Lecture 4 Roots, Rhizosphere and Resource Capture

Why do roots matter?

• Water

• Nutrients

• Anchorage

• Storage

• Hormone production and regulation

• Biological interactions

Root Anatomy

• Root forms from radicle of embryo

• Root tip protected by root cap

• Root tip divided into 3 main zones

• Root hairs account for up to 70-90% of total root surface area

Nutrient Acquisition

• Fertile soils contain negatively charged clay particles

• Positively charged minerals bind to the soil

• Plants require uptake

  • Mg

  • Na

  • K

  • Po4

• Some ions can passively diffuse into roots

• Most are actively transported

  • protons pumps in roots cells

Apoplastic pathway

Moves through cell wall

Symplastic pathway

Through cytoplasm

Rhizosphere

Area of high microbial activity

Other areas around the root

• Rhizoplane

• Bulk soil

Root exudates

• Mucilage from the actively growing root tip

• Sugars and other carbohydrates

• Amino acids and proteins

• Secondary metabolites

  • Phenolics

  • Strigolactones

  • Tannins and terpenoids

• Growth regulators

  • Hormones

Mycorrhizal Networks

Most widespread symbiosis on earth

Ectomychorrhizae (ECM): type of Mycorrhizal network

• Do not penetrate plant cell walls

• Form a branched web called HARTIG NET

Arbuscular Mycorrhizae (AMF)

• Hyphae penetrate cell wall of the root cortex

• Create highly branched structures ARBUSCULES

• Allows plants to capture more nutrients and increase surface area

Nitrogen Fixing Bacteria

• More exclusive than mycorrhizal relationships  

• Between legumes and Rhizobia bacteria  

  
  

• Nitrogen plentiful in the air, most limiting nutrient in most land ecosystems  

  
  

• Plant and bacteria communicate with each other to fill needs of each  

  
  

What can Low Iron cause

• Iron can be highly insoluble and therefore unavaliable

• Roots are able to reduce soil pH

Salinity tolerance

• High salts cause osmotic stress

• Root quickly thicken Casparian strip to block Na+ from getting into xylem

• Active ion pumps

Drought adaptation

• Topsoil dries out

• Roots bend toward moisture

  • Hydrotoprism

• Hydraulic redistribution

3 types of symbiotic relationships

• Mutualism (mutual)

• Parasitism (Not mutual)

• Commensalism (Neutral)

What happens when nodule forms in soil

• Limited nitrogen causes legume root to exude flavonoids in the soil

• Rhizobia bacteria detects the flavonoids, and turn on their nodulation genes

• Plant forms root hairs, root hairs sense genetic factors from rhizobia. Called (Nod factors)

• Root hairs curl around bacteria

• Plant builds an infection thread ( a tunnel down the root hair) allowing entry for bacteria

• Bacteria goes through thread, and to the cortex

• Cortex divides rapidly and creates the nodule

• Within the nodule the plant produces protein called leghemoglobin

• leghemoglobin binds oxygen to protect the nitrogenase

Function of Nodules

• Symbiotic partnership: Soil bacteria (such as Rhizobium) enter the plant's roots and trigger the formation of these tiny swellings. [1

• Nutrient exchange: The bacteria take nitrogen gas from the air trapped in the soil and convert it into ammonia, a usable form of nitrogen for the plant. In return, the plant provides the bacteria with sugars and carbon produced through photosynthesis