IMG-20240328-WA0017

Outcome of Host- Pathogen Relationship 1. Compatible response: Results in disease 2. Incompatible response: Results in little or no disease at all The response depends upon Disease resistance of individual plant species that exists as a range of responses a. Immunity (the complete lack of any disease symptoms) b. Highly resistant (some disease symptoms) c. Highly susceptible (significant disease symptoms).

Host — Pathogen Relationship • Biotrophs: Pathogens that keep their host alive and feed on living plant tissues Ex. Powdery mildew fungus Blumeria graminis and the bacterial rice pathogen Xanthomonas oryzae • Necrotrophs: Pathogens often produce toxins or tissue-degrading enzymes that beat plant defenses and promote the quick release of nutrients Ex. Gray mold fungus Botrytis cinerea and the bacterial soft-rot pathogen Erwinia carotovora • Hemibiotrophs: Pathogens are biotrophic during the early stages of infection but become necrotrophic during the latter stages of disease Ex. Fungus Magnaporthe grisea, the causative agent of rice blast disease.

Need for Defense in Plants • Adjustment is probably, one of the most important advantage of a system that ensures it survival • Resistance to Exploitation as a Biological System forced by co- existence with pathogen, has led to development of defense mechanism in plants • Survival of plants in spite of living amongst some of the potentiality disturbing pathogens in addition to abiotic stresses + Thus, defense means resistance against any 'damaging act' or response of plant system.

(iii) Structure of Epidermal Cell Walls: • Tough and thick outer walls of epidermal cells • The presence or absence of lignin and silicic acid in the cell walls • Most outer walls of epidermal cells of many plants are lignified Ex. The resistant varieties of potato tubers the epidermal cells contain higher fiber contents Ex. Rice leaves against Pyricularia oryzae causing Rice blast (iv) Structure of Natural openings: • The size and shape of stomata surrounded by broad lipped raised structures prevent entry of water drops containing pathogens • The small lenticels in many plants may play a defensive role against the pathogens • Nectaries provide openings in the epidermis and may play a defensive role due to high osmotic concentration of the nectar Ex. Resistant varieties of Citrus for Citrus canker bacteria X. citri.

(B) Defense Structures Developed after the Attack of the Pathogen (i) Defense Reactions in the Cytoplasm: • The cytoplasm of the invaded cell surrounds the hyphae of the pathogen and the nucleus of the host cell gets stretched and enlarge • The cytoplasm becomes granular and dense resulting the disintegration of the pathogen mycelium and thus the further invasion of påthogen stops (ii) Cell Wall Defense Structures: Three types of cell wall defense structures are generally observed (i) Cell walls thicken in response to the pathogen by producing a cellulose material (ii) The outer laver of cell walls of the arenchvma cells in contact with invading acterial cells produce an amorphous fibrillar material that traps the bacteria (iii) Callose papillae get deposited on the inner layers of the cell walls s Ex. Hyphal sheathing by F. oxysporum.

(c) Tyloses: Tyloses are out growths of protoplasts of adjacent live parenchyma cells protruding into xylem vessels through pits under stress or in response to attack by the vascular pathogens Their development blocks the Xylem vessels, obstructing the flow of water and resulting in the development of uilt symptoms Ex. In most of the plants against vascular wilt pathogen pp v Tyloses formation in Vessels.

Plant Resistance • Basal Resistance (Innate Immunity): Some Plants have surveillance mechanisms that recognize potentially dangerous pathogens and rapidly respond before those organisms have a chance to cause serious damage Ex. Plant cells recognize microbe-associated molecular patterns like specific proteins, lipopolysaccharides, and cell wall components commonly found in microbes • Ilypersensitive Response (IIR): Plant cells deliberately induce suicidal response at the site of infection Ex. Plant cell recognize the presence of specific disease-causing effector molecules introduced by the Bacteria, fungi, viruses, and nematodes • Systemic Acquired Resistance (SAR): Plant tissues become highly resistant to a broad range of pathogens for an extended period of time by producing plant activators Ex. These substances are much less toxic to humans and wildlife and their protective effects can last much longer • RNA Silencing: Plants can defend themselves by a refined genetic system through recognizing foreign molecules(dsRNA or DNA) and respond by digesting the genetic strands into useless fragments.

(iii) Defense Structures Developed by the Tissues: (a) Gum Deposition: Plants produce a variety of gummy substances around lesions or spots as a result of infection Ex. In resistant Rice plants against Helminthosporium infection (b) Abscission Layers: These layers develop in their young leaves in response to infection by several fungi, bacteria or viruses resulting formation of a gap between two circular layers of cells surrounding the point of infection Abscission layer formation protects the healthy leaf tissue from the attack of the pathogen Ex. By Xanthomonas pruni on Peach leaves.

(d) Formation of Cork Layers: • Some pathogens like certain bacteria, some fungi and even some viruses and nematodes stimulate the host to form multilayered cork cells in response to infection These layers inhibit the further invasion by the pathogen and also block the flow of toxic substances secreted by the pathogen Cork layers also stop the flow of nutrients of the host thus also depriving the pathogen of the nutrients Ex. Potato tuber infected "ith Rhizoctonia sp..

(A) Pre Existing Defense Structures (i) Cutin and Wax: • Cutin is composed of fatty acids and Mlax is a mixtures of long chain aliphatic compounds get deposited on the cuticular surface of plants causing a hydrophobic surface where water is repelled and the pathogen does not get sufficient water to germinate or multiply • Due to the presence of fatty acids a negative charge develops on the leaf surface which prevents/reduces the chance of infection by many pathogens Ex. Infection against many Bacteria and Fungi (ii) Thickness of Cuticle: • The thickness of cuticle obstructs the path of pathogen and also checks the exit of the pathogen from inside the host, thus reducing the secondary infection Ex. Linseed cuticle act as a barrier against Melampsora lini.

(iv) Necrosis or Hypersensitive Type of Defense: • In this case, the host nucleus moves toward the pathogen and soon disintegrates into broun granules • Soon the cell membrane swells and finally the cell bursts and dies • These cause the pathogen nucleus to disintegrate into a homogenous mass and its cytoplasm dense resulting the pathogen fails to grow beyond the necrotic or dead cells and the further growth of the pathogen is stopped Ex. Synchytrium endobioticum causing wart disease of potato, Phytophthora infestans causing late blight disease of potato and Pyricularia oryzae causing blast of rice G. NC..