L40 Plants and Pathogens

Crop Loss Due to Biotic Factors

• Crop losses from biotic factors (diseases, pests, weeds) are estimated to exceed 40% of yields, translating to more than $200 billion annually in monetary terms.

Key Contributors to Crop Losses

• Pests: Insects and other organisms that harm plant health.

• Diseases: Pathogenic attacks that can lead to decreased yield and quality.

• Weeds: Competing plants that can hinder crop growth and yield.

Environmental Impacts

• Tree Diseases: Recent cases of tree diseases such as Phytophthora ramorum, which prompted the Woodland Trust to cleave 500 acres of infected larches to prevent further spread.

  • Phytophthora ramorum: A severe pathogen causing considerable damage to forests.

Types of Plant Pathogens

1. Bacteria

2. Viruses

3. Fungi and Fungal-Like Organisms

Bacterial Diseases in Crop Plants

• Plant pathogenic bacteria result in average annual crop losses of over 5%, financially impacting around $50 billion.

• Bacterial infections can directly affect crop yields or indirectly contribute to spoilage during storage.

• Challenges include limited availability of resistance genes and environmental impacts from chemical treatments (e.g., copper sprays).

Notable Bacterial Pathogens

• Pseudomonas syringae: A widespread pathogen causing significant losses across various crops including tomatoes, kiwifruit, and olives.

• Typical diseases include:

  • Olive Knot disease

  • Bacterial speck of tomato

  • Blackleg disease: Caused by Pectobacterium spp in potatoes, leading to losses in the UK of up to £50 million annually.

Viral Diseases in Plants

• Major economic threat especially in Africa & Asia.

• Typically transmitted by vectors such as aphids and whiteflies.

• E.g., viruses in cassava and sweet potatoes can lead to yield losses of 25-50%.

Fungal and Fungal-Like Diseases

• Phytophthora infestans is notable for causing the Irish Potato Famine.

• Under ideal conditions, it can devastate a full crop in 2-3 weeks.

Strategies Utilized by Plant Pathogens

• Necrotrophs (Murderers): Thrive in dead tissue, secrete enzymes to kill host cells, and use nutrients released for growth.

• Biotrophs (Confidence Tricksters): Need living tissue, interact with the host to extract nutrients.

Plant Defense Mechanisms

• Plants can use structural barriers and toxic substances for protection.

• They activate innate immunity, requiring recognition of specific pathogens to curb their spread.

• Innate Immunity: Unlike mammals, plants do not have an adaptive immune response; their defenses are inherited and passed down through genes.

Components of Plant Immune System

• plants lack an adaptive immune system, instead rely on innate immunity of each cell and mobile signals

1. Basal Defense: Recognizes pathogen-associated molecular patterns (PAMPs) like flagellin and cell wall components.

   • PAMP-triggered immunity (PTI)

     • Conserved, exposed molecules from many microbe species. Indispensable to the microbe (critical function) Not present in the host (nonself)

2. Gene-for-Gene Resistance: Uses specific resistance (R) genes to recognize pathogen effectors and activate defense responses.

   • Gene-for-Gene Hypothesis (1940): Resistance in plants is achieved when complementary genes in host and pathogen interact.

     • Plants can recognise proteins associated with individual pathogens (often specific isolates of individual pathogens) and enact defence strategies to limit their spread.

Defense Responses Include:

• Programmed Cell Death (localized response to infection).

• Activation of Systemic Acquired Resistance (SAR): Initiated by gene-for-gene responses, whereby salicylic acid (SA) acts as a signaling molecule.

Resistance (R) Genes and Plant Breeding with Avirulence

• Breeding R-genes leads to increased field resistance; however, pathogens can evolve to overcome this resistance over time (Boom or Bust scenario).

• For instance, resistance to P. infestans exemplifies this cycle, leading to losses due to genetically drifted pathogens.

Engineering for Disease Resistance

• Conventional Resistance: Introduction of existing R-genes into crops.

• Novel Resistance: Introducing genes that limit pathogen growth through different mechanisms.

Case Study: Engineering Against Late Blight

• Innate Potatoes (Simplot): Genetically engineered potatoes resistant to late blight using R-genes from wild Solanum species.

Conclusion

• Addressing plant diseases is critical for securing agricultural output and preventing economic losses. Effective strategies rely on understanding pathogen characteristics and developing innovative resistant strains through genetic engineering and breeding practices.