Defensive strategies in plants

Plants have evolved a diverse array of defenses to combat pathogens, similar to the immune strategies found in animals. Pathogens such as bacteria, fungi, viruses, nematodes, and insects can all affect plants, and the specific defensive mechanisms vary across plant species. Unlike animals, plants lack antibodies and mobile immune cells. Instead, they rely on a range of complex metabolic responses and defensive compounds to mitigate infections and deter herbivores.

Pre-formed Defenses Plants use several structural and chemical defenses to protect themselves from pathogens:

• Structural Barriers: The plant cuticle and cell walls act as physical barriers to pathogen entry.

• Chemical Defenses: These include antimicrobial chemicals (e.g., glucosides and saponins), antimicrobial proteins, and enzyme inhibitors. Detoxifying enzymes break down toxins produced by pathogens.

• Receptors: Pattern-recognition receptors detect pathogens and initiate a basal immune response.

Inducible Defenses When a plant is infected, it activates various inducible defenses:

• Cell Wall Reinforcement: Plants enhance cell walls with substances like callose, lignin, suberin, and specialized proteins.

• Antimicrobial Chemicals: Reactive oxygen species (e.g., hydrogen peroxide) and complex phytoalexins (e.g., genistein, camalexin) are produced.

• Antimicrobial Proteins and Enzymes: Defensins, thionins, chitinases, beta-glucanases, and peroxidases play roles in fighting pathogens.

• Hypersensitive Response (HR): This involves rapid programmed cell death at infection sites to limit pathogen spread.

Plant Hormones and Responses

• Auxins: These hormones regulate the formation and organization of vascular tissues and aid in tissue regeneration after wounding.

• Traumatin: Produced in response to wounds, this hormone is a precursor to traumatic acid and helps in tissue repair.

• Jasmonic Acid (JA): Derived from linolenic acid, JA regulates growth inhibition, senescence, leaf abscission, and responses to wounding. It also inhibits insect digestion and is converted into derivatives like methyl jasmonate.

Systemin

Systemin is a peptide hormone found in the Solanaceae family, discovered in tomato leaves. It coordinates defense responses against insect herbivores and affects plant development. Systemin induces the production of protease inhibitors and other defensive proteins. Its role includes activating systemic acquired resistance and influencing plant responses to abiotic stresses, such as salt and UV radiation.

Dehydrins 

Dehydrins are proteins that help plants tolerate low temperatures and drought. Induced by abscisic acid (ABA) and salt stress, they protect cell membranes and contribute to high salt tolerance.

Plant Immune System and Signal Transduction 

Plants have evolved immune systems with similarities to those of animals but exhibit unique features:

• Pattern Recognition: Plants use pattern-recognition receptors to detect pathogen-associated molecular patterns (PAMPs) and initiate basal defense responses.

• Resistance (R) Proteins: Encoded by R genes, these proteins recognize specific pathogen effectors and activate targeted defense responses.

• Signal Transduction: Key mediators include salicylic acid, jasmonic acid, ethylene, reactive oxygen species, and nitric oxide. These signals trigger various defensive responses and adjust the plant’s immune response based on the pathogen threat.

• Systemic Responses Plants can mount systemic responses to an infection in one part of the plant, enhancing defense in other parts. This includes systemic acquired resistance (SAR), mediated by salicylic acid, and induced systemic resistance (ISR), mediated by jasmonic acid. Plants also use RNA interference to silence pathogen-specific genes, a form of adaptive immunity.

Broad-Spectrum Resistance 

In some cases, plants have genes that confer broad-spectrum resistance against entire pathogen species. Examples include barley’s MLO gene against powdery mildew, wheat’s Lr34 against leaf rust, and Yr36 against stripe rust. Effective plant immunity may arise from a lack of co-adaptation between the plant and pathogen or from highly effective pre-formed defenses.

Example diseases

Bacteria

Most bacteria that are associated with plants are actually apostrophic, and do no harm to the plant itself. However, a small number, around 100 species, are able to cause disease. Bacterial diseases are much more prevalent in sub-tropical and tropical regions of the world.

Most plant pathogenic bacteria are rod shaped. In order to be able to colonize the plant they have specific pathogenic factors. Five main types of bacterial pathogenic factors are known:

1. Cell wall degrading enzymes – used to break down the plant cell wall in order to release the nutrients inside. Used by pathogens such as Erwinia to cause soft rot.

