx is amplified.
  • Small Auxin gradient switches on the expression of monopterous.

Summary

  • The root is specified in the embryo through establishment of auxin and cytokinin gradients towards the base of the embryo.
  • Monopterous is an auxin response protein that is activated by auxin and amplifies the auxin gradient.
  • In a monopterous mutant, the root will not be properly formed due to the lack of auxin gradient.

Root Apical Meristem

  • Quiescent center and stem cells.
  • Columella stem cells make root cap stem cells.
  • Root cap protects the cells.
  • Epidermis, Cortex, Endodermis and Pericycle are the cells that make up the structure.

Cell Type Specification

  • Like in animal embryo development, different cell types are created by gradients of expression of different genes that specify cell types.
  • The auxin gradient specifies their expression.
  • Initials of cells will be seem very early on, specifying different types of tissue within the cells.
  • These stem cells in root apical meristem specify the different cells.

Root Apical Meristem

  • Very predictable lineages originating from different stem cells.
  • Cell divisions for cell fate within the apical meristem.
  • Endodermis (gatekeeper) is a pink layer between the cortex and pericycle.
  • Endodermis is highly supervised, making it water impermeable.
    *Force water and nutrient import into the vascular tissue through active transport.

Mutants

  • Scarecrow and short root genes are obviously candidates for endodermal cell specification.
  • Scarecrow: one cell layer that is both cortex and endodermis.
  • Short root: one layer that is only the cortex.
  • Use GFP to determine where short root is expressed, the expression of this gene is in the vascular tissue.
  • The function of the vascular tissue occurs in the endodermis.
  • Experiments are performed to see where messenger RNA is made in the cell.
    • Promoter is taken and fused to GFP to see where the expression is being performed.
  • To localize a protein, something is called a translational fusion is performed.
    • Promoter fused to coding region of gene and GFP behind the protein to localize the protein inside the cell.
  • Scarecrow is in the endodermis, already in the initials.
  • With a GFP scarecrow, it is transformed into the short root to make up for its mutant state.
  • Short root enhances the expression of scarecrow.
  • Short root is expressed in the vasculature, transcribed and translated into short root protein.
  • The protein is then moved through gaps in the cell way called plasmadesmata.
  • The protein ends up in the endodermis.
  • The protein interacts with scarecrow to get specified into endodermal sulfate.
  • If either scarecrow or short root are knocked out, that interaction can no longer happen.

Cell Fate

  • Cell fate is determined by multiple difference ways.
    • Parentage: cells are like their parents (precursor cells).
    • No influence: Do not listen to predecessors, influenced by neighbors during signal exchange, neighbors influence other cells.
    • Mixture of initial influence and some lineage dependency.
  • Position dependence is required to restart root growth if cells are destroyed.
  • Ablation done via lasers to kill a cell of interest, usually a cortical cell.
  • Neighboring pericycle cells starts to divide into a cortical cells.
    • Meristematic pericycle cells causes positional effects.
    • Signals diffuse to the surroundings to specify different cell specifications.