Plant Biology: Root Development

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Questions and Answers

What is the primary role of nitrogen fixation in plant development?

  • To increase soil temperature
  • To promote root elongation
  • To absorb carbon from the atmosphere
  • To convert N2 into valuable nitrogen compounds for plants (correct)

Which of the following statements about root nodules is correct?

  • They are mandatory for all plants.
  • They exclusively contain free-living bacteria.
  • They are found on the roots of leguminous plants. (correct)
  • They do not play a role in nitrogen fixation.

What is the function of nitrogenase in the process of nitrogen fixation?

  • To produce leghemoglobin
  • To catalyze the reduction of dinitrogen to ammonia (correct)
  • To aid in the absorption of nutrients
  • To bind oxygen within root nodules

Which type of genes are involved specifically in the formation of root nodules in plants?

<p>Nodulation (nod) genes (C)</p> Signup and view all the answers

How do legumes and Rhizobium bacteria initiate their symbiotic relationship under nitrogen-limited conditions?

<p>Through chemical signaling (A)</p> Signup and view all the answers

What is the primary function of roots in plants?

<p>Absorption of water and nutrients (D)</p> Signup and view all the answers

Which structures are involved in the protection and direction of root growth?

<p>Root cap (B)</p> Signup and view all the answers

Which region of the root tip is primarily responsible for cell elongation?

<p>Region of Elongation (C)</p> Signup and view all the answers

What is the function of the quiescent center in the root apical meristem?

<p>Synthesis of plant hormones (B)</p> Signup and view all the answers

In which type of root system does the radicle form a primary root with lateral branches?

<p>Taproot system (B)</p> Signup and view all the answers

What occurs in the Region of Maturation of the root tip?

<p>Root hair formation and maturation of cells (C)</p> Signup and view all the answers

What type of relationship do many roots have in the rhizosphere?

<p>Mutualistic relationships with bacteria and fungi (B)</p> Signup and view all the answers

Which primary meristem is responsible for developing the outer layer of the root?

<p>Protoderm (B)</p> Signup and view all the answers

What function do common nod genes (nodA, nodB, nodC) serve in rhizobial strains?

<p>Facilitate root hair curling and infection thread formation (B)</p> Signup and view all the answers

Which proteins are associated with host-specific nod genes that define the host range of rhizobial species?

<p>nodH and nodF (D)</p> Signup and view all the answers

What triggers the root hairs to curl during the symbiotic process?

<p>Chemical attractants secreted by the bacteria (B)</p> Signup and view all the answers

What role does NodD play in the nodulation process?

<p>It regulates the transcription of other nod genes (C)</p> Signup and view all the answers

What structural feature is formed when bacteria penetrate the root hair?

<p>Infection thread (A)</p> Signup and view all the answers

Which of the following correctly describes how bacteria are transported through the plant cells during infection?

<p>Through fusion of membrane vesicles in the infection thread (A)</p> Signup and view all the answers

How do bacteria access the apoplast after reaching the cortical cells?

<p>By releasing enzymes to penetrate cell walls (A)</p> Signup and view all the answers

Which statement best describes the process of nodulation in plants?

<p>It relies on interaction between rhizobial proteins and root cell membranes. (C)</p> Signup and view all the answers

Flashcards

Nitrogen Fixation

The process of converting atmospheric nitrogen (N2) into usable nitrogen compounds for biochemical processes in plants.

Root Nodule

A swelling on a legume root containing nitrogen-fixing bacteria (Rhizobium).

Rhizobium

Bacteria capable of nitrogen fixation, found in root nodules of legumes.

Nitrogenase

A bacterial enzyme that reduces atmospheric nitrogen to ammonia.

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Leghemoglobin

Oxygen-binding protein in legume nodules. Important in creating low-oxygen environment good for nitrogenase.

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Nodulin genes

Plant genes specific to nodule development.

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Nodulation (nod) genes

Rhizobium genes involved in nodule formation.

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Symbiosis (Legumes and Rhizobium)

A non-obligatory relationship between legumes and Rhizobium. Legume may live independent of Rhizobium.

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Root Function

Roots absorb water and nutrients, conduct absorbed materials to the plant, provide anchorage, and store food.

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Rhizosphere

The thin layer of soil around a root where roots interact with soil bacteria and fungi.

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Radicle

The first root that emerges from a seed.

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Taproot System

A root system in which the radicle grows into a primary root, which then develops lateral(branch) roots.