2. Toxins - These can be non-host specific, and damage all plants, or host specific and only cause damage on a host plant.

3. Effecter proteins - These can be secreted into the extracellular environment or directly into the host cell, often via the Type three secretion system. Some effectors are known to suppress host defense processes.

4. Phytohormones – for example Agrobacterium changes the level of auxins to cause tumours.

5. Exopolysaccharides – these are produced by bacteria and block xylem vessels, often leading to the death of the plant.

Significant bacterial plant pathogens include:

• Burkholderia

• Proteobacteria

  • Xanthomonas spp.

  • Pseudomonas spp.

Phytoplasmas and spiroplasmas

Phytoplasmas and Spiroplasmas are a genre of bacteria that lack cell walls, and are related to the mycoplasmas which are human pathogens. Together they are referred to as the mollicutes. They also tend to have smaller genomes than true bacteria. They are normally transmitted by sap-sucking insects, being transferred into the plants phloem where it reproduces.

Fungi

• The majority of phytopathogenic fungi belong to the Ascomycetes and the Basidiomycetes.

• The fungi reproduce both sexually and asexually via the production of spores. These spores may be spread long distances by air or water, or they may be soil borne. Many soil borne spores, normally zoospores and capable of living saprotrophically, carrying out the first part of their lifecycle in the soil.

• Fungal diseases can be controlled through the use of fungicides in agriculture, however new races of fungi often evolve that are resistant to various fungicides.

• Significant fungal plant pathogens:

Ascomycetes

• • Fusarium spp. (causal agents of Fusarium wilt disease)

  • Thielaviopsis spp. (causal agents of: canker rot, black root rot, Thielaviopsis root rot)

  • Verticillium spp.

  • Magnaporthe grisea causes blast of rice and gray leaf spot in turf grasses

Plant pathology department of Infectious diseases

Basidiomycetes

• • Rhizoctonia spp.

  • Phakospora pachyrhizi Sydow; causes soybean rust

  • Puccinia spp.; causal agents of severe rusts of virtually all cereal grains and cultivated grasses

Oomycetes

The oomycetes are not true fungi but are fungal-like organisms. They include some of the most destructive plant pathogens including the genus Phytophthora which includes the causal agents of potato late blight and sudden oak death.

Despite not being closely related to the fungi, the oomycetes have developed very similar infection strategies and so many plant pathologists group them with fungal pathogens.

Significant oomycete plant pathogens

• Pythium spp.

• Phytophthora spp.

Viruses and Viroids

• There are many types of plant virus, and some are even asymptomatic. Normally plant viruses only cause a loss of crop yield. Therefore it is not economically viable to try to control them, the exception being when they infect perennial species such as fruit trees.

• Most plant viruses have small, single stranded RNA genomes. These genomes may only encode three or four proteins: a replicase, a coat protein, a movement protein to allow cell to cell movement though plasmodesmata and sometimes a protein that allows transmission by a vector.

• Plant viruses must be transmitted from plant to plant by a vector. This is often by an insect, but some fungi, nematodes and protozoa have been shown to be viral vectors.

Nematodes

Nematodes are small, muilticelluar wormlike creatures. Many live freely in the soil, but there are some species which parasitise plant roots. They are a problem in tropical and subtropical regions of the world, where they may infect crops. Potato cyst nematodes (Globodera pallida and G. rostochiensis) are widely distributed in Europe and North and South America and cause £160 million worth of damage in Europe every year. Root knot nematodes have quite a large host range whereas cyst nematodes tend to only be able to infect a few species. Nematodes are able to cause radical changes in root cells in order to facilitate their lifestyle.

Protozoa

There are a few examples of plant diseases caused by protozoa. They are transmitted as zoospores which are very durable, and may be able to survive in a resting state in the soil for many years. They have also been shown to transmit plant viruses. When the motile zoospores come into contact with a root hair they produce a plasmodium and invade the roots.

Parasitic Plants

Parasitic plants such as mistletoe and dodder are included in the study of phytopathology. Dodder, for example, is used as a conduit for the transmission of viruses or virus-like agents from a host plant to either a plant that is not typically a host or for an agent that is not graft-transmissible.

Insects

Types of Plant-Damaging Insects:

• Chewing insects, such as caterpillars, beetles, and grasshoppers, are among the most destructive plant pests. They feed by chewing on plant tissues, which can lead to significant defoliation and reduced plant vigor.

