2.3 Mineral uptake and transport

Mineral uptake and transport are crucial processes for plant survival. Plants use active and passive transport mechanisms to move essential minerals from the soil into their roots. Ion channels and carrier proteins play key roles in facilitating this movement across cell membranes.

Once inside the roots, minerals travel through symplastic and apoplastic pathways. The Casparian strip in the endodermis regulates mineral movement, while mycorrhizal associations enhance uptake. Phloem loading and unloading processes distribute nutrients throughout the plant.

Mineral Transport Mechanisms

Active and Passive Transport

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• Active transport moves minerals against their concentration gradient from areas of low concentration to high concentration
  • Requires energy input, typically from ATP hydrolysis
  • Allows plants to accumulate essential minerals even when external concentrations are low
  • Enables selective uptake of specific minerals
• Passive transport moves minerals down their concentration gradient from areas of high concentration to low concentration
  • Does not require energy input
  • Includes diffusion and facilitated diffusion through ion channels and carrier proteins
  • Occurs when mineral concentrations are higher in the soil solution than in plant cells

Ion Channels and Carrier Proteins

• Ion channels are membrane proteins that form pores allowing specific ions to pass through
  • Facilitate rapid passive transport of ions down their electrochemical gradient
  • Exhibit selectivity for specific ions based on size and charge (potassium channels)
  • Can be gated by voltage, ligands, or mechanical stimuli to regulate ion flow
• Carrier proteins bind to specific minerals and undergo conformational changes to transport them across membranes
  • Include both passive transporters (facilitate diffusion) and active transporters (use energy to move against gradient)
  • Exhibit high specificity for particular minerals (phosphate transporters)
  • Can be regulated by factors such as pH, mineral concentration, and plant signaling molecules

Pathways of Mineral Movement

Symplastic and Apoplastic Pathways

• Symplastic pathway involves mineral movement through the cytoplasm of interconnected cells via plasmodesmata
  • Allows minerals to bypass the Casparian strip in the endodermis
  • Provides a selective route for mineral transport controlled by the plant
  • Requires minerals to cross plasma membranes to enter and exit the symplast
• Apoplastic pathway involves mineral movement through the cell walls and intercellular spaces
  • Allows rapid, non-selective transport of minerals in the root cortex
  • Restricted by the Casparian strip in the endodermis, which forces minerals into the symplast
  • Plays a role in delivering minerals to the xylem for long-distance transport

Casparian Strip

• The Casparian strip is a band of suberin deposited in the radial and transverse cell walls of the endodermis
  • Acts as a barrier to the apoplastic movement of water and minerals
  • Forces minerals to enter the symplast before reaching the xylem
  • Helps maintain root pressure by preventing water loss from the xylem to the soil
• The Casparian strip is not a perfect barrier and some apoplastic bypass flow may occur
  • Varies among plant species and environmental conditions
  • Can be enhanced by the deposition of additional suberin lamellae in the endodermis (maize under drought stress)

Mineral Uptake Enhancements

Mycorrhizae

• Mycorrhizae are symbiotic associations between plant roots and fungi
  • Formed by approximately 80% of land plant species
  • Improve plant mineral uptake, particularly phosphorus and nitrogen
  • Provide plants with access to a larger soil volume through the extensive fungal mycelium
• Arbuscular mycorrhizae (AM) are the most common type
  • Involve fungi from the phylum Glomeromycota
  • Form highly branched structures called arbuscules within root cortical cells for nutrient exchange
  • Deliver phosphorus directly to the plant via specialized fungal phosphate transporters
• Ectomycorrhizae (EM) are formed by certain tree species with basidiomycete and ascomycete fungi
  • Fungal hyphae form a dense sheath around the root and a Hartig net between root cells
  • Improve uptake of nitrogen, phosphorus, and other minerals from the soil organic matter
  • Provide host plants with protection against pathogens and environmental stresses (drought)

Phloem Transport

Phloem Loading

• Phloem loading is the process of moving sugars and other organic compounds into the phloem for long-distance transport
  • Occurs in the source tissues where photosynthates are produced (mature leaves)
  • Can be passive (symplastic) or active (apoplastic) depending on the plant species and environmental conditions
  • Apoplastic loading involves the use of sucrose-proton symporters to move sugars against their concentration gradient
• Phloem loading is regulated by the activity of enzymes and transporters involved in sugar metabolism and transport
  • Sucrose phosphate synthase (SPS) plays a key role in synthesizing sucrose for export
  • Sucrose transporters (SUTs) facilitate the uptake of sucrose into companion cells and sieve elements
  • Potassium channels and proton pumps establish the osmotic and electrochemical gradients necessary for phloem loading

Phloem Unloading

• Phloem unloading is the process of moving sugars and other organic compounds out of the phloem in sink tissues
  • Occurs in the non-photosynthetic tissues that require photosynthates for growth and metabolism (roots, young leaves, fruits)
  • Can be symplastic or apoplastic depending on the sink tissue and developmental stage
  • Symplastic unloading involves the movement of sugars through plasmodesmata into the recipient cells
• Phloem unloading is influenced by the strength of the sink and the activity of sugar transporters and enzymes
  • Invertases hydrolyze sucrose into glucose and fructose, creating a concentration gradient for unloading
  • Hexose transporters (HXTs) facilitate the uptake of glucose and fructose into sink cells
  • Starch synthases and other enzymes convert the imported sugars into storage compounds (starch in potato tubers)