bio lecture 03/12

Interaction of Legumes and Rhizobia

Overview of Experiment

• Legumes are plants that interact symbiotically with rhizobia, bacteria that fix nitrogen in the soil.

• An experiment was conducted to observe the growth of legumes with or without nitrogen fertilizer and with or without rhizobia bacteria.

• The aim was to assess how these variables affect the number of root nodules formed.

Nodules Formation

• Nodules are structures on the roots where rhizobia multiply and take nitrogen from the atmosphere, converting it into a form the plant can use.

• Plants receiving nitrogen fertilizer exhibited fewer nodules than those without, suggesting a redundancy in nitrogen sources.

Implications of Fertilization

• When nitrogen is readily available from fertilizer, plants do not require rhizobia for nitrogen; hence, they invest less energy in forming nodules.

• Carbon-Nitrogen Balance: Plants manage a critical balance between carbon (from sugars) and nitrogen.

  • High carbon levels from photosynthesis don't require additional nitrogen from rhizobia, leading to reduced nodule formation.

• Plants may regulate rhizobial interactions at various levels (e.g., signaling pathways) to minimize unnecessary energy expenditure.

Ecological Relationship

Commensalism vs. Mutualism

• The relationship between legumes and rhizobia can shift depending on environmental conditions such as nitrogen availability.

• Originally a mutualistic interaction (both benefit), it becomes more like commensalism or parasitism when legumes provide energy (sugars) without receiving a proportional nitrogen benefit.

• When rhizobia take sugars without contributing sufficient nitrogen, this leads to a parasitic dynamic where the plant suffers.

Importance of Nitrogen

• Nitrogen is a critical nutrient, necessary for producing:

  • Proteins (composed of amino acids that include nitrogen)

  • Nucleic acids (DNA and RNA contains nitrogenous bases)

Introduction to CRISPR

Background

• A revolutionary genetic tool derived from the immune defense systems of bacteria that can edit DNA.

• Allows for manipulation of genetic material in various organisms.

Mechanism of CRISPR

1. Acquisition Phase:

   • Bacteria encounter a phage (bacterial virus) and capture a piece of its DNA (spacer DNA).

   • This spacer is incorporated into the bacterial genome, serving as a genetic memory of previous infections.

2. Expression Phase:

   • The bacterial cell transcribes spacer DNA into RNA.

   • RNA binds to Cas proteins (like Cas9) that carry out the next step.

3. Interference Phase:

   • The RNA guides Cas9 to matching viral DNA, where it cuts (cleaves) the foreign DNA, preventing the virus from replicating.

Distinction between Self and Non-self

• PAM (Protospacer Adjacent Motif) sequences are crucial for the recognition of foreign DNA versus bacterial DNA, ensuring self-DNA remains untouched.

• This mechanism underlines the CRISPR system's efficiency and specificity in targeting foreign invaders.

Application of CRISPR Technology

• Used in labs for genetic research, gene editing, and potential de-extinction projects (e.g., reintroducing traits of extinct species like woolly mammoths into living relatives).

• Ethical discussions exist around gene editing, especially concerning ecological impacts and the preservation of species.

Summary of CRISPR Process

• Cas9 acts as a molecular scissor that excises specific DNA sequences.

• A guide RNA is designed to match the DNA region intended for alteration, ensuring precision in the cutting action.

• Resulting edits can either deactivate genes or replace them with desired sequences, depending on the introduction of donor DNA that encodes for the desired traits.