• Caterpillars: Larvae of moths and butterflies, such as the cabbage looper and corn earworm, are notorious for their voracious appetite. They can consume large amounts of foliage, flowers, and fruits, severely impacting crop yield and quality.

• Beetles like the Colorado potato beetle and the Japanese beetle target a variety of plants, including economically important crops like potatoes, tomatoes, and grapes. Their feeding can lead to defoliation, reduced photosynthesis, and increased susceptibility to diseases.

• Grasshoppers: These insects can cause widespread damage to crops and natural vegetation, especially during outbreaks. Their feeding habits can strip entire fields of their foliage, leading to significant agricultural losses.

• Sucking insects, such as aphids, whiteflies, and spider mites, feed on plant sap by piercing plant tissues with their specialized mouthparts. This feeding behavior can weaken plants, transmit diseases, and cause distortion.

Aphids

Aphids are small, soft-bodied insects that cluster on plant stems and leaves. They extract sap, which can stunt plant growth and transmit plant viruses like the cucumber mosaic virus.

Whiteflies

Whiteflies are tiny, winged insects that suck sap from the undersides of leaves. Their feeding can cause yellowing and wilting of leaves and they are known vectors for various plant viruses.

Spider mites

These minute arachnids feed on plant cells, causing stippling, discoloration, and leaf drop. Heavy infestations can lead to significant plant damage and reduced crop yields.

Boring insects 

Boring insects, including certain beetles and larvae of insects like the emerald ash borer, attack plants by boring into stems, trunks, and roots. Their feeding behavior can compromise the structural integrity of plants, leading to death.

Leaf mining insects 

Leaf miners are insects that create tunnels or mines within the leaf tissue. Their feeding can reduce the photosynthetic capacity of the plant and make it more vulnerable to other pests and diseases. Leaf Miners include the larvae of certain flies and moths that burrow into leaves, creating visible trails. The damage reduces the plant's ability to photosynthesize effectively, impacting overall health and productivity.

Plant Disorders

Significant abiotic disorders can be caused by:

·         Flooding and poor drainage

·         Frost damage by snow and hail

·         Drought

·         Nutrient deficiency

·         Salt deposition and other soluble mineral excesses

·         Wind (windburn, and breakage by hurricanes and tornadoes)

·         Lightning, wildfire and manmade

·         Man-made (arguably not abiotic, but usually regarded as such)

·         Soil compaction

·         Pollution of air, soil, or both

·         Salt from winter road salt application or irrigation

·         Herbicide over-application

·         Poor education and training of people working with plants

Management of diseases

Quarantine

Wherein a diseased patch of vegetation or individual plants are isolated from other, healthy growth. Specimens may be destroyed or relocated into a greenhouse for treatment/study. Another option is to avoid introduction of harmful non-native organisms by controlling all human traffic and activity although legislation and enforcement are key in order to ensure lasting effectiveness.

Cultural

Farming in some societies is kept on a small scale, tended by peoples whose culture includes farming traditions going back to ancient times. (An example of such traditions would be lifelong training in techniques of plot terracing, weather anticipation and response, fertilisation, grafting, seed care, and dedicated gardening.) Plants that are intently monitored often benefit not only from active external protection, but a greater overall vigour as well. While primitive in the sense of being the most labour-intensive solution by far, where practical or necessary it is more than adequate.

Plant resistance

Sophisticated agricultural developments now allow growers to choose from among systematically cross-bred species to ensure the greatest hardiness in their crops, as suited for a particular region's pathological profile. Breeding practices have been perfected over centuries, but with the advent of genetic manipulation even finer control of a crop's immunity traits is possible. The engineering of food plants may be less rewarding however, as higher output is frequently offset by popular suspicion and negative opinion about this tampering with nature.

Chemical

Many natural and synthetic compounds exist that could be employed to combat the above threats. This method works by directly eliminating disease-causing organisms or curbing their spread; however it has been shown to have too broad an effect, typically, to be good for the local ecosystem. From an economic standpoint all but the simplest natural additives may disqualify a product from organic status, potentially reducing the value of the yield.

Biological

Crop rotation may be an effective means to prevent a parasitic population from becoming well established, as an organism affecting leaves would be starved when the leafy crop is replaced by a tuberous type, etc. Other means to undermine parasites without attacking them directly may exist.

Integrated

The use of two or more of these methods in combination offers a higher chance of effectiveness.

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