Monoclonal Antibodies and Plant Diseases

Monoclonal Antibodies

Monoclonal antibodies are identical copies of a single type of antibody, produced in large quantities in the lab. They are designed to target one specific antigen (protein).

How Monoclonal Antibodies Are Produced

1. A mouse is injected with the target antigen (e.g. a protein found on cancer cells)

2. The mouse produces lymphocytes that make antibodies against that antigen

3. These lymphocytes are removed from the mouse

4. The lymphocytes are fused with tumour cells (which divide rapidly and indefinitely)

5. The resulting hybrid cells are called hybridoma cells

6. Hybridoma cells divide rapidly to produce large quantities of the same antibody — these are monoclonal antibodies

7. The antibodies are collected and purified

The tumour cells are used because normal lymphocytes do not divide for very long. The fusion gives the hybridoma both the ability to produce antibodies (from the lymphocyte) and the ability to divide indefinitely (from the tumour cell).

Uses of Monoclonal Antibodies

1. Pregnancy tests

• The hormone HCG (human chorionic gonadotropin) is present in the urine of pregnant women
• The test strip contains monoclonal antibodies that bind specifically to HCG
• If HCG is present, the antibodies bind to it and produce a colour change (positive result)

2. Diagnosing disease

• Monoclonal antibodies can be attached to a fluorescent dye or radioactive marker
• They bind to specific antigens on disease cells (e.g. cancer cells)
• This allows doctors to locate and image the disease using scans

3. Cancer treatment

• Monoclonal antibodies can be designed to bind to antigens on cancer cells only
• They can carry a radioactive substance, a toxic drug, or a chemical that stops cells dividing directly to the cancer cells
• This reduces damage to normal cells compared to chemotherapy
• However, they have more side effects than expected and are not yet as widely used as initially hoped

4. Research and lab testing

• Used to detect and measure specific molecules in samples

Advantages and Limitations

Advantages	Limitations
Highly specific to one target	Can cause side effects (fever, itching, low blood pressure)
Can be produced in large quantities	Expensive to develop and produce
Useful for diagnosis, treatment, and research	Not yet as effective for cancer treatment as hoped

Plant Diseases

Plants can also be affected by diseases caused by pathogens, mineral deficiencies, and pests.

Detecting Plant Diseases

• Stunted growth — may indicate mineral deficiency or disease
• Spots on leaves — e.g. rose black spot (fungal)
• Areas of decay (rot) — caused by bacteria or fungi
• Growths or lumps — abnormal growths (galls) on stems or roots
• Malformed stems and leaves — distorted growth
• Discolouration — yellowing (chlorosis), mosaic patterns (TMV)
• Presence of pests — e.g. aphids on shoots

Identification methods:

• Gardening manuals or websites — compare symptoms
• Laboratory testing — using monoclonal antibodies or DNA analysis to identify the specific pathogen
• Testing kits — similar to those used for human diseases

Plant Defence Responses

Plants cannot run away from threats, so they have evolved their own defence mechanisms:

Physical defences:

• Cellulose cell walls — strong barrier against pathogens
• Tough waxy cuticle on leaves — waterproof barrier
• Bark on trees — thick protective layer
• Thorns and spines — deter herbivores (e.g. roses, cacti)
• Leaf curling — reduces surface area exposed to pests
• Drooping or mimicry — some plants mimic dead leaves or other species to avoid being eaten

Chemical defences:

• Antibacterial chemicals — produced to kill invading bacteria (e.g. mint, witch hazel)
• Poisons — deter herbivores (e.g. foxgloves produce digitalis; deadly nightshade produces atropine)
• Tannins — bitter-tasting chemicals that deter herbivores

Mechanical defences:

• Thorns — modified leaves or stems
• Hairs — on stems and leaves to deter insects
• Leaf drop — infected leaves are shed to prevent disease spread

Mineral Deficiency in Plants

Mineral	Needed for	Deficiency symptoms
Nitrate	Making amino acids and proteins	Stunted growth, yellowing of older leaves
Magnesium	Making chlorophyll	Chlorosis (yellowing) of leaves — cannot photosynthesise properly
Potassium	Enzyme function and disease resistance	Poor flower/fruit growth, yellow/brown leaf edges
Phosphate	DNA and cell membranes, root growth	Poor root growth, purple/red discolouration of leaves

Exam Tips

• The key to monoclonal antibodies is the hybridoma cell — know that it is a fusion of a lymphocyte and a tumour cell
• Plant defences are categorised as physical, chemical, and mechanical — give specific named examples
• Nitrate deficiency causes stunted growth and magnesium deficiency causes yellow leaves — these are the two most commonly examined