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Fibrous Root System

A root system in which several embryonic roots develop from the radicle and form a network of branching roots.

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Root Tip Regions

The root tip is organized into the Root Apical Meristem (RAM), Region of Elongation, and Region of Maturation.

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Root Cap

A protective layer at the root tip that protects the root apical meristem, perceives gravity, and is constantly replaced.

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Root Apical Meristem (RAM)

The actively dividing region at the root tip that produces all cells and tissues.

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Quiescent Center (QC)

A small part of the RAM with slow cell division; its function is not completely understood, but it might regulate plant hormones.

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Nod genes

Genes involved in rhizobium-plant symbiotic interactions, specifically in nodulation.

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Common Nod genes

Nod genes present in all strains of Rhizobium, and they code for the signaling molecules involved in triggering root responses to bacteria.

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Host-specific Nod genes

Differ among Rhizobium species and affect which specific plant species are colonized.

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NodD protein

A protein from nodD gene, which regulates the transcription of other nod genes.

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Infection thread

A tubular structure formed by the plant root hair cell's plasma membrane during Rhizobium infection that allows bacteria to travel to the root cortex.

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Chemical attractants (iso)flavonoids and betaines

Chemicals produced by the plant's roots that attract Rhizobium bacteria.

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Recognition

The first step in the symbiotic relationship between rhizobium and plants, where plants recognize and respond to bacteria.

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Establishment of Symbiosis

The process by which a symbiotic relationship is formed between rhizobium and plant.

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Infection

The process where bacteria penetrates inside plant cells.

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Study Notes

Root Development

  • Roots absorb water (Hâ‚‚O) and nutrients (dissolved salts and minerals).
  • Absorbed materials are transported throughout the plant.
  • Roots anchor plants in soil.
  • Roots store food.
  • Many roots have relationships with bacteria and fungi in the soil (rhizosphere).
  • The radicle is the first root, initiating penetration into the soil and branching.
  • Plants have different root types.
    • Taproot systems have a primary root and lateral roots.
    • Fibrous root systems branch from the primary root.
  • Root tips have a protective root cap.
  • Root caps protect the root apical meristem (RAM).
  • The RAM is the site of gravity perception, directing root growth.
  • The RAM continuously sheds cells of the root cap.
  • The root tip is organized into three regions: RAM, region of elongation, and region of maturation.
  • RAM produces cells and tissues for the root.
  • The elongation region is where root cells lengthen.
  • The maturation region is where root hairs form, and other cells mature.
  • A quiescent center (QC) is a small part of the RAM, dividing slowly and possibly involved in plant hormone synthesis for root control.
  • RAM produces three primary meristems (protoderm, ground meristem, procambium).
  • The three primary meristems develop into primary tissues (epidermis, cortex, xylem, phloem).

Roots and Nitrogen Fixation

  • Nitrogen fixation is the conversion of atmospheric nitrogen (Nâ‚‚) into usable compounds.
  • Biological nitrogen fixation is a crucial soil process conducted by bacteria.
  • Bacteria absorb atmospheric nitrogen and convert it into usable nutrients for plants.
  • Legumes have root nodules containing nitrogen-fixing bacteria (often Rhizobium).
  • The nodules are swellings on the roots.
  • A complex bacterial enzyme (nitrogenase) carries out the nitrogen conversion.
  • Nitrogenase is inhibited by oxygen (Oâ‚‚).
  • Leghemoglobin binds oxygen, maintaining low oxygen levels in the nodules.
  • Symbiosis between legumes and Rhizobium is not obligatory.
  • Legumes can exist unaffected by Rhizobium.
  • Rhizobium exists as free-living organisms in the soil.
  • Nitrogen-limited environments promote symbiosis.
  • Nod genes in plants, and rhizobial genes, interact to facilitate nodule development.
    • Nodulin genes help develop plant-specific nodules.
    • Nodulation genes guide nodule formation in rhizobia.
  • Common nod genes are present in many strains.
  • Host-specific nod genes influence root hair curling.
  • NodD is constitutively expressed to regulate transcription in nod genes.
  • Chemical attractants help Rhizobium recognize legumes and establish symbiosis.
  • Bacteria penetrate the root.
  • Infection threads carry bacteria into the root cortex.
  • Bacteria differentiate into bacteroids in the cortical cells.
  • The nodule develops, providing nitrogen to the plant.